A Dictionary of APL Acknowledgments I am indebted to numerous colleagues for minor corrections and major ideas arising from their reviews of various drafts of the mss., chief among them being R. Pesch, E.E. McDonnell, R. Hui, D.L. Forkes, J.A. Cramer, K.B. Iverson, P. Cooper, R.H. Chamberlain, L.J. Dickey, and J. Sansom. © 1987, by Kenneth E. Iverson. Reproduced with permission from the Iverson estate. APL is a formal, imperative language. Because it is imperative, a sentence may be called an instruction, and may be executed to produce a result. In illustrations used here an instruction will be indented, and the result of its execution will be shown on the following line without indentation. Thus: 3+4 7 2×(3+4) 14 Because it is formal and unambiguous, APL can be executed mechanically by a machine called a computer or, strictly speaking, an APL computer, or APL system. For this reason it is also called a programming language, a phrase from which its name derives. Because it shares the analytic properties of mathematical notation, it is also called an analytical programming language. Like any living language, APL has dialects, resulting in part from evolution that has rendered certain constructs obsolete, but also from the limitations and special characteristics of the many different computers on which APL systems are implemented. Although this introduction includes a brief discussion of dialects, the reader may find it necessary to consult manuals for specific systems, or the APL Standard promulgated by the International Standards Organization [1]. APL originated in an attempt to provide consistent notation for the teaching and analysis of topics related to the application of computers, and developed through its application in a variety of topics, and through its implementation in computer systems. Extensive discussions of the history of the language may be found in The Design of APL [2] and The Evolution of APL [3], and in the publications cited therein. A dictionary should not be read as an introduction to a language, but should rather be consulted in conjunction with other material that uses the language in some context of interest to the reader. Even the general section on grammar, which may be intelligible even to the beginner, should perhaps be studied only after a certain amount of other exposure to the language. On the other hand, a dictionary should not be used only
to find the meanings of individual words, but should also
be studied to gain an overall view of the language. In particular,
the grammar may be profitably reviewed again and
again in the light of increased knowledge of the language,
and the study of groups of related verbs and adverbs can
reveal important relationships otherwise easily overlooked.
I: Letters and Words In addition to both cases of the letters of some native alphabet such as English (used for names) and the ten decimal digits (used in numbers and in names), the APL alphabet includes six Greek letters, fiftyfour graphic symbols, and about twenty composite symbols formed by superposing a pair of graphic symbols. Nearly all of the graphic symbols occur (in at least one orientation) in some table of commonlyused symbols in English dictionaries, such as the Table of Symbols and Signs in The American Heritage Dictionary [4]. At least half are familiar from their use in English and in arithmetic. On most APL systems, the name ⎕av denotes a list whose elements include the alphabet. Many systems print the two English alphabets in other than lowercase and uppercase italics, often using uppercase and uppercase underscored. Every “primitive” verb and adverb (i.e., those defined in the dictionary) is formed from a single graphic, composite symbol, or Greek letter. Consequently, symbols do not conflict with the choice of names for pronouns, as do the reserved words (names of primitives represented by letters of the English alphabet) commonly used in programming languages. In any use of symbols it is convenient to have pronounceable names for them. Because of the many different uses commonly made of the same symbol in different endeavours, the bestknown names usually reflect a particular use of the symbol rather than a name for the symbol itself. For example, in the cited table from [4], “resistivity”, and “multiplied by” are given for ⍴ and × , rather than “Greek rho” and “St. Andrew’s cross”. In this dictionary, we attempt to provide both a name for the symbol (in Table 1: Alphabet) and for its general use in APL (Sections IVVI). Table 4 provides synonyms for possible use in particular contexts. The three columns of Table 1 provide the APL symbol, a name for the symbol, and a transliteration scheme for use with the ASCII character set [5] widely provided on computer terminal devices. The last twentyfive APL symbols (beginning with ⍝) are called composite symbols because they are commonly entered by superposing two simpler symbols (as in ∩ with ∘ for ⍝), and are often named by the pair composing them (as in circlebar for ⊖). A student of any language should be alert to, and exploit, clues provided by knowledge of other languages, as well as clues provided by the appearance of the symbols themselves. For example:
Words are formed by scanning a sequence of letters of the APL alphabet from left to right according to the following rules:
Wordformation is defined formally by the function ⊥ in Section IV. Any of the words ⍺ , ⍵ , → , and $ , or a word that begins with the symbol ⎕ or a letter of the native alphabet and continues with further letters, digits, macrons or dots is a name, and may be assigned a value by means of a copula (←) as discussed in the section on grammar. A word that is not a name is called a token. A token is either meaningless or has a fixed referent. For example, the tokens 3.14 and 314e¯2 and 2j3 are meaningful, but 3..14 and ¯¯2 and 'pi and 2k3 are not. A meaningful numeric token may be a list formed of two or more numbers separated by spaces, where a number may be a real number or a complex number formed by two real numbers separated by a j . A real number is either simple or
an exponential number
formed from two simple numbers separated
by an e .
A simple number may be formed of
one or two macrons (representing infinity and minus infinity),
or contain at least one
digit and at most one dot and one macron (which denotes
a negative number and must be in the leading position).
II: Grammar APL has six grammatical elements:
A. Nouns Nouns may be classified in four independent ways: numeric or literal; token or pronoun; open or boxed; arrays of various ranks. Arrays of ranks 0 , 1 , and 2 have the special names, item, list, and table, or, alternatively (in mathematics) scalar, vector, and matrix. The four classifications are elaborated below. Numeric and literal. Numbers are written as 2 and 2.3 and 23e2 (for 2300) and 2j5 (for a complex number), and a negative number has a leading macron, as in ¯3.2 ; literals are enclosed in quotes, as 'A' and 'a' and '+' . Tokens and pronouns. A name that has been assigned to a noun will be called a pronoun. Tokens that refer to nouns include ¯ and ¯¯ and ∘ (denoting infinity, minus infinity, and the boxed empty list <''), as well as other numeric items and lists (such as 3.4e2 and 2 3 4), and literal items and lists (such as 'a' and 'cab'). A phrase that produces a noun (such as 3×4 or ⌽'abcd') may be called a noun phrase. Arrays. A single entity such as 2.3 or 2.3j5 or 'A' or '+' is called an item. The verb denoted by a comma (and called catenate or before) chains its arguments to form a list whose shape (given by the verb ⍴) is equal to the number of items combined. For example: date←1,7,7,6 ⍴date 4 word←'s','a','w' ⍴word 3 ⌽word (⌽ is the verb reverse) was ⌽date 6 7 7 1 The expression s⍴l produces an array of shape s from the list l. For example: (3,4)⍴date,1,8,6,7,1,9,1,7 1 7 7 6 1 8 6 7 1 9 1 7 table←2 3⍴word,'b','a','t' ⍴table table 2 3 saw bat The number of elements in the shape of a noun is called the rank of the noun; thus a noun of rank 2 is a table, of rank 1 is a list, of rank 0 is an item, of rank 3 is a 3dimensional or rank3 array, etc. Moreover, each position of the shape is called an axis of the array, and axes are referred to by indices 0 , 1 , 2 , etc. For example, axis 0 of table has length 2 and axis 1 has length 3 . The last k axes of an array a determine rankk cells or kcells of a . For example, if: a←2 3 4⍴'abcdefghijklmnopqrstuvwx' a abcd efgh ijkl mnop qrst uvwx then the list 'abcd' is a 1cell of a , the two separate 3 by 4 tables are 2cells of a , and the individual letters are each 0cells of a . The rest of the shape vector is called the outer shape
or frame of the array relative to the cells
of rank k .
For example, if ⍴b
is The number of cells in a frame is the product over its shape, and if one or more of the elements in the shape is zero, the number of cells is zero; the frame is then said to be a zero frame. Since the product across an empty list is 1 , an empty frame is not a zero frame. A cell of rank one less than the rank of a is called a major cell of a , and major cells play an important role in the discussion of nouns and the application of verbs to them. For example, the verb from (denoted by {) selects major cells from its argument, as in: 0{a 1{a abcd mnop efgh qrst ijkl uvwx 0{0{a 2 1{0{a abcd ijkl efgh 1{2{0{a j Moreover, the verb grade (denoted by ⍋) provides indices to { that bring major cells to “lexical” or “rowmajor” order. For example: n←4 3⍴3 1 4 2 7 9 3 2 0 3 1 4 n 3 1 4 2 7 9 3 2 0 3 1 4 g←⍋n g 1 0 3 2 g{n 2 7 9 3 1 4 3 1 4 3 2 0 'abcdefg' ⍋ 'care' 1 0 3 2 Negative numbers (as in ¯1cell and ¯2cell) are also used to refer to cells whose corresponding frames are of the rank indicated by the magnitude of the number. For example, the list 'abcd' may be referred to either as a ¯2cell or as a 1cell of a , and each of the two separate 3 by 4 tables are called either ¯1cells or 2cells of a . The ¯1cells of an array are its major cells, and an item has a single major cell, itself. Open and boxed. The nouns discussed thus far are called open, to distinguish them from boxed nouns produced by the verb box (denoted by <). The result of box is an item, and boxed nouns are commonly (but not necessarily) displayed in boxes. For example: <'here' ¯¯¯¯ here ____ Box allows one to treat any array (such as a list of letters that represent a word) as a single entity. For example: letters←'I was here' ⍴letters 10 ⌽letters ereh saw I words←(<'I'),(<'was'),(<'here') ⍴words 3 ⌽words (2,3)⍴words, ⌽words ¯¯¯¯¯¯¯¯ ¯ ¯¯¯¯¯¯¯ herewasI I washere ________ _ _______ ¯¯¯¯¯¯¯¯ herewasI ________ B. Verbs Most verbs are limited in their normal application, and the class of nouns to which a verb normally applies is called its domain. For example, the verb minus (in the expression n) applies only to a numeric argument n , and the expression 'n' is meaningless; the domain of minus is limited to numeric arguments. Monads and Dyads. Most verbs have two definitions, one for the monadic case (one argument), and one for the dyadic case (two arguments). If one of these definitions is omitted, the corresponding case of the verb has an empty domain. In any sentence, the dyadic definition of a verb applies if it is preceded by a suitable left argument, that is, any noun that is not itself an argument of a conjunction. Otherwise the monadic definition applies. The monadic case of a verb is also called a monad, and we may speak of “the monad ÷” used in the expression ÷⍵ , and of “the dyad ÷” used in the expression ⍺÷⍵ . Ranks of verbs. The notion of verb rank is closely related to that of noun rank: a verb of rank k applies to each of the kcells of its argument. This notion will be introduced by an example using the verb ravel (denoted by a comma), which ravels its entire argument to produce a list of the elements. Thus: a←2 3 4⍴'abcdefghijklmnopqrstuvwx' a abcd efgh ijkl mnop qrst uvwx ,a abcdefghijklmnopqrstuvwx Since ravel applies to its entire argument, its rank is said to be unbounded, or infinite; it can also be applied to cells of a specified rank r by using the rank conjunction ⍤ in the expression ,⍤r . Thus: ,⍤2 a abcdefghijkl mnopqrstuvwx ⍴,⍤2 a 2 12 The last result illustrates a general rule: the shape of a result is the frame of the argument (relative to the cells to which the verb applies) catenated with the shape produced by applying the verb to the individual cells. Commonly these individual shapes agree, but if not, they are first brought to a common shape as follows:
The case of a zero frame (which has no cells to which
the function may be applied)
is normally treated as follows:
The shape of the individual result is determined by
applying the function to a surrogate argument having the
shape required for the argument cell.
For example, if ⍴m
is The dyadic case of a verb has two ranks, a left rank that governs the rank of the cells of its left argument, and a right rank that governs the rank of cells of its right argument. For example: p←'abc't q←3 4⍴'wakereadlamp' q wake read lamp p,⍤0 1 q awake bread clamp Finally, each verb has three intrinsic ranks, a monadic rank, a left rank, and a right rank. This fact often simplifies the definition of a verb. For example, the monadic case of ⌽ is defined to have rank 1, and it therefore suffices to define its behaviour on lists, perhaps by example, as in ⌽'abc' ←→ 'cba' and ⌽1 2 3 ←→ 3 2 1 . The application of ⌽ to an argument of higher rank is therefore completely defined. For example: ⌽a dcba hgfe ikji ponm tsrq xwvu Degenerate cases. The rank of a verb merely places an upper limit on the ranks of the cells to which it applies, and its domain may include arguments of rank lower than its nominal rank. For example, ⌽ has rank 1 , but its domain also includes an item s as follows: s≡⌽s . Similarly, ⌹ (matrix inverse) has rank 2 , but is extended to list and item arguments as follows: (⌹a)≡⌹⍪⊃a , where the table function ⍪ forms a onecolumn table from a list or item argument. Agreement. The two arguments of the dyadic case of a verb must agree in the following sense: the left frame and the right frame (relative to the particular verb) must be identical, except that if one frame is an empty list, the single corresponding cell is used as argument together with each cell of the other argument. For example, if q←3 4⍴'wakereadlamp'
and p←'abc'
(as in the earlier example
using the derived verb ,⍤0 1),
then in the expression The same arguments in the expression p,⍤1 1 q abcwake abcread abclamp illustrate the exceptional case; the left frame is empty, and the single cell 'abc' is extended to apply to each of the three cells of the right argument. If one of the ranks is unbounded, the extension of the corresponding argument will always occur. For example, since the indexing verb from, { , has zero left rank and unbounded right rank, we have: p←3 4⍴'abcdefghijkl' p 2 0{p abcd ijkl efgh abcd ijkl The tie conjunction (denoted by a period) can be used to relax the normal agreement constraints by specifying the number of leading axes of the frames that must agree, and “freeing” the remaining frame axes to interact independently. For example: p←l 2 3 g←l 2 3 m←p 0 .+ q m 2 3 4 5 3 4 5 6 4 5 6 7 ⍴m 1 .+m 3 4 4 ⍴m 0 .+m 3 4 3 4 C. Adverbs and conjunctions Unlike verbs, adverbs and conjunctions have a fixed valence; an adverb is monadic (applying to a single argument to its left), and a conjunction is dyadic (applying to two arguments, one on each side). Each argument of a conjunction may be either a noun or a verb, and it may therefore produce as many as four distinct classes of results. For example, v⍤n produces a rank n function that applies the verb v to each cell of rank n ; n⍤v produces a cut that applies the verb v to each of a set of segments cut along the first axis of the argument in a manner determined by the noun n ; v1⍤v2 produces the composition of the verbs v1 and v2 ; and n1⍤n2 yields a constant verb of rank n2 , whose result for each cell is n1 . The conjunction ⍤ may therefore be referred to variously as the rank, cut, composition, or constant according to its use, or may be referred to by the single term on, which is vague enough to roughly cover all of the cases, as in “ravel on reverse” for ,⍤⌽ , and “ravel on 2” for ,⍤2 . The most common result of an adverb or conjunction is a verb, but it may also be a noun, adverb, or conjunction. D. Copula In addition to certain distinguished names (discussed in the following section), the names that may be assigned by the copula are ⍺ and ⍵ and → and $ and those that begin with a letter of the native alphabet and (may) continue with letters or digits, as in ABc←3 and a2←3 , and A2b3←3 , or with a macron or dot, as in A¯b¯2 and a.b . This restriction prohibits the redefinition of tokens, as in 3←2 or +←2 or +←× . In the expression (1{'abc')←3+4 , the parentheses force evaluation of the expression 1{'abc' before the assignment, and the name represented by the result is assigned the result of 3+4 . Thus the name b is assigned the value 7 . Name assignments of the form (n)←x are called indirect. More generally, if the shape of n
agrees with the outer shape
of x ,
then (n)←x assigns the name represented by
the open of each element of n
to the corresponding cell
of x .
For example, if x←3 4⍴⍳12
and n1←'abc'
and n1 n2 x abc ¯¯¯¯¯¯ 0 1 2 3 p0p1p2 4 5 6 7 ______ 8 9 10 11 (n1)←x (n2)←x 'abc'←3 4 5 b p0 b 4 5 6 7 0 1 2 3 4 In other words, (n)←x implies that (>i{n)←>i{x for each item i{n of n , and corresponding cell i{x of x . However, if>i{n is itself boxed (and therefore not a proper name), it and the corresponding cell are both opened, and assignment is re attempted. Thus, if n←(<'p'),(<<'q'),<<<'r' then (n)←3⍴<<'cat' assigns <<'cat' to p and <'cat' to q , and 'cat' to r . An alternate form of the copula (denoted by ⍅) is discussed in Section J. E. Dialogue Any user of a language will normally wish to control his own pronouns, in the sense that the value assigned by him to any name will not be affected by assignments made by others to the same name. On the other hand, one partner in a dialogue may use a pronoun assignment made by the other, as in:
In general, a mutual agreement by two users to share a specific name can be used to provide communication between them, and such communication (via one or more shared names) can provide the basis for arbitrarily complex collaboration. If one partner wishes to ensure that the other partner has actually consulted the current value assigned to a shared name before he assigns a new value, the partners can achieve this by sharing one or more further names, and using them in a strict protocol to signal and acknowledge dispatches (assignments) and receipts (uses) of the values of the primary shared name. APL computer systems that provide for sharing names also provide protocols that control delivery and receipt of values assigned to shared names, and manuals for specific systems should be consulted. A good basic statement is provided by [7]. Most systems limit sharing to names of nouns, and prohibit the sharing of names of verbs and adverbs. A set of distinguished names (beginning with ⎕ or ⍞) is reserved for communicating with the APL system that executes APL sentences. Three such names, used uniformly in dialects, merit consideration here. As may have been apparent from earlier examples of
the execution of APL sentences, the result of a sentence
such as ⎕←a←1 2 3 3 2 1 the name ⎕ denoting, in effect, the display mechanism of the system. The name ⍞ is shared with the terminal driver (that controls the inputoutput device). When referred to (as in b←⍞) it, in effect, denotes the keyboard, and assumes the literal value of the list of symbols next entered on it. For example: a←⍞ eva can i stab evil live bats in a cave a evac a ni stab evil live bats i nac ave ⍴a 39 When assigned a value (as in ⍞←d), ⍞ behaves like ⎕ , except that it signals the terminal driver to suppress the final “carriage return”, that is, it leaves the cursor at the end of the output displayed. The detailed behaviour of ⍞←d varies considerably between different APL systems. Finally, ⎕io (called index origin) is not used here, but merits comment because it occurs in all dialects, affecting the behaviour of the verbs ⍋ , ⍒ , ⍉ , ? , and ⍳ . If ⎕io is assigned the value 0 , the behaviour in any dialect agrees with that described here. If ⎕io←1 , then each element of ⍳⍵ is increased by 1 , and the other functions (which yield or use indices drawn from ⍳⍵) are affected accordingly. F. Comparatives In the everyday use of comparisons, a reasonable relative tolerance is implied. For example, a statement that two threefoot shelf boards are equal in length would normally imply that they agree to a fraction of an inch, whereas a statement that two cities are equidistant from a third would normally imply that the distances agree to within a mile or so. In APL the comparison ⍺=⍵ is treated similarly, yielding 1 (for true) not only if ⍺ and ⍵ are identical, but also if the difference ⍺⍵ falls relatively close to zero. The relative tolerance used in comparisons is specified by the system variable ⎕ct (called comparison tolerance). If the magnitude of the difference ⍺⍵ does not exceed ⎕ct times the larger of the magnitudes of ⍺ and ⍵ , then ⍺=⍵ yields 1 . The application of tolerance in other comparisons (such as < , ≤ , and ≠) is detailed in the discussion of these verbs. Tolerance also applies to the verbs ⌊ (floor or integer part) and ⌈ (ceiling), which yield integer results, effectively by comparing the argument with neighbouring integers. Exact comparisons may be obtained by setting ⎕ct to 0 . G. Terminology Standard names and synonyms. Because of the rather large number of verbs, adverbs, and conjunctions in APL, it is important to choose names for them that are both distinctive and suggestive. Standard names are used in Sections IVVI, and Table 4 lists synonyms that may be appropriate in narrower contexts. For example, the verb ⍴ (called shape) yields the number of elements of a list to which it is applied and is therefore often called “length”, although such a term would be inappropriate in geometry, where the length of a list b (better called a vector in this context) is defined as (+/b*2)*.5 . Although many mathematical terms provide standard names (such as plus, minus, and times), others are unsuitable for various reasons:
Distinct names for the monadic and dyadic cases of a verb are desirable, but not essential. Thus, a*b and a×*b are clear when read as “a power b” and “a times power b”, and the phrases “the dyad power” and “the monad power” are as convenient as “the power function” and “the exponential function”. When working in a narrow and familiar context, one might find the suggested standard names bizarre and unnecessary, but in wider contexts come to accept them as reasonable compromises. Other considerations. In mathematics the terms scalar, vector, and matrix are used for what we have here called item, list, and table; function and operator are used for what we have called verb and adverb. We will use these synonyms wherever they seem appropriate. In verbalizing a written APL sentence,
clarity may be gained by observing that
the simple verbs ⊢ and ⊣
(called right and left)
have the effect of coordinating or subordinating
conjunctions.
Thus, The occurrence of a copula without ⊣
may also be read as “where”,
as in “a is x
times x ,
where x is 1 2 3”,
for a←x×x←1 2 3 .
Because of the common use of “and”
for the verb ^ in logic,
it should probably not be used
instead of “where”,
even though it may seem appropriate.
H. Identities and Proofs Formal identities between sentences can play an important role in the use of formal languages. In the body of the dictionary we will adopt a rather widely used scheme for expressing identities: writing one sentence immediately below another will imply that the second is equivalent to the first. For example: +/⍳n +/⌽⍳n .5×(+/⍳n)+(+/⌽⍳n) .5×+/((⍳n)+(⌽⍳n)) .5×+/n⍴(n1) .5×n×(n1) The foregoing six sentences state five identities, all of which may be tested by executing them after assigning some value to n . Moreover, the five identities together imply an identity between the first and the last sentences, and therefore provide the wellknown efficient calculation (.5×n×n1) for the sum of a sequence of n successive integers beginning with zero. The foregoing example can be elaborated to provide a proof of the identity between the first and last sentences by writing beside each sentence the basis for asserting its identity with the preceding sentence. For example:
Identities are also expressed by placing ←→ between sentences, as in +/⍳n ←→ .5×n×n1 , or by using the verb ≡ , as in (+/⍳n)≡.5×n×n1 . I. Parsing and Execution A sentence is executed by executing its parts in a sequence determined by the parsing rules of the language. For example, the sentence 10÷3+2 is executed by first executing 3+2 to obtain a result that is then used to divide 10 . The parsing rules can be summarized as follows:
One important consequence of these rules is that in an unparenthesized sentence the right argument of any verb is the result of the entire phrase to the right of it. A sentence such as 3×p⌈q*r5 can therefore be read from left to right: the overall result is three times the result of the remaining phrase, which is the maximum of p and the part following the ⌈ , and so on. It is also instructive to examine the explicit parsing process. Parsing proceeds by moving successive elements (or their values in the case of pronouns) from the tail end of a left stack (originally the given sentence prefixed by a marker ⍝) to the front of a right stack, and eventually “executing” some eligible portion of the right stack and replacing it by the single result of the execution. For example, if a←l 2 3 , and if ⋄ is used to separate the stacks, then the sentence b←+/2xa would be parsed and executed as follows: ⍝ b ← + / 2 × a ⋄ ⍝ b ← + / 2 × ⋄ 1 2 3 ⍝ b ← + / 2 ⋄ × 1 2 3 ⍝ b ← + / ⋄ 2 × 1 2 3 ⍝ b ← + ⋄ / 2 × 1 2 3 ⍝ b ← + ⋄ / 2 4 6 ⍝ b ← ⋄ + / 2 4 6 ⍝ b ⋄ ← + / 2 4 6 ⍝ b ⋄ ← 12 ⍝ ⋄ b ← 12 ⍝ ⋄ 12 ⋄ ⍝ 12 The foregoing illustrates two important points: 1) Execution
of the phrase The executions in the right stack are confined to the first four elements only, and eligibility for execution is determined only by the class of each of these elements (noun, verb, adverb, conjunction, copula, parenthesis, name, and left marker). Consequently, the parsing process can be made clearer by replacing each element in a sentence by a single chosen member of its class: 1 , + , / , and period for noun, verb, adverb, and conjunction; and the letters a , b , c , d , and e for names. The earlier example would then begin as follows: ⍝ b ← + / 1 + 1 ⋄ ⍝ b ← + / 1 + ⋄ 1 ⍝ b ← + / 1 ⋄ + 1 ⍝ b ← + / ⋄ 1 + 1 ⍝ b ← + ⋄ / 1 + 1 ⍝ b ← + ⋄ / 1 Thus:
J. Verb Definition The conjunction ∇ provides a general means for defining a new verb, as discussed in Section VI. A simpler informal scheme called direct definition [6] will be adopted for definitions used for exposition in this dictionary. It defines a function by either one or three sentences, as illustrated below: sqrt: ⍵*.5 sqrt 5 5 6 2 2.2361 2.4495 root: ⍵*÷⍺ 2 root 64 3 root 64 8 4 f: ⍵:⍺=1:~⍵ 0 f 0 1 1 f 0 1 0 1 1 0 The symbols ⍺ and ⍵ denote the left and right arguments. In a threesentence definition, the middle sentence is a conditional which is executed first; the first or last sentence is then executed according to whether the result of the conditional is 0 or 1 . Since direct definition is informal, its use on any APL system requires a translation provided by the functions translate and ∆ , themselves defined in the canonical form discussed in Section III. They appear in Table 3, together with examples of use. Local and global names. A name may be local to a function in the sense that its use in the execution of the function has no relation to its use outside the function. For example, in direct definition, the argument names ⍺ and ⍵ are local, as are any names that occur immediately to the left of a copula. For example: f:axa ←1+b←2×⍵ a←b←⍵←3 f 2 25 a,b,⍵ 5 3 3 A name that is not local to a function is said to be global to it; a name not local to any function is said to be a global name. In functions produced
by the conjunction ∇ ,
the names ⍺ , ⍵ , → ,
and $
are local, and other localizations may be
produced dynamically:
when any name a is to be assigned
by an expression of the
form a←b or (a)←b ,
then a is first
made local if it is not already so.
Expressions of the
form a⍅b and (a)⍅b
do not produce localization.
III: Dialects The vexing question of what to include in a dictionary as standard, and what to relegate to dialects is settled here as follows: a construct is excluded if a) it is anomalous, and itself requires special rules, and b) is obsolescent, in the sense that its use can be avoided by the use of other (usually newer) constructs that are at least as convenient. Variations in wordformation were discussed in Section I. Anyone beginning to write for a particular APL system should consult the manual for it, and should probably do so rather early so as to avoid the use of constructs that it does not include. On the other hand, the use of such alien constructs may prove beneficial, since they may lead to an improved style of programming, and may, in effect, be partially or fully incorporated into the dialect by designing functions to simulate them. A sentence may fail to execute either because it is illformed (for example, 2+ or a← or a←←3) or because a verb is applied to arguments not in its domain. Most dialects provide a set of error reports which are used to indicate the type of failure in a sentence. The error report is normally followed by a display of the sentence, with a caret marking the point at which execution stopped. Dialectal definitions are included in Sections IVVI, together with explicit references to the relevant manuals. Certain dialectal constructions are excluded from such discussion because they depart too strongly from the grammar defined here. These excluded topics include strands (which, in effect, allow elision of the verb link), ambiguous symbols (which allow certain symbols such as / to denote either a verb or an adverb), and selective specification (such as (c/[a]v)←x). Certain important constructs excluded from standard APL occur in nearly all dialects and are therefore discussed here, even though such discussion cannot completely obviate the consultation of other manuals. In each case the major reasons for exclusion are presented, as are alternative phrasings in the standard language. A. BracketSemicolon Indexing Brackets and semicolons are commonly used for indexing in dialects, and a good definition may be found in [7]. For example: ⊢a←2 3 4⍴⍳24 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 a[1;2;1] a [1;1 2;1] a [1;;1] 21 17 21 13 17 21 Deficiencies of bracketsemicolon indexing include:
This form of indexing can easily be expressed in terms of { . For example: a[i;j;k] ←→ (<i⊃j⊃k){ a a[i;j; ] ←→ (<i⊃j){ a a[i; ;k] ←→ (<i⊃∘⊃k){ a On the other hand, an expression such
as B. Indexed Assignment The effect on the value of the name a produced by an expression of the form a[i]←b , is to assign to it a merge of the values of a and b controlled by the index i . The same effect can be obtained by applying the merge adverb } to the selection function i¨{ , that is, a←b i¨{} a . Indexed assignment shares the deficiencies already
noted for the related bracketsemicolon indexing. Moreover,
it applies only to the particular form of indexing provided
by the brackets, rather than to any function,
as in C. Bracket Axis Notation Expressions such as ⌽[i]a and +/[i]a and +\[i]a apply the (possibly derived) function preceding the brackets “along axis i” of the argument a . For example: ⊢a←2 3⍴⍳6 0 1 2 3 4 5 ⌽[0]a +/[0]a +\[0]a 3 4 5 3 5 7 0 1 2 0 1 2 3 5 7 Phrases of the form f[i]a can be reexpressed in terms of the rank conjunction. For example, ⌽[i]a and +/[i]a and +\[i]a can be expressed as ⊖⍤k a and +⌿⍤k a and +⍀⍤k a , respectively, where k←(⍴⍴a)i . Similar remarks apply to a dyad,
that is, a,[i] b is equivalent
to A fractional value of i in the expression a,[i]b provides lamination, inserting a new axis of length 2 between axes ⌊i and ⌈i . For example: a←3+b←⍳3 a,[.5]b a,[¯.5]b 3 0 3 4 5 4 1 0 1 2 5 2 The verb ,¨< is equivalent to ,[¯.5] , and ,¨<⍤k ←→ ,[¯.5+(⍴⍴a)k] . D. Canonical Function Definition In most dialects, the system
function ⎕fx applies to
a character matrix argument that represents a function in
canonical form, and establishes the definition of the function.
The first row of the matrix is called a header;
it is a paradigm of the use of the function
(such as m ⎕fx m z←a root b;c root c←÷a z←b*c 2 root 64 3 root 64 8 4 The major anomaly to be noted in canonical definition is that although ⎕fx produces a function (verb), it is itself a function rather than an operator (adverb or conjunction). Moreover, it produces a defined function not as an explicit result to which a name may be assigned, but rather as a covert effect, a function with a specific name determined by the header of the argument of ⎕fx . The major deficiencies are:
Most APL systems provide special editing facilities to
make convenient the revision of functions
defined in canonical form.
Corresponding editing facilities can be provided
for direct definition by writing editing functions in APL
using APL systems also provide facilities for monitoring and controlling the execution (trace and stop) of canonically defined functions. Editing to insert expressions such as ⎕← or m (where m is any desired monitoring function whose explicit result is its argument) can provide equivalent facilities for functions produced by the conjunction ∇ . Any function definition of the form ⎕fx t can be mimicked by an expression of the form f←m∇d , and the appropriate values of m or d can be obtained from t rather simply. For example, in any assignment to a global variable, the arrow ← must be replaced by ⍅ and a branch of the form →u/l1,12,13 may be replaced by →←>0{u/l1⊃l2⊃l3 . Niladic functions. The function f
defined by The behaviour of a niladic function is therefore the same as that of a normal function provided with an argument. If the result of a function does not depend on the explicit argument, then any argument may be used. For example, if g←'○a'∇'' , then the phrase g∘ is equivalent to the niladic function f defined above. Adverbs and Conjunctions. Some dialects provide for the definition of adverbs and conjunctions, using the canonical scheme used for defining functions, but allowing the use of parentheses in the header as follows:
E. Comments and Statement Separators Anything following a comment
symbol (⍝) in an expression
is ignored in its execution. Comment can be similarly
added to the end of a line by appending
a←⍵× ⊢'count to n'⊢ ⍳n← ⊢'length of r' ⊢ ⍴r Expressions using the statement separator (⋄) can be mimicked by expressions using the verb left. The primary difference is that the separation imposed by ⊣ follows the normal rules for order of execution. For example, either of the following expressions will determine the coefficients c of a polynomial equivalent to a polynomial with roots r : n←⍴r⋄b←n⍴2⋄t←b⊤⍳×/b⋄s←(⍳1+n)∘.=+⌿t⋄c←s+.×p⊣p←r×.*t c←s+.×p⊣p←r×.*t⊣s←(⍳1+n)∘.=+⌿t⊣t←b⊤⍳×/b⊣b←n⍴2⊣n←⍴r Since ⊣ is a normal verb with simple properties, the phrase b⊣b can be simplified to b , and (since p is used nowhere else) the phrase p⊣p← can be omitted entirely, allowing the second expression to be simplified to: c←s+.×r×.*t⊣s←(⍳1+n)∘.=+⌿t←b⊤⍳×/b←n⍴2⊣n←⍴r The difficulty with the seeminglysimple statement separator is that it raises questions about many issues, such as the behaviour of a branch between separators, and the interaction between separators and comments. An indication of the complexity is given by the index entry for “diamond” in Berry [8]; references are made to eight distinct sections of the manual. F. Permissive Treatment of OneElement Arrays Most dialects are permissive in allowing oneelement lists (and sometimes oneelement arrays of any rank) to be treated exactly as the corresponding scalar. For example, the shapes of (,2)⌽⍳5 and 2⌽⍳5 are both 5 , although the shape of the former should be 1 5 . In dialects which assign ranks to primitive verbs, agreement with the established permissive definition is obtained by making the rank unbounded, thus permitting any desired behaviour. In such cases, standard behaviour for a primitive p can be obtained by imposing the proper rank r , as in p⍤r . For example, the fundamental definition of antibase (⊤) is on a list left argument and a scalar right argument, and it therefore has rank 1 0 . However, because the extra result axis was traditionally placed first rather than last, dialects assign it unbounded right rank. Standard behaviour can thus be obtained by using ⊤⍤1 0 . For example, most dialects would yield: ⊢m←2 2 2⊤⍳2*3 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 whereas IV. Verbs The ranks specified for a function are very important in
reading its definition, since the basic definition is given only
for cells of the indicated rank, and extension to higherrank
arrays follows the general rules for verbs stated
in Section II.B.
For example, the ranks of the verb ⌽
are 2⌽⍳5 1 3⌽2 5⍴⍳10 2 3 4 0 1 1 2 3 4 0 8 9 5 6 7 Unless otherwise clear from the definition of a verb, the rank of the result it produces is the same as the rank of its right argument. For example, the functions + ,  , × , ⌊ , and * , all produce item results, but ⍺?⍵ (as stated in its definition) does not. The first line of each main entry contains:
It should not be surprising that many of the names do not have the form of English verbs; English phrases such as 3 and 4 , 3 plus 4 , 3 times 4 , and p or q denote the action by prepositions or conjunctions rather than by verbs, and even where a verb is available (as in deny p for ~p) it may be more appropriate to use an adjective (as in not p).
Dyad. These functions are defined as in elementary arithmetic, except that 0÷0 is defined as 0 , for reasons presented by McDonnell [9]. Most dialects define 0÷0 as 1 , and most restrict the domains to real numbers.
Monad. The exponential [10] denoted by *⍵ is equivalent to e*⍵ , where e is the base of the natural logarithms, given approximately by: *1 2.718281828 The natural logarithm ⍟ is inverse to * , that is, ⍵ ←→ ⍟*⍵ ←→ *⍟⍵ . Moreover, ⍟⍵ ←→ e⍟⍵ . Dyad. a*2 and a*3 and a*.5 are the square, cube, and square root of a . The general definition of ⍺*⍵ is *⍵*⍟⍺ , and applies for complex numbers as well as real. For the simple case of an integer right argument it is equivalent to ×/⍵⍴⍺ ; in particular, ×/ applied to an empty list yields 1 , and ⍺*0 is 1 for any ⍺ , including the case where ⍺ is zero. The expression ⍺*⍵ is often read as “⍺ to the power ⍵”. The baseb logarithm b⍟⍵ is inverse to power, in the sense that ⍵ ←→ b⍟b*⍵ ←→ b*b⍟⍵ .
Monad. The result of <⍵ is a scalar encoding of ⍵ in the sense that it has rank 0 and can be decoded (by >). Moreover <⍵ differs from ⍵ . See the discussion of boxed nouns in Section II, and of their display under ⍕ . Dyad. The result of ⍺<⍵ is 1 if ⍺ is less than ⍵ , and is 0 otherwise. Also see ≤ .
Monad. If s is a mix (as defined under ]), and if s{a replaces i{a by j{a , and j{a by k{a , and k{a by i{a , then the list i,j,k is said to be a cycle of the mix s . Every mix can be conceived as a collection of disjoint cycles of various lengths, including length one for those elements that do not move. For example, if A mix c is said to be a cycle representation of a mix s if the cycles of s are the boxed elements of the vector b←(a=⌈\a)1⍤<a←(⍴c)c . For example: c←2 4 0 5 3 1 (c=⌈\c)1⍤<c ¯¯¯¯¯¯¯¯¯ 24 05 3 1 _________ and c is therefore the cycle representation
of the mix If s is a mix, then ≤s yields its cycle representation; ≥ is the inverse function, and ≥≤s is the standard form of s . The results of both ≥ and ≤ are mixes in standard form. More generally,
if the elements of ⍵ are distinct
nonnegative integers,
then (≤⍵)≡≤⍵,(⍳⌈/⍵)~⍵ .
For example, Dyad. The result of ⍺≤⍵ is 1 if ⍺ is less than or equal to ⍵ and 0 otherwise. However, the comparison is made with a tolerance specified by the system variable ⎕ct as follows: ⍺≤⍵ is 1 if ⍺⍵ does not exceed ⎕ct multiplied by the larger of their magnitudes, that is, if ⍺⍵ is less than or equal to ⎕ct×(⍺)⌈⍵ . Similar comparisons apply to the other relations. For example, ⍺≥⍵ is 1 if ⍺⍵ is greater than or equal to ⎕ct×⌈/⍺,⍵ , and ⍺>⍵ is 1 if (⍺≥⍵)^~⍺=⍵ . The custom conjunction (: , which see) may be used with any of the relations, that is, ⍺<:t ⍵ provides comparison with a tolerance t , regardless of the value of ⎕ct . The relations are commonly applied to boolean arguments. For example, ⍺≠⍵ , is the exclusiveor of boolean ⍺ and ⍵ , and ⍺≤⍵ is implication.
Monad. The function ≥ is the inverse of ≤ , and ≥⍵ produces the standard representation of the mix whose cycle representation is ⍵. See ≤ and ] . More generally, if the elements of ⍵
are distinct nonnegative
integers, then ≥⍵ ,
is the mix whose elements are
determined by ⍵ ,
in the normal manner, but whose missing
elements (⍳⌈/⍵)~⍵ are treated as cycles
of length 1 , and
therefore stay fixed.
For example, Dyad. The result of ⍺≥⍵ , is 1 if a is greater than or equal to ⍵ , and is 0 otherwise. Also see ≤ .
Monad. Open (>) is the inverse of box (<) , that is, ⍵≡><⍵ . When applied to an open array (that contains no boxed elements), open has no effect. For example: ><1 2 3 >1 2 3 1 2 3 1 2 3 The opened elements are brought to a common shape as discussed in Section II.B. Dyad. The result of ⍺>⍵ is 1 if ⍺ is greater than ⍵ , and is 0 otherwise. Also see ≤ .
Monad. The function = classifies the major cells of the nub of ⍵ according to equality with the major cells of ⍵ , producing an m by n boolean table, where m and n are the number of major cells of the nub of ⍵ (that is, ↑⍵) and of ⍵ , respectively. For example: a ↑a =a allah alh 1 0 0 1 0 0 1 1 0 0 0 0 0 0 1 b ↑b =b abc abc 1 0 1 def def 0 1 0 abc Formally, =⍵ ←→ (MC↑⍵)∘.=MC⍵ , where MC:,<⍤¯1 ⍵ boxes the major cells of its argument. Hence, =s ←→ =,s for any scalar argument s . Dyad. The function =
is a rank 0 form
of match (≡);
formally, ⊢a←3 5⍴'abcdefghijklmno' abcde fghij klmno a=⌽a ⊢b←<⍤1 a 0 0 1 0 0 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 0 0 1 0 0 abcdefghijklmno 0 0 1 0 0 _______________ b=⌽b 0 1 0 Dialects. Some define =
differently on boxed arrays;
it may therefore be necessary to
use
Monad. Nubsieve (≠⍵) provides a boolean list that selects the nub of ⍵ , that is, ↑⍵ ←→ (≠⍵)⌿⍵ . For example, ≠'abacus' ←→ Dyad. The result of ⍺≠⍵ is 1 if ⍺ is unequal ⍵ , and is 0 otherwise. Also see ≤ .
Dyad. The expression ⍺≡⍵ yields a scalar boolean result; 1 if the arguments match completely, in shapes, in boxing structure, and in elements. The comparison of numeric elements is made under the normal rules of comparison tolerance described under ≤ . In particular, the custom conjunction applies to ≡ .
Monad. ~ applies only to boolean arguments, and negates them: ~0 1 ←→ 1 0 . Dyad. The result of ⍺~⍵ is the array whose major cells are the major cells of ⍺ less the major cells of ⍵ . For example: (⍳6)~7 2 4 0 1 3 5 ⍺←3 4⍴'abcdefghijkl' ⍵←2 4p 'mnopabcd' ⍺~⍵ efgh ijkl Formally, ⍺~⍵ ←→ (~(MC ⍺)∊MC ⍵)⌿⍺ , where MC: ,<⍤¯1 ⍵ . The result of a~a~b is called the intersection of a and b . For example, if a←⍳6 and b←7 4 2 , then a~a~b is 2 4 . The result for any scalar argument s is the same as for ,s .
Dyad. p^q is the least common multiple of p and q , and p∨q is the greatest common divisor. For example: 12∨30 12∧30 6 60 For boolean arguments (0 and 1) the ∨ is equivalent to the logical or, and the ∧ to the logical and. Thus: p←0 0 1 1 q←0 1 0 1 p^q p∨q 0 0 0 1 0 1 1 1 Finally, ⍲ and ⍱ (called nand and nor) are defined only on boolean arguments, and are negations of ^ and ∨ , that is: (⍺⍲⍵)≡(~⍺∨⍵) (⍺⍱⍵)≡(~⍺∨⍵)
Monad. Both ⊢ and ⊣ are identity functions, that is, ⊢⍵ ←→ ⍵ , and ⊣⍵ ←→ ⍵ . Dyad. Left yields the left argument,
and right the right,
that is,
Monad. ⊥ provides the word formation described in Section I. For example: ⊥'3 4.5×ABC DE⍴Q' ¯¯¯¯¯¯¯¯¯¯¯¯¯ 3 4.5×ABCDE⍴Q _____________ It is equivalent to the function words defined below: words:f(c g(q∧¯1↓0,q←∨⌿3↑c)⍱q∧¯1↓0,q) 1⍤<,⍵⊣ q←q∨≠\q←,¯1↑c←(<,⍵)∊⍤>p f:(0≠,⍴⍤c)/c←(r⊥⍤r←⌽¨>a)↓¨>(a⊥⍤>a←⍵=¨>' ')↓¨> ⍵ g:⍵\q⍲¯1↓0,q←⍵/(∨⌿⍺[1 3;])∨(⍺[2;])∧1↓(∨⌿⍺[1 2;]),0 The parameter p is a 5element list of boxed lists consisting of the quad (⎕) and the native alphabet, the macron (¯) and the decimal digits, the period (.), the space, and the quote ('). Dyad. ⍺⊥⍵ is the item +/W×⍵ ,
where W is the list of weights
such that k{W is ×/(k+1)↓(⍴⍵)⍴⍺ .
For example if ⍺⊥⍵ ⊢W←1+⌽×\⌽⍺,1 +/W×⍵ 3723 3600 60 1 3723 Dialects. Most dialects differ in their treatment of higher rank arguments, for both ⊥ and ⊤ .
Dyad. For simple cases, ⊤ is inverse to the base ⊥ . For example, if
Monad. The result of lea is the result of executing the character list =. For example, l'a+3+t~' assigns the value 7 to the name a. The result of t' ' is *. Dyad. The result of alto is the result of lo if ~a is a valid expression (that is, its execution completes properly). Otherwise, the result of ale0 is the result of ia, which may itself invoke an error report. Effects may also be produced by any partial execution of ~ that has occurred.
Briefly stated, the format function produces a character array that represents numeric results in familiar forms. However, the exact rules for rounding the numbers, for inserting spaces to align columns, and for replacing leading and trailing zeros by spaces, become rather intricate. Monad. If ⍵ is an open character array, then ⍵≡⍕⍵ . Although the monadic rank of ⍕ is unbounded, we will first restrict attention to arguments of rank 2 . If ⍵ is an open numeric, then ⍕⍵ is, with the exceptions stated below, equivalent to ((m+1),⎕pp)⍕⍵ , where m is the greatest width required for any of the fields, and where ⎕pp is the system variable that controls printing precision. Leading zeros preceding the decimal point and trailing zeros following it are replaced by spaces, except that if all digits are zero, the one before the decimal point is retained, and the decimal point itself is replaced by a space. In extending the definition of ⍕ to arguments of rank greater than 2 , it is necessary to choose a value of m for use in the expression ((m+1),⎕pp)⍕⍵ that is large enough for all of the individual rank 2 cells. We therefore define a dyadic function F of rank 0 2 such that n F ⍵ is equivalent to ⍕⍵ if 2=⍴⍴⍵ and if n is the maximum field width required. Moreover, if om is the overall maximum width required for all rank 2 cells of an argument ⍵ , then om F ⍵ provides a useful extension to arguments of higher rank. We adopt this extension as a definition of ⍕ , with a modification that inserts rows of spaces between the results for individual cells. Using the subsequent definition of ⍕ on boxed arrays, these insertions may be expressed as follows: If 3=⍴⍴⍵ , and if r←om F ⍵ , then the result of ⍕⍵ matches ⍕⍪<⍤2r , evaluated with ⎕ps←0 0 1 0 . If 3<n←⍴⍴⍵ , then ⍕⍵ matches ⍕⍪<⍤2⍕⍤(n1)⍵ . The degenerate cases of vector and scalar arguments are defined by the expression ,⍕⍉⍪,⍵ . The normal display of any result r matches the display of ⍕r . For example: 1 100∘.+.1 1 ∘.×¯1 0 1 0.9 1 1.1 0 1 2 99.9 100 100.1 99 100 101 If the argument of ⍕ is a boxed array, a width sufficient for all elements in a column of the display is allotted for each column, and sufficient height is similarly allotted to each row. The justification within the allotted space is then determined by the first two elements of the “position and spacing” system variable ⎕ps as follows: justify top, centre, or bottom in each row if 0{⎕ps is ¯1 , 0 , or 1 , and left, centre, or right in each column if 1{⎕ps is ¯1 , 0 , or 1 . Space between successive rows and columns is allotted as follows: 2{⎕ps spaces between successive rows, and 3{⎕ps spaces between successive columns. Finally, each element is bordered by horizontal lines (¯ and _) immediately above and below if 2{⎕ps is negative, and by vertical lines immediately to the left and right if 3{⎕ps is negative. For example: b←2 2⍴(3 5⍴'ab')⊃1 2⊃'cd'⊃'efghij' ⎕ps←1 1 3 3 b ¯¯¯¯¯ ¯¯¯ ababa 1 2 babab ___ ababa _____ ¯¯ ¯¯¯¯¯¯ cd efghij __ ______ ⎕ps←0 0 0 0 b ababa babab 1 2 ababa cd efghiff All other examples of boxed arrays in the dictionary
are shown as if The custom conjunction (which see) applies to ⍕ to
specify the position and spacing independently
of ⎕ps , as
in Dialects. Display, and the result of ⍕ , differ somewhat among dialects. However, most control display and the result of w by the system name ⎕pp (printing precision), whose value determines the maximum space allotted to the printing of each number. They also use ⎕pw to control the printing width (the point at which the line is broken to continue indented on a subsequent line); this applies to normal display, but not to the result of ⍕ . Dyad. The expression ⍺⍕⍵ yields a character array; the form of the result is controlled by ⍺ in two different ways, depending upon whether ⍺ is numeric or character. If ⍺ and ⍵ are both numeric tables, the result of ⍺⍕⍵ is a table of 1↑⍴⍵ rows, in which groups of adjacent columns (called fields) are determined by each column of ⍵ together with a twoelement control vector c . The control for field j is given by c←j{⍺ ; the element 0{c determines the width of the field, and 1{c determines the number of decimal places used. If k←1{c is negative, the result is represented in exponential form, as illustrated below: (⍉⍪8 ¯3)⍕ ⍉123.45678 .00876543 1.23E2 8.77E¯3 To obtain the exponential form of a number n , it is first normalized to the equivalent m×10*p , where p is an integer, and 0≤m , and 10>m . The number m is then rounded to k places (as described below) and is suffixed by one of the forms e¯qr or e qr , where qr denotes the representation of p . The rounding of a number n is determined by k←1{c as follows: (10*k)×⌊.5+(n)×10*k The character string that represents this result is then prefixed by enough spaces to bring it to the width determined by 0{c , after being prefixed by the symbol ¯ if n is negative. The width of a field is equal to 0{c , unless 0{c is zero, in which case the width is one more than the maximum width required to represent any one of the numbers in the entire column of ⍵ . The degenerate case of a scalar or vector right argument is treated as ,⍺⍕⍉⍪,m . The degenerate case of a (2element) vector left argument is treated as (((1↑⍴⍵),2)⍴⍺)⍕⍵ , and the case of a scalar left argument ⍺ is treated as the vector 0,⍺ . Dialects. Dialects commonly use the ravel of the table left argument described above, rather than the table itself. For example: ⊢a←3 2⍴24.34 ¯57.684 ¯0.45 ¯134.27 24.34 ¯57.684 ¯0.45 ¯134.27 24.34 ¯57.684 12 3⍕a 24.340 ¯57.684 ¯0.450 ¯134.270 24.340 ¯57.684 5 2 8 0⍕a 8 2 0 ¯2⍕a 24.34 ¯58 24.34 ¯5.8e1 ¯0.45 ¯134 ¯0.45 ¯1.3e2 24.34 ¯58 24.34 ¯5.8e1 If ⍺ is a character array, the expression ⍺⍕⍵ is called format by example, since ⍺ provides a pattern for the result. In the standard case of a single cell, ⍺ is a onerow table, and the last dimension of the table ⍺⍕⍵ agrees with the last dimension of ⍺ . Since the left rank of ⍕ is 2 , the standard case requires a table left argument ⍺ ; since that table consists of a single row, the equivalent degenerate case (,⍺)⍕⍵ is commonly used. Degenerate cases of the right argument ⍵ are treated as ,⍺⍕⍉⍪,⍵ . For example: ⍺←'Balance is $(55,510.50) on 05/55/55' ⍺⍕23456.714 61184 Balance is 23,456.71 on 06/11/84 ⍺⍕456.78, 100⊥6 11 84 Balance is 456.78 on 06/11/84 ⍺⍕¯23456.714 61184 Balance is $(23,456.71) on 06/11/84 ⍺⍕¯456.78 61184 Balance is $(456.78) on 06/11/84 ⊢r←'555.55'⍕1 0 10.1 100 ⍴r 1 10.1 100 24 A numerical field in ⍺ is bounded by blanks or (on the right) by the first noncontrol character following a six, and must contain at least one digit. The digits in a field are both placeholders and control characters for that field. Nondigits are decorators, simple, controlled, or conventional:
These conventional dots and commas print as the elements 0{⎕fc and 1{⎕fc of the format control variable ⎕fc . (The normal value of 0 1{⎕fc is '.,' ; reversal of these values provides printing according to a common European convention.) A decimal point not followed by a fractional value is a part of the trailing decorator, and as such is suppressed according to the rules that apply to any trailing decorator. For example: '5125.POS'⍕ 123 ¯123 123.POS 123 The control characters have the following significance:
Notes.
Monad. The expression ○⍵ is equivalent to pi×⍵ , where pi is the ratio of the circumference of a circle to its diameter, and is given approximately by: ○1 3.141592654 Dyad. The expressions k○⍵
and (k)○⍵
produce several families of related functions,
circular or trigonometric
(for k∊1 2 3),
hyperbolic (for k∊5 6 7),
pythagorean (for k∊0 4 8),
and complex (for Corresponding positive and negative cases are inverse in the sense that at least one of the identities ⍵≡k○(k)○⍵ or ⍵≡(k)○k○⍵ hold (at least within the principal domains, as defined in [10]). The cases 9○⍵ and 11○⍵ give the real and imaginary parts, and 12○⍵ , gives the arc of a polar representation. As a mnemonic aid it may be noted that (except for k=12) the expression k○⍵ is even (that is, k○⍵ ←→ k○⍵) if k is even, and is odd (that is, k○⍵ ←→ k○⍵) if k is odd. The definitions follow:
The custom conjunction (which see) can be used to specify the number of units per quadrant, as in 1○:90 d to determine the sine of an angle given in degrees. Formally, k○:q ⍵ ←→ k○.5×○⍵÷q .
Monad. Reverse reverses the order of a list. For example: ⌽'abc' ⌽2 3⍴'abcdef' cba cba fed Dyad. Rotate cycles the elements of a list as illustrated below: 2⌽l←'abcdef' cdefab ¯2⌽l efabcd For example: ⊢t←2 3⍴l 1 2⌽t 1⌽t abc bca bca def fde efd Dialects. Dialects commonly depart from this definition in two significant ways:
Monad. Upset overturns its argument as illustrated below: ⊢t←3 4⍴'abcdefghijkl' ⊖t abcd ijkl efgh efgh ijkl abcd It may be expressed in terms of reverse as follows: (⊖⍵)≡⍉⌽⍉⍵ . Dyad. Rowel is equivalent to ⌽¨⍉ . For example: 0 1 2 3⊖t afkd ejch ibgl
Monad. This function reverses the order of axes of its argument. Thus: ⊢m←3 4⍴⍳12 ⍉m ⍴⍉m 0 1 2 3 0 4 8 4 3 4 5 6 7 1 5 9 8 9 10 11 2 6 10 3 7 11 It may be defined in terms of the dyad cant: (⍉⍵)≡(⌽⍳0{⍴⍴⍵)⍉⍵ . Dyad. If p is any permutation of the axes of array a , then b←p⍉a is similar to a except that its axes are permuted; axis i of a becomes axis i{p of b . For example: ⊢b←(p←1 2 0)⍉a←2 3 4⍴⍳24 0 4 8 ⍴b 12 16 20 4 2 3 1 5 9 p{⍴b 13 17 21 2 3 4 2 6 10 ⍴a 14 18 22 2 3 4 3 7 11 ⍴1 0 4 2 3⍉2 3 4 5 6⍴9 15 19 23 3 2 5 6 4 More generally, q⍉a is defined if (⍴q)=⍴⍴a and if the nub of q is a permutation (of ⍳n , for some n). For example, each of the following values of r , s , and t are valid left arguments of ⍉ for a right argument of rank 4: ↑r←2 0 1 2 ↑s←1 0 1 1 ↑t←0 0 0 0 2 0 1 1 0 0 Just as for the case b←p⍉a where p is a permutation, the i th axis of a becomes the i{q axis of b←q⍉a . However, in this case, two or more axes of a may map into a single axis of b , providing a diagonal section of a . For example: a←3 3⍴⍳9 c←3 5⍴⍳15 0 1 2 0 1 2 3 4 3 4 5 5 6 7 8 9 6 7 8 10 11 12 13 14 0 0⍉a 0 0⍉c 0 4 8 0 6 12 Formally, if i is any complete index of q⍉a (that is, (<i){q⍉a selects a scalar element of q⍉a), then (<i){q⍉a is equivalent to (<q{i){a. Dialects. Since the elements of the left argument of ⍉ are drawn from ⍳n , they commonly depend on the value of ⎕io , as discussed in Section II.E.
Monad. The definition (⍵×+⍵)*.5 yields the size or “absolute value” not only for a real argument, but also for a complex argument, in which case the result is the length of the hypotenuse of a rightangled triangle whose sidelengths are the real and imaginary parts of the number. For example, 3j4 is 5 , and ¯6 is 6. Dyad. The most familiar use of residue is to determine the remainder resulting from dividing a nonnegative integer dividend by a positive integer divisor. For example: 30 1 2 3 4 5 6 7 8 0 1 2 0 1 2 0 1 2 The definition ⍵⍺×⌊⍵÷⍺+0=⍺ extends this notion to a zero left argument (giving the right argument unchanged), to non integer right arguments, and to negative and fractional left arguments. For a negative integer left argument, the result ranges between the argument and zero, as it does for a positive left argument. For example: ¯3¯4 ¯3 ¯2 ¯1 0 1 2 3 4 ¯1 0 ¯2 ¯1 0 ¯2 ¯1 0 ¯2 12.5 3.64 2 ¯1.6 0.5 0.64 0 0.4 However, in order to produce a true zero (rather than a small fraction) for cases such as (÷3)2÷3 , the residue is made “tolerant” by the following definition: ⍺⍵ ←→ ⍵⍺×⌊s if (⍺≠0)^(⌈s)≠⌊s←⍵÷⍺+⍺=0 ←→ ⍵×⍺=0 otherwise For example: .12.5 3.64 2 ¯1.6 0 0.04 0 0 This definition also applies to complex arguments, using the complex properties of the floor function ⌊ .
Monad. For a nonnegative integer argument, the factorial is defined by !⍵ ←→ ×/1+⍳⍵ . For example, !4 is Dyad. The dyadic case of ! is defined in terms of the beta function [10]; subject to the interpretation given below, this definition is equivalent to ⍺!⍵ ←→ (!⍵)÷(!⍺)×(!⍵⍺) . For nonnegative integer arguments, ⍺!⍵ yields the number of ways of selecting ⍺ things from ⍵ things; this accounts for the name “out of”, and for its use in producing binomial coefficients. For example, (⍳n+1)!n←3 ←→ 1 3 3 1 . For a negative integer i , the expression !i is not defined, because near a negative integer the magnitude approaches infinity. Nevertheless, the definition of ⍺!⍵ can be understood by assuming that these infinite values occur in the expression for the dyadic definition in the following sense:
The definitions of floor and ceiling on complex numbers are rather involved, and the interested reader should experiment with them on an APL system, or consult McDonnell [11]. Dyad. a⌊b yields the lesser of a and b , and a⌈b yields the greater. For example, 3⌊4 is 3 , and 3⌊¯4 is ¯4 . Complex numbers are not in the normal domain of ⌊ and ⌈ .
Monad. ↑⍵ selects the nub of ⍵ , that is, all of its distinct major cells. For example: ⍵ ↑⍵ ↑3 ⍴↑3 ABC ABC 3 1 ABC DEF DEF Formally, ↑⍵ ←→ (≠⍵)⌿⍵ . The raze function is defined by (↓⍵)≡⍪¨>⌿⍵ , and therefore assembles along a leading axis the opened elements of ⍵ . For example: ⊢⍵←2⍤<'Now is the time ' ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Now is the time  ________________ ↓⍵ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Now is the time  ________________ Dyad. If (⍴⍺)=⍴⍴⍵ , and if all elements of ⍺ are nonnegative, then the shape of r←⍺↑⍵ is ⍺ ; if i is any complete (boxed) index of both r and ⍵ , then (i{⍵)≡i{r ; if i is an index of r only, then i{r is an array of fill elements, zeros if ⍵ is entirely open numeric or empty (0=×/⍴⍵) , spaces (' ') if ⍵ is entirely open character, and <'' otherwise. For example: ⊢⍵←3 4⍴⍳12 2 2↑⍵ 3 6↑⍵ 0 1 2 3 0 1 0 1 2 3 0 0 4 5 6 7 4 5 4 5 6 7 0 0 8 9 10 11 8 9 8 9 10 11 0 0 More generally, ⍺ may have negative items, which cause selection from the trailing end of the corresponding axes, and if (⍴⍺)<⍴⍴⍵ then ⍺ takes the (⍴⍺)cells of ⍵ as defined by ⍺↑⍵ ←→ (⍺,(⍴,⍺)↓⍴⍵)⍴⍵ . For example: 2 ¯2↑⍵ 3 ¯6↑⍵ 2↑⍵ 2 3 0 0 0 1 2 3 0 1 2 3 6 7 0 0 4 5 6 7 5 6 7 8 0 0 8 9 10 11 As illustrated by the case 3 ¯6↑⍵ above, the fill elements are placed in the leading positions in the case of a negative argument. Drop is defined similarly, and ⍺↓⍵
drops elements from ⍵ ,
from the trailing end if the element
of ⍺ is negative.
If (⍴⍺)=⍴⍴⍵ ,
then the shape of ⍺↓⍵
is 0⌈(⍴⍵)⍺ .
For example, Dialects. Because of permissive treatments of left arguments such as (1 1,⍴v)⍴v as a vector, dialects commonly treat the left rank of these verbs (↑ and ↓) as unbounded. Moreover, k↑0⍴a usually treats the empty argument 0⍴a like a itself, “filling” with 0 or ' ' . For example, 3↑0⍴⍳5 ←→ 0 0 0 , and 3↑0⍴'abc' ←→ 3⍴' ' .
⊢g←⍋⍵←3 1 4 2 1 3 3 g{⍵ 1 4 3 0 5 6 2 1 1 2 3 3 3 4 Moreover, elements of g
that select equal elements of ⍵
(such as 1 4 and If ⍵ is a table, ⍋⍵ grades the rows, that is, it grades the base value of the rows, using a base larger than the magnitude of any of the elements. For example: ⍵←4 3⍴3 1 4 2 7 9 3 2 0 3 1 4 ⍵ ⊢g←⍋⍵ g{⍵ 3 1 4 1 0 3 2 2 7 9 2 7 9 3 1 4 3 2 0 3 1 4 3 1 4 3 2 0 Higher rank arguments are graded as if their major cells were ravelled, that is, ⍋⍵ ←→ ⍋,⍤¯1⍵ . The definition of downgrade (⍒) is like that of upgrade, except that (⍒⍵){⍵ is in descending order. Dialects. In most dialects the results of ⍋ and ⍒ depend upon the index origin ⎕io , as discussed under dialogue in Section II.E. Dyad. If ⍺ is a list, then ⍺⍋⍵ grades ⍵ according to the “collating sequence” specified by ⍺ . Formally, ⍺⍋⍵ ←→ ⍋⍺⍳⍵ . For example: ⍺←'abcde'⊣⍵←'labelled' ⍺⍳⍵ 5 0 1 4 5 5 4 3 ⍋⍺⍳⍵ ⍺⍋⍵ 1 2 7 3 6 0 4 5 1 2 7 3 6 0 4 5 (⍺⍋⍵){⍵ abdeelll In particular, since ⍺⍳s yields ⍴⍺ for any scalar s not in ⍺ , then (⍺⍋⍵){⍵ places all elements of ⍵ not in ⍺ at the end, in the same relative order that they have in ⍵ . If ⍺ is a table of two fonts such as: ⍺ abcdefghijklmnonqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ then ⍺⍋⍵ grades ⍵ in dictionary order, that is, using the ordering of the fonts (as specified by the columns) as a secondary ordering within that specified by the rows. For example: ⍵ (⍺⍋⍵){⍵ cabal baker Baker Baker baker cabal Since dyadic grade is extended to higher rank arguments in the same systematic manner that monadic grade is, we will state its general behaviour in terms of a list right argument only. The result of ⍺⍋⍵ is based upon a ranking of the complete indices of ⍺ in ⍵ . Because earlier axes of ⍺ provide secondary ordering within the ordering imposed by later axes, the ranking is based upon the monadic grade of the table of reversed complete indices. For example, using the twofont alphabet ⍺ and the value ⍵←'abcABC' , the indices, reversed indices, and ranking are: i ⌽i ⍋⌽i 0 0 0 0 0 3 1 4 2 5 0 1 1 0 0 2 2 0 1 0 0 1 1 1 1 1 1 2 2 1 For any argument not in ⍺ , the complete index is taken as ⍴⍺ . Formally, then, ⍺⍋⍵ ←→ ⍋⌽⍺I⍵ where Finally, if a value s
occurs more than once in ⍺ ,
its index is taken to be the minimum
over all of its possible indices.
For example, if ⍴⍴⍺
is 3 ,
and the indices of s
are 2 3 4 and 1 5 2 ,
the index is taken
as Alternative formal definitions of dyadic ⍋ and examples of its use may be found in [14], and in the original paper [15] cited therein. Dyadic downgrade (⍒) is defined analogously. Dialects. Most dialects exclude a scalar right argument from the domains of ⍋ and ⍒ , and also introduce a dependence upon the index origin ⎕io , as discussed under dialogue in Section II.E.
Monad. ∊⍵ is an autoclassification that determines for each element of ⍵ whether its open contains each element of the open of the raze of ⍵ , that is, ∊⍵ ←→ ⍵∊⊂⍤>↓⍵ . The result is a boolean of shape (⍴⍵),⍴↓⍵ . For example: ⍵←'abc'⊃'dc'⊃'a' ↓⍵ ∊⍵ ¯¯¯¯¯¯ 1 1 1 0 1 1 abcdca 0 0 1 1 1 0 ______ 1 0 0 0 0 1 Dyad. The result of ⍺∊⍵ is 1 if ⍺ belongs to ⍵ in the sense that ∨/⍺0 .=,⍵ . For example: 'cat'∊'abcd' 1 1 0
Dyad. If b←⍺⍷⍵ , then the ones in b indicate the beginning points of occurrences of the pattern ⍺ in ⍵ . For example: 'co' ⍷ 'cocoa' (0 1∘.+⍳3) ⍷ 4i∘.+i←⍳5 1 0 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 Formally, ⍺⍷⍵ ←→ ((⍉1,⍪⍴⍺) 3⍤<⍵)∊<⍺ .
Monad. The verb ⍳ “counts” in the sense that ⍳n yields a list of the first n integers beginning with zero. Thus, ⍳3 is the list 0 1 2 . More generally, ⍳⍵ is an array of shape ⍵,⍴⍵ such that (⍴⍵)>i ←→ i{⍳⍵ for any index i that selects a 1cell of ⍳⍵ . Dialects. Many dialects treat any oneelement argument as
a scalar, rather than allowing it to provide an outer shape.
Thus Dyad. If j←⍺⍳⍵, then j is the index of the first occurrence of ⍵ in ⍺ (that is, ⍵≡j{⍺), unless ⍵ does not occur in ⍺ , in which case j equals ⍴⍺ . Formally, (⍺⍳⍵)≡+/∧\⍺∘.≠⍵ , except that a scalar left argument is not permitted.
Dyad. The function ⍸ is defined in terms of ⍷ , and i←⍺⍸⍵ yields the index in b←⍵⍷⍺ of the first 1 , or the value ⍴b if ⍵ does not occur in ⍺ . Thus: A B A⍸B 0 1 2 0 1 2 4 5 1 1 3 4 5 3 4 5 2 0 1 2 0 1 2 0 A⍸⌽B 4 5 3 4 5 3 4 6
The monadic shape and dyadic reshape functions are simply related; ⍺⍴⍵ produces a result of shape ⍺ from the elements of ⍵ , and therefore ⍺≡⍴⍺⍴⍵ . For example: ⊢r←2 3⍴q←'abcdef' ⍴r abc 2 3 def q≡,r ⍴⍴r ⍴⍴3 1 2 0 The last examples (⍴⍴r and ⍴⍴3) illustrate the fact that the rank of an array is the shape of its shape; and that the rank of an item is 0 , implying that its shape is an empty list; and that (⍳0)⍴,3 produces an item as a result. If the number of elements in p equals the number of elements in an array of shape s (that is, ×/s) , then (,s⍴p)≡,p . If (⍴,p)>×/s , then only the first ×/s elements of ,p are used; if (⍴,p)<×/s , then ,p is used cyclically. For example: p←2 3⍴'abcdef' 2 2⍴p 3 5⍴p ab abcde cd fabcd efabc
Monad. The function roll is named from the analogy with rolling a die to choose one of a set of numbers with equal probability; the set used for ?⍵ is ⍳⍵ . Dyad. The function deal is named by analogy with dealing from a pack of cards. The result of ⍺?⍵ is a list of length ⍺ , all elements being distinct. The population drawn from in each case is ⍳⍵ . See Berry [8] for details of the most commonly used pseudorandom number generator. Dialects. See Section II.E concerning dependence on ⎕io.
Monad. Ravel (,) ravels the elements of its arguments in “normal” order. For example: ⊢⍺←2 3 4⍴' abcdefghijklmnopqrstuvwx' abcd efgh ijkl mnop ,⍺ qrst abcdefghijklmnopqrstuvwx ugwx Normal order means that the results are ordered by cells; that is, the elements of i{⍵ precede those of j{⍵ if i<j ; the ¯2cells (0{0{⍵ , 1{0{⍵ , etc.) are similarly ordered within the ¯1cells, and so on. The result of ,⍵ is a list of all elements of ⍵ ; its shape is 1⍴×/⍴⍵, tn particular, the ravel of an item is a oneelement list. For a nonscalar argument, the expression ⍪⍵ produces a ravel of each of the major cells of ⍵ , that is, (⍪⍵)≡,⍤¯1 ⍵ . For a scalar argument, the result is a table of shape 1 1 ; the result for any argument is therefore a table. The complete formal definition therefore involves two applications of ,⍤¯1 , the second one producing an effect only in the case of a scalar. Thus: (⍪⍵)≡,⍤¯1,⍤¯1 ⍵ . For example: ⍪0 1 2 ⍪ 2 2 2 ⍴⍳8 ⍴⍪2 0 0 1 2 3 1 1 1 4 5 6 7 2 Dyad. The expression ⍺⍪⍵ catenates the major cells of ⍺ and ⍵ . For example: ⊢⍺←2 4⍴'abcdefgh' ⍺⍪⍵ abcd abcd efgh efgh ijkl ⊢⍵←3 4⍴'ijklmnopqrst' mnop ijkl qrst mnop qrst If either argument is an item, it is first “extended” by the expression (1,1↓⍴⍺)⍴⍵ or (1,1↓⍴⍵)⍴⍺ . If either argument is of rank 1 less than the other, its shape is first augmented by a leading 1 , that is, by application of the expression >,< . The resulting major cells must then agree in shape. For example: ⍺⍪'A' 'ABCD'⍪⍵ abcd ABCD efgh ijkl AAAA mnop qrst The related function denoted by the comma is best defined in terms of ⍪ that is: ⍺,⍵ ←→ ⍺⍪¨⍉⍵ ←→ ⍉(⍉⍺)⍪(⍉⍵) For example: ⍺,⍺ 'AB',⍺ ⍺,'A' abcdabcd Aabcd abcdA e£ghefgh Befgh efghA Dialects. Most dialects restrict catenation to arguments of the same type, and therefore avoid the introduction of “heterogeneous” arrays that may contain both numbers and alphabetics, but some [12 13] do not.
Monad. ⊃ boxes an open argument and has no effect on a boxed argument. The expression ⍵≡⊃⍵ can therefore be used to determine whether ⍵ is open or boxed. Dyad. ⍺⊃⍵ links ⍺ and ⍵ , by boxing ⍺ and catenating it to ⍵ , first boxing ⍵ if it is open. Thus, (⍺⊃⍵)≡(<⍺),⊃⍵ . For example: 'now'⊃'is'⊃'the'⊃'time' ¯¯¯¯¯¯¯¯¯¯¯¯ nowisthetime ____________ Dialects. In dialects [12 13], ⊃⍵ is a form of open (>), and ⍺⊃⍵ is a form of indexing of ⍵ by ⍺ .
Monad. If ^/(i=⌊i),(i<n),i≥n←1↑⍴a , then i is an index of a , and the major cells of (,i){a are major cells of a . If (n←⍴i)i is also a reordering or permutation of ⍳n , then i is called a mix of order n , and ni is its standard form. There are exactly !n distinct standard mixes of order n , and if S is a !n by n table of them in lexical order (such that (⍋S)≡⍳!n), then i is the mix index of i{S. If ⍵ is a mix,
then ]⍵ yields the mix index
of (n←⍴⍵)⍵ ;
if it is not, the result is !n .
For example, Dyad. If i is an index
of ⍳n←''⍴⍴a , then i]a
is p{a ,
where ]p is ni .
For example,
Monad. The expression {⍵ forms a catalogue from the elements of its argument. For example: ⍵←'ht'⊃'ao'⊃'gtw' {(2 3⍴⍳6)⊃10 11 {⍵ ¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯ 0 100 11 haghathaw ________ _________ ¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯ 1 101 11 hoghothow ________ _________ ¯¯¯¯¯¯¯¯ 2 102 11 ¯¯¯¯¯¯¯¯¯ ________ tagtattaw _________ ¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯ 3 103 11 togtottow ________ _________ ¯¯¯¯¯¯¯¯ 4 104 11 ⍴{⍵ ________ 2 2 3 ¯¯¯¯¯¯¯¯ 5 105 11 ⍴{(2 3⍴⍳6)⊃10 11 ________ 2 3 2 As may be seen in the foregoing examples, the shape of the catalogue {⍵ is the catenation of the shapes of the disclosed elements of ⍵ , that is, >↓⍴¨>⍵ , and the common shape of the disclosed elements of the catalogue is ⍴⍵ . Expressions of the form {⍳¨>s are useful for producing a complete array of indices of an array of shape s . For example: ⍳¨>s←2 3 ¯¯¯¯¯¯¯¯ 0 10 1 2 ________ {⍳¨>s ¯¯¯¯¯¯¯¯¯ 0 00 10 2 _________ ¯¯¯¯¯¯¯¯¯ 1 01 11 2 _________ Portions of such an array of indices are useful in conjunction with the dyadic case of { ; in particular, ⍵≡({⍳¨>⍴⍵){⍵ . The case {(<⍺),<⍵ is called the cartesian product of ⍺ and ⍵ . Dyad. For an integer scalar left argument in the range from 1↑⍴⍵ to ¯1+1↑⍴⍵ , ⍺{⍵ selects the major cell of ⍵ indexed by (1↑⍴⍵)⍺ , as in 2{a←5 3⍴⍳15 ←→ 6 7 8 , and ¯2{a ←→ 9 10 11 ; in other words, negative indices select from the tail end. Since the left rank of { is zero, the shape of ⍺{⍵ for any array of integers ⍺ is (⍴⍺),1↓⍴⍵ . For example: i{'abcde'⊣i←2 2⍴1 ¯1 0 ¯2 i{a be 3 4 5 ad 12 13 14 0 1 2 9 10 11 More generally, each element of ⍺ may be a boxed vector, whose successive elements are (possibly) boxed arrays of integers which specify selection along successive axes of ⍵ . For example: a←<(<i0←1 2),(<i1←2 3⍴⍳6) b←3 6⍴'abcdefghijklmnopqr' b a{b abcdef ghi ghijkl jkl mnopqr mno pqr ⊢c←i0{⍤(¯,2) b i1{⍤(¯,1) c ghijkl ghi mnopqr jkl mno pqr For the case of a singleelement (i.e., scalar) left argument, this selection along successive axes can be stated formally as follows (using only the simple definition of { given originally for an integer left argument): FROM: (>⍺)FR ⍵⊣n←1+⍴⍴⍵ FR: (1↓⍺)FR (>0{⍺){⍤(¯,n ←n1) ⍵:0=⍴⍺:⍵ It maybe noted that (<''){⍵ ←→ ⍵ ←→ ∘{⍵ , even for a scalar ⍵ . If any of the values of >0{⍺
occurring in the execution
of FR is not an open array,
then the selection is made
using the complement
(with respect to all indices along that axis)
of the indices in its open,
that is in >>0{⍺ .
In other words, the indices selected
are (⍳l)~l>>0{⍺⊣l←(n){⍴⍵ .
For example, if, Since the middle element in the open of i←<I⊃∘⊃K is the boxed boxed empty vector, the expression i{b selects all along the middle axis, and is equivalent to the dialectal form b[I;;K] discussed in Section III.A.
Monad. For a nonsingular matrix m , ⌹m is the inverse of m , that is, (⌹m)+.×m is the identity matrix i∘.=i←⍳0{⍴m . More generally, ⌹m is defined in terms of the dyadic case, as (i∘.=i←⍳0{⍴m)⌹m or, equivalently, by the relation ⍺⌹⍵ ←→ (⌹⍵)+.×⍺ . The shape of ⌹m is ⌽⍴m . The degenerate cases (vector and scalar) are defined by using the table ⍪⍵ instead of ⍵ , although most dialects differ in yielding a result of the same shape as ⍵ , rather than a matrix. For a nonzero vector v , the result of ,⌹v is v÷+/v×+v ; that is, a vector collinear with v . For a scalar s , the result of ,⌹s is ,s×÷s . Dyad. If the columns of ⍵
are linearly independent, and if
the first elements of ⍴⍺
and ⍴⍵ agree,
then ⍺⌹⍵ is defined so
as to minimize the elements
of Geometrically, ⍵+.×⍺⌹⍵ is, for vector ⍺ , the projection of ⍺ on the column space of ⍵ , that is, the point in the space spanned by the columns of ⍵ that is nearest to ⍺ . The most common uses of ⍺⌹⍵ are in the solution of linear equations, and in the approximation of functions by polynomials. As in the monadic case, the degenerate cases
of ⍵ are treated as ⍪⍵ ,
and disagreements with most dialects arise
as noted under the monadic case.
V. Adverbs An adverb produces two different classes of results
(usually verbs), one when it is applied to a noun and the
other when applied to a verb.
The derived results are therefore
referred to by the symbol for the adverb preceded
by n (for noun) or v (for verb).
For example, n⌿ refers to
the derived verb copy
(for which The ranks are given as they are in the verb table, except that a rank may depend upon the monadic, left, or right rank of the argument verb v , indicated by my , lv , and rv . As for the verbs, the definitions show the results for individual ceils, and the derived verb applies to arguments of higher rank in the standard manner discussed in Section II.B.
Monad. In the expression n⌿⍵ , the argument ⍵ is split into its major cells, and cell i{⍵ is copied i{n times. Thus: ⊢⍵←3 3⍴'abcdefghi 2 0 2⌿⍵ abc abc def abc ghi ghi ghi 2 0 2⌿⍳3 0 0 2 2 Scalar arguments. If n
has a single element, it is treated
as (1↑⍴⍪⍵)⍴n ,
for example, 2⌿⍳3 ←→ Dyad. In the dyadic case of n⌿ , the argument n may contain negative elements; a negative element copies major cells from the left argument of the derived verb rather than from the right. For example: 'ABCDEFG' ¯1 0 1 0 1 0 0⌿'abcdefg' Ace '⍝'(¯1*s=' ')⌿s←'now is the winter' now⍝is⍝the⍝winter
Monad. In the expression v⌿⍵ , the argument ⍵ is split into its major cells, and the verb v is applied between them. Thus if 1↑⍴⍵ is 3 , the result is (0{⍵)v(1{⍵)v(2{⍵) . Consider, for example, the following identities: ⊢⍵←3 2⍴⍳6 +⌿⍵ 0 1 (0{⍵)+(1{⍵)+(2{⍵) 2 3 0 1 + 2 3 + 4 5 4 5 0 1 + 6 8 6 9 ∘.+⌿⍵ +.×⌿⍵ 0 1 ∘.+ 2 3 ∘.+ 4 5 0 1 +.× 2 3 +.× 4 5 0 1 ∘.+ 6 7 0 1 +.× 23 7 8 23 6 7 7 8 7 8 8 9 Identity elements. If the leading axis
of ⍵ has zero length
(that is, 0=1↑⍴⍵),
the result of v⌿⍵ is the identity element
of verb v .
The left identity of v
is a noun l such that l v x
yields x for any x
in the right domain of v ,
the right identity of v
is a noun r
such that
The definition of each verb indicates its identity element (if any), listed after the symbols v⌿: . An element is included even though it is strictly a right identity or a left identity, and in some cases (such as for = and ≠) if it applies only over a subdomain (boolean). Identity elements extend relations of the form (+/a)≡(+/k↑a)+(+/k↓a) to include the cases k=0 and k=⍴a . Thus: a←2 3 5 7 11 k←3 k←0 (+/k↑a)+(+/k↓a) (+/k↑a)+(+/k↓a) (+/2 3 5)+(+/7 11) (+/⍳0)+(+/a) 10 + 18 0 + 28 28 28 Dyad. The
expression ⊢m←5 4⍴⍳20 2 +⌿ m 0 1 2 3 4 6 8 10 4 5 6 7 12 14 16 18 8 9 10 11 20 22 24 26 12 13 14 15 28 30 32 34 16 17 18 19 3 +⌿ m 12 15 18 21 24 27 30 33 36 39 42 45
Monad. The expression n⍀⍵ expands the argument ⍵ along the first axis, inserting at each point corresponding to a zero of n , a cell of zeros if ⍵ is numeric, a cell of spaces if ⍵ consists of characters, and a cell of the elements <⍳0 if is boxed or heterogeneous. For example: 1 0 1 0 1⍀2 3 5 1 0 1 0 1⍀3 4⍴⍳12 2 0 3 0 5 0 1 2 3 0 0 0 0 1 0 1 0 1⍀'abc' 4 5 6 7 a b c 0 0 0 0 8 9 10 11 In other words, n must be a boolean list, and if r←n⍀⍵ , then ⍵≡n⌿r , and (~n)⌿r is an array of zeros, spaces, or boxed empty vectors, of shape (+/~n),1↓⍴⍵ .
Monad. The expression v⍀⍵ assembles, along the leading axis, the 1↑⍴⍵ results (0{⍵) , (v⌿0 1{⍵) , (v⌿0 1 2{⍵) , etc. For example: ⊢m←3 5⍴0 1 0 0 v⍀m 0 1 0 0 0 0 1 0 0 0 1 0 0 0 1 1 1 0 0 1 0 0 0 1 0 1 1 0 1 1
The expression v/⍵ is equivalent to v⌿⍵ except that the split is made along the last axis rather than the first. In other words v/⍵ is equivalent to v⌿a , where a is obtained from ⍵ by transposing the last axis to the leading position, that is, a←(1⌽⍳0{⍴⍴⍵)⍉⍵ , or a←¯1⍥⊢⍵ . For example: ⊢⍵←3 4⍴⍳12 ⊢a←(1⌽⍳0{⍴⍴⍵)⍉⍵ 0 1 2 3 0 4 8 4 5 6 7 1 5 9 8 9 10 11 2 6 10 3 7 11 +/⍵ +⌿a 6 22 38 6 22 38 A similar correspondence holds for the other three cases, except that the leading axis of the result (which, in effect, results from the splitting axis) must be returned to last position. For example: 2 0 1 0/⍵ 2 0 1 0⌿a 1 0⍉2 0 1 0⌿a 0 0 2 0 4 8 0 0 2 4 4 6 0 4 8 4 4 6 8 8 10 2 6 10 8 8 10 The relations can be summarized as follows: (n/⍵) ≡ ⊢⍥0 n⌿ ¯1⍥⊢⍵ (v/⍵) ≡ v⌿ ¯1⍥⊢⍵ (n\⍵) ≡ ⊢⍥0 n⍀ ¯1⍥⊢⍵ (v\⍵) ≡ ⊢⍥0 v⍀ ¯1⍥⊢⍵ (⍺n/⍵) ≡ ⊢⍥0 ⍺n⌿ ¯1⍥⊢⍵ (⍺v/⍵) ≡ ⍺v⌿ ¯1⍥⊢⍵
Monad. The result of v}⍵ is a selection from ⍵ of the form i{⍵ , where the index i is obtained by applying v to {⍳¨>⍴⍵ , the “complete index table” of ⍵ . For example: ⊢⍵←2 3 3⍴⍳18 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 t←{⍳¨>⍴⍵ ⍴t 2 3 3 0{t ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 0 0 00 0 10 0 2 _______________ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 0 1 00 1 10 1 2 _______________ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 0 2 00 2 10 2 2 _______________ ⊢i←0 1 1⍉t ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 0 0 00 1 10 2 2 _______________ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 1 0 01 1 11 2 2 _______________ ⊢r←i{⍵ r≡0 1 1¨⍉}⍵ 0 4 8 1 The foregoing illustrates
how the adverb } can apply
to any selection function
(in this case However, verbs other than selection can serve meaningfully as arguments of } . For example: ⊢q←1¨⌊¯>}⍵ ⊢j←1 ⌊¨>t 0 1 1 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 3 4 4 0 0 00 0 10 0 1 3 4 4 _______________ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 9 10 10 0 1 00 1 10 1 1 12 13 13 _______________ 12 13 13 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 0 1 00 1 10 1 1 _______________ q≡j{⍵ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 1 1 0 01 0 11 0 1 _______________ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 1 1 01 1 11 1 1 _______________ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 1 1 01 1 11 1 1 _______________ Dyad. If ⍺ has the same shape as the index array v}⍳¨>⍴⍵ , then ⍺ v} ⍵ produces a merge of ⍺ and ⍵ , by inserting the elements of ⍺ in the selected positions of ⍵ , more specifically, in the positions of ⍵ indexed by the array v{⍳¨>⍴⍵ . Continuing the example used in the monadic case: ⊢⍺←2 3⍴100+⍳6 ⊢m←⍺ 0 1 1¨⍉}⍵ 100 101 102 100 1 2 103 104 105 3 101 5 6 7 102 103 10 11 12 104 14 15 16 105
Monad. v⊂⍵ ←→ ⍵v⍵ Dyad. ⍺v⊂⍵ ←→ ⍵v⍺ Dialects. Some dialects
[12
13]
use ⊂ for a function
called enclose;
in these, ⊂⍵ is equivalent to <⍵
except that ⍵≡⊂⍵ if ⍵ is an open item.
VI. Conjunctions A conjunction produces four main classes of results, referred to by using m and n for left and right noun arguments, and u and v for left and right verb arguments. Ranks are shown as for adverbs.
3⍕2¨⍟⍵←1 2 3 4 5 6 7 8 0.000 1.000 1.585 2.000 2.322 2.585 2.807 3.000 *¨2 ⍵ 1 4 9 16 25 36 49 64
This function is equivalent to composition (u⍤v) except that the function inverse to v is applied to the result of each cell. For example, since < and > are inverses, as are * and ⍟ : ⍴⍤>a←'abc'⊃1 3⊃'abcd' ⍴¨>a 3 ¯¯¯ 2 324 4 ___ ⍴⍤>b←1 2 3⊃''⊃'abcd' ¯¯¯¯¯ 302 3 _____
3 +⍤⍟ 4 3 +¨⍟ 4 2.48490665 12 The function u¨v is often called “the dual of u with respect to v”, but the phrase “u under v” is probably better, suggesting that u is performed after preparatory work by v , and before the task is sewn up by reversing the effect of v . The expression u¨v is valid only if v possesses an inverse. The following list shows inverse pairs commonly used in dialects: * +  ÷ ⌽ ⊖ ⍉ ⊢ < ⌹ ~ ≤ ⍟ +  ÷ ⌽ ⊖ ⍉ ⊢ > ⌹ ~ ≥
See u⍤n for a discussion of the expression n←⌽3⍴⌽n . Monad. The derived verb m⍤n has rank n , and produces a constant individual result m for each cell to which it applies. For example: m←'abc' ⍵←2 3⍴⍳6 m⍤0 ⍵ m⍤1 ⍵ m⍤2 ⍵ abc abc abc abc abc abc abc abc Dyad. The dyadic case differs only in that agreement of the outer shapes of the arguments is imposed. For example: ⍺←'PQ' ⍺ m⍤0 1 ⍵ ⍺ m⍤0 0 ⍵ abc domain error abc
Monad. The expression 1⍤v ⍵ applies v to each of a set of “segments” cut along the first axis, and assembles the results along a leading axis of length equal to the number of segments. Each segment is of the form s←(k+⍳n){⍵ , and k and n are chosen so that each segment begins at the occurrence of the delimiter 0{⍵ . In other words, for each segment s , (j{s)≡0{⍵ for j=0 and for no other value. For example: 1⍤<⍵←' worlds on worlds' ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯  worlds on worlds _________________ ⊢m←5 4⍴⍳6 1⍤< m 0 1 2 3 ¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 4 5 0 1 0 1 2 30 1 2 3 2 3 4 5 4 5 0 14 5 0 1 0 1 2 3 2 3 4 5_______ 4 5 0 1 _______ 1⍤⍴⍵ 1⍤(+⌿) m 7 6 9 6 9 3 4 6 2 4 7 The expression ¯l⍤v ⍵ differs only in that the delimiters are excluded from the segments. For example: ¯1⍤<⍵ ¯1⍤⍴⍵ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 6 worldsonworlds 2 ______________ 8 ¯1⍤<m ¯1⍤(+⌿) m ¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 6 8 4 6 4 5 0 14 5 0 1 4 5 0 1 2 3 4 5_______ _______ The verbs 2⍤v and ¯2⍤v differ from the corresponding cases 1⍤v and ¯1⍤v only in that the delimiter is the last cell, and marks the end of segments rather than the beginning. For example: 2⍤<⍵ 2⍤<m ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯  worlds on worlds 0 1 2 32 3 4 5 _________________ 4 5 0 10 1 2 3 _______4 5 0 1 _______ The case 0⍤v ⍵ applies v
after reversing ⍵ along each
axis, and is equivalent
to
The monads 3⍤v and ¯3⍤v produce tessellation by “maximal cubes” using size (⍴⍴⍵)⍴⌊/⍴⍵ and shift (⍴⍴⍵)⍴1 in the corresponding dyads. Dyad. For (k)∊1 2 , the dyadic cases ⍺ k⍤v ⍵ differ from the corresponding monadic cases only in that the delimiters are the 1’s in the boolean list ⍺ . For example: ⍺←0 1 0 0 1 ⍺ 1⍤< m ⍺ 2⍤< m ¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯ 4 5 0 14 5 0 1 0 1 2 32 3 4 5 2 3 4 5_______ 4 5 0 10 1 2 3 0 1 2 3 _______4 5 0 1 _______ _______ The case 0⍤v has left rank 2 , and ⍺ 0⍤v ⍵ applies v to a “rectangle” whose beginning point in ⍵ is determined by 0{⍺ , and whose size is determined by 1{⍺ . For example: a←2 3⍴0 1 1 2 2 3 b←2 3⍴0 1 1 2 2 ¯3 ⍵←2 3 5⍴⍳30 a b ⍵ 0 1 1 0 1 1 0 1 2 3 4 2 2 3 2 2 ¯3 5 6 7 8 9 10 11 12 13 14 a 0⍤⊢ ⍵ b 0⍤⊢ ⍵ 6 7 8 8 7 6 15 16 17 18 19 11 12 13 13 12 11 20 21 22 23 24 25 26 27 28 29 21 22 23 23 22 21 26 27 28 28 27 26 The beginning point is determined by (⍴⍵)(⍴⍴⍵)↑0{⍺ ; in other words, negative indexing may be used, and 0{⍺ is extended by zeros to provide a full index to ⍵ . The size is determined by s,(⍴s)↓⍴⍵⊣s←1{⍺ . Finally, before application of the verb v , the rectangle is reversed along each axis for which 1{⍺ is negative, as illustrated by the example using b above. The case 3⍤v also has left rank 2 , and ⍺3⍤v⍵ applies v to each element produced by a tessellation of ⍵ , using a size 1{⍺ , and beginning points that are multiples of the “shift” 0{⍺ . For example: a (>3 2⊃2 3) 3⍤<a abcdef ¯¯¯¯¯¯¯¯ ghijkl abccdeef mnopqr ghiijkkl stuvwx ________ yzABCD ¯¯¯¯¯¯¯¯ stuuvwwx yzAABCCD ________ The table of beginning points of the elements of the tessellation is given by: s×¨>{⍳¨>⌈(⍴⍵)÷s←0{⍺ The case ¯3⍤v is equivalent to 3⍤v except that shards of shape less than 1{⍺ are omitted.
If n is a aelement list, then u⍤n is a monadic function of rank 0{n , and a dyadic function of left rank 1{n and right rank 2{n . In general, n is treated as if it were ⌽3⍴⌽n ; in other words a single element specifies all ranks, but if 2=⍴n , the first element specifies the left rank, and the last specifies the right ranks, both dyadic and monadic. Monad. A monadic rank of k implies that the function applies to kcells of its argument, except that the rank of the cell will not exceed the rank of the argument, as discussed under degenerate cases in Section II.B. For example: ⍵←2 3 4⍴'abcdefghijklmnopqrstuvwx' ⍵ ,⍤2 ⍵ ,⍤¯1 ⍵ abcd abcdefghijkl abcdefghijkl efgh mnopqrstuvwx mnopqrstuvwx ijkl ,⍤4 ⍵ mnop abcdefghiffklmnopqrstuvwx qrst uvwx Dyad. In the expression ⍺ u⍤(l,r) ⍵ , the outer shapes of ⍺ and ⍵ (complementary to the shapes of the lcells and rcells) must agree unless one of them is empty, in which case the single corresponding cell is extended to apply to each of the cells of the other argument. For example, if ⍺←'yz' , then: ⍺,⍤0 2 ⍵ ⍺ ,⍤1 1 ⍵ yabcd yzabcd yefgh yzefgh yijkl yzijkl zmnop yzmnop zqrst yzqrst zuvwx yzuvwx
Monad. In the simplest case u⍤v ⍵ is equivalent to u v ⍵ . For example: ⊢y←⊖⍤⍉ ⍵←4 3 2⍴'abcdefghijkYmnopqrstuvwx' bhnt djpv flrx agms ciou y≡⊖⍉⍵ ekqw 1 However, this relation holds only because the monadic rank of ⍉ is unbounded; more generally, the rank of the derived function u⍤v is the rank of v ; that is, the expression u v is applied to each of the cells of ⍵ relative to v . For example: ⊢z←⊖⍤2 ⍉⍤2 ⍵ z≡⊖⍉⍤2 ⍵ bdf 0 ace z≡⊖⍤(⍉⍤2) ⍵ hjl 1 gik npr moq tvx suw Dyad. The left and right ranks
of u⍤v are both the monadic
rank of v .
Therefore ⍺ u⍤v ⍵ is equivalent
to
The verbs mt~v and ut~n apply the verbs u and v to their argument or arguments after transpositions to defer the axes specified by n (in the case of ut~n) to the tail end, or to prefer the axes specified by m (for mt~v) to the front. For example: a←2 3 4⍴'abcdefghijklmnopqrstuvwx' ⊢b← 2 l⍥⊢a am eq iu bn fr jv co gs kw dp ht lx 2 1⍥,a ameqiubnfrjvcogskwdphtlx (⊢⍥0 a)≡(1 2⍥⊢a) 1 These axis movements are prescribed by: PR: (⍋⍺,(⍳0{⍴⍴⍵)~⍺←(⍴⍴⍵)⍺)⍉⍵ DE: (⍋((⍳0{⍴⍴⍵)~⍺),⍺←(⍴⍴⍵)⍺)⍉⍵ in the following definitions: m⍥v⍵ ←→ v(>0{a)PR⍵⊣a←⌽3⍴⌽⊃m u⍥n⍵ ←→ u(>0{a)DE⍵⊣a←⌽3⍴⌽⊃n ⍺m⍥v⍵ ←→ ((>1{a)PR⍵)v(>2{a)PR⍵⊣a←⌽3⍴⌽⊃m u⍥n⍵⍵ ←→ ((>1{a)PR⍵)u(>2{a)DE⍵⊣a←⌽3⍴⌽⊃n Thus, the axes moved (for the cases of a monadic argument, left argument, and right argument, respectively) by v⍥(1⊃2⊃3) are 1 , 2 , and 3 , by v⍥(2⊃3) are 3 , 2 , and 3 , and by v⍥1 2 3 or v⍥(<1 2 3) are 1 2 3 . Compare with the rank adverb ⍤ for use of the phrase ⌽3⍴⌽ .
The monad u is applied to the result of v , that is: u⍥v ⍵ ←→ u v ⍵ ←→ u⍤v ⍵ ⍺ u⍥v ⍵ ←→ u ⍺ v ⍵ For example: 7 5 3 ⍥ 3 5 7 4 0 4
Dyad. The left argument of the tie adverb specifies the number of outer axes of the arguments that must agree, leaving any remaining axes free to contribute independently to the overall shape of the result, as illustrated by the examples in the discussion of agreement in Section II.B. If the ranks of v are l and r , and if z←a t.v b , then the outer shapes are osa←(l)↓⍴a and osb←(r)↓⍴b . The argument t determines a split of these outer shapes into tied and free shapes as follows: ta←t↑osa fa←t↓osa tb←t↑osb fb←t↓osb The tied shapes must agree, that is, ta≡tb ; the overall outer shape of the result z is given by ta,fa,fb . If i is an index that selects a single cell of z , then ⍴>i must be ⍴ta,fa,fb . Moreover, each cell of z is defined by: (i(z)≡((<(⍴ta,fa)↑>i){a) v (<(⍴ta)⌽(⍴fa)↓(⍴ta)⌽>i){b Dialects. Dialects commonly permit ∘.v for 0 .v .
Monad. The function u is applied n times. For example: ⍟. 2 ⍵ ←→ ⍟⍟ ⍵ ⌿. 4 ⍵ ←→ ⌿⌿⌿⌿ ⍵ 1¨○ .3 ⍵ ←→ 1○1○1○ ⍵ The function u.¯ is the limit of the application of u ; that is, u.¯⍵ is equivalent to u.k ⍵ , where u.k ⍵ matches u.(k1)⍵ . Finally, a negative value of n denotes n applications of the inverse function; that is, u.¯1 is the function inverse to u , and u.(n) is inverse to u.n , and u.¯¯ is inverse to u. ¯ .
Monad. The expressions .×⍵ and +.×⍵ are, for square matrix arguments ⍵ , the determinant and the permanent of mathematics. More generally, u.v is defined in terms of a recursive “expansion by minors”, given by the following expression from [17]: (0{⍉⍵) u.v $ (<¨>¨>{0⊃⊂⍳0{⍴⍵){⍵ : 1=¯1{⍴⍵ : u⌿,⍵ Dyad. The expression 1 2 3 +.× 3 4 5 26 a←2 3⍴⍳6⊣b←3 4⍴⍳12 a+.×b 20 23 26 29 56 68 80 92 a+.×1 2 3 8 26 For verbs other than + and × that share the property of applying to items (i.e., of rank 0) and producing items, the same definition holds for argument ranks not greater than 2 . For example: ⍺⌊.+⍵ ⍺+.⌊⍵ 0 1 2 3 3 3 3 3 3 4 5 6 9 10 11 12 The general definition for arbitrary functions and arguments of arbitrary rank is: ⍺ u.v ⍵ ←→ u⌿(¯1⍥⊢⍺) 1 .v ⍵ In other words, the result is obtained by u over the result of applying 0 .v to the major ceils of ¯1⍥⊢⍺ and ⍵ . For example: ⊢c←¯1⍥⊢a b 0 3 0 1 2 3 1 4 4 5 6 7 2 5 8 9 10 11 (0{c) ∘.× 0{b (1{c) ∘.× 1{b 0 0 0 0 4 5 6 7 0 3 6 9 16 20 24 28 (2{c) ∘.× 2{b 16 18 20 22 40 45 50 55 The final result of a+.×b is the sum over these tables, which agrees with the example of a+.×b given earlier. Since ¯1⍥⊢⍺ moves the last axis of ⍺ to the leading position, its overall effect in the definition of u.v is to “split ⍺ along the last axis”, just as ⍵ is split along the leading axis. This asymmetric treatment of the arguments rests on the desire to make the simple case of +.× on matrix arguments agree with the matrix product of mathematics, whose definition exhibits the same sort of asymmetry.
The function u∪n is equivalent to u , except that it has a combining rank of n (relevant to the application of the conjunctions ∪ and ∩ to verbs). The combining rank of all primitive verbs is zero.
Monad. The result of u∪v is a “catenation” of u and v determined by the combining ranks of u and v (denoted here by cu and cv) as follows: u∪v ⍵ ←→ (u⍵),¨<(v⍵) if cu=cv ←→ (u⍵)⍪(v⍵) if cu≠cvThe combining rank of u∪v is (cu⌈cv)+cu=cv . For example: ∪∪× ¯2 ¯1 0 1 2 2 ¯1 1 1 ¯1 0 0 0 ¯1 1 1
Monad. The combining rank (defined under ∪) is cu1 , where cu denotes the combining rank of u . Moreover: u∩v⍵ ←→ v⌿⍤cu u⍵ For example, f∪g∩+ and f∪g∩× denote what is commonly denoted in mathematics by f+g and f×g .
Verb definition. If m and d are (possibly boxed) character nouns, then m∇d yields a verb of unbounded rank whose monadic and dyadic cases are determined by m and d respectively. For example: ⊢m←,⊃'⍵×⍎(⍵≤2)↓''$⍵1''' ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ⍵×⍎(⍵≤2)↓'$⍵1' _______________ ⊢d←'s←⍴⍵'⊃'c⍅⍺,⍵'⊃'(2,s)⍴c' ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Is←⍴⍵c⍅⍺,⍵(2,s)⍴c ________________ s←c←⍵←2 r←'abc' m∇d 'def' r abc def s 2 c abcdef ⍵ 2 The major aspects of ∇ can be inferred from this example: the derived function executes by first assigning to ⍺ and ⍵ the values of the arguments, then executing the boxed sentences in d in sequence, and then providing as the explicit result of the function the result of the last sentence executed. The names ⍺ , ⍵ , → , and $ are local to the function, and other names may be localized dynamically, as discussed in Section II.J. The monadic case of m∇d illustrates the use of the symbol $ for selfreference, that is, reference to the derived function being produced. For example: m∇d 2 m∇d 3 m∇d 4 2 6 24 The sequence of execution of the sentences in evaluating m∇d ⍵ , is controlled by a system variable → (local to a function) as follows: → is first assigned the value of ⍳0{⍴m ; sentence >(0{→){m is selected for execution, → is respecified by →←1↓→ , the selected sentence is executed, and the sequence is repeated until → is exhausted, or until a value of 0{→ occurs that is not an index to m . Similar remarks apply to the dyadic case. Since → may be respecified within any sentence, any
sequence of execution can be achieved.
For example, if: p ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ b←,1→←>(⍵≥⍴b){2⊃3 1bb←(0,b)+b,0 _______________________________ f←p∇∘ f 0 f 1 f 3 1 1 1 1 3 3 1 If sentence k (that is, >k{a) begins with a name followed by a right parenthesis, that name (called a label) is localized and assigned the value k↓⍳⍴a . Labels are useful in branching, that is, in expressions of the form →←>j{l1⊃l2⊃l3 or →←l1~l2 . Finally, if a is any argument of ∇ , it is treated as ,⊃a . Consequently, open arguments can be used, as in '⍵*÷2'∇'⍵*÷⍺' . Adverb definition. The conjunction ∇ accepts certain integer left arguments: in particular 3∇(m⊃d) produces a function, and is equivalent to m∇d . More generally, 1∇n yields an adverb, 2∇n a conjunction, and 3∇n a verb, provided that n is a suitable representation of the corresponding result. The representation of a conjunction differs from that for a function only in that it may include the names ⍺ and ⍵ , which refer to the left and fight arguments of the resulting conjunction. Similar remarks apply to an adverb. For example, COM←1∇('(⍺⍵)'⊃'⍵⍺⍺') is a “commute” adverb such that a F COM b is equivalent to b F a , and F COM b is equivalent to F b. Similarly, TIL←2∇('(⍵⍵)⍺⍵'⊃'(⍵⍵)⍺⍺') is a conjunction such that if H←F TIL G , then a H b is equivalent to (G b)F a , and H b is equivalent to (G b)F b . Representations. The expression 0∇a yields a noun which is the representation of the argument a . If a is a verb, adverb, or conjunction defined by a←k∇(m⊃d) (for any k∊1, 2, 3), then 0∇a yields the “standard” representation (,⊃m),(,⊃d) . More generally, the representation of a verb, adverb, or conjunction may comprise more than two elements. For example, if H←F.G , then the representation of H must incorporate the complete representations of the verbs (or nouns) F and G . This is necessary to ensure that subsequent reassignments of the names F and G will not affect the definition of H . This is analogous to the fact that the value of the noun c←a×b is not affected by subsequent reassignments of the values of a and b . For k∊⍳4 , the expression k∇r applies to any suitable representation r . Consequently a is equivalent to k∇(0∇a) for any a .
Most adverbs and conjunctions apply to all functions in a single manner; the custom conjunction : applies to particular functions in ways defined for the specific function, and its effects are discussed under each function affected. For example, For each of the relations (<≤=≥>≠≡), the custom conjunction specifies the tolerance. For example, <:1E¯6 is equivalent to < used with ⎕ct set to 1E¯6 .
Withe (u⍩v) is similar to (u⍤v) , but applies v only to the right argument: ⍺ u⍩v ⍵ ←→ ⍺ u v ⍵ u⍩v ⍵ ←→ ⍵ u v ⍵ References
Table 1: APL Alphabet and ASCII Transliteration
The ASCII
[5]
transliteration scheme in the last column is based upon similarity,
EnglishGreek correspondences,
and variants, denoted by
an extra delimiter (@)
and varying by rotation about a horizontal or vertical axis.
Each transliteration which begins
with a delimiter must end with a space.
Table 2: Parsing Process
Table 3: Translation to Canonical Form
Table 4: Standard Names and Synonyms
First appeared in APL Quote Quad, Volume 18, Number 1, 198709.
