0: y (Constant Function)
0: y ignores its argument, y (which can therefore have any shape or type), always returning the value 0.
0: is equivalent to (0"_). Using Rank in this way makes a verb out of a noun (viz. a string or numeric constant).
0: has sister-primitives _: (=infinity), 1:, 2:, 3:, ... 9: and _1:, _2:, _3:, ... _9:, returning the corresponding integers.
YY=: ;:'The quick brown fix is ignored by the lazy dog' 0: YY 0 $$ 0: YY 0
Old J code makes more use of 0: (1: 2: etc.) than up-to-date code. You'll often find in these examples that 0 works just as well. This is because at some stage the syntax of J was altered to treat the train: (NOUN VERB VERB) as meaning (NOUN"_ VERB VERB).
Common uses
This family of primitive verbs is more useful than it first appears, especially when defining a verb tacitly. Tacit definitions don't like embedded variables and constants. They behave best when built from verbs, adverbs and prepositions. Trains of primitives form hooks and forks: they receive data from arguments x and y in a complex way. Use 0: like a verb, knowing that it will mop up unwanted values presented as arguments and reliably return a scalar 0.
Example: pad the ends of a numeric vector with zeros.
pad=: 0: , ] , 0: pad i.9 0 0 1 2 3 4 5 6 7 8 0
Note: 0 will work in place of the first 0: here, but not the second. This is because the train of verbs applied to argument y:
(a b c d e) y
is equivalent to the explicit expression:
(a y) b ((c y) d (e y))
from which you can see that 0: (standing in for a and e here) is called with an unwanted argument y which must be lost.
Example: avoid an accidental vector constant, when what you want is to parameterize an adverb or a conjunction:
0 1} i.6 NB. Try to amend 1{i.6 to zero
|rank error
| 0 1}i.6
0 1:} i.6
0 0 2 3 4 5Example: Pascal's Triangle.
bc =: < 0&(, + ,~) 1: NB. binary coeff bc 6 1 0 0 0 0 0 1 1 0 0 0 0 1 2 1 0 0 0 1 3 3 1 0 0 1 4 6 4 1 0 1 5 10 10 5 1
This example won't work if 1 is used in place of 1:.
See Also
x 0: y (Constant Function)
x 0: y ignores its arguments, x and y (which can therefore have any shape or type), always returning the value 0.
XX=: ;:'The quick brown fox is ignored by the lazy dog' YY=: i. 5 3 XX 0: YY 0 $$ XX 0: YY 0
Common uses
See above: 0: y