>>  <<  Usr  Pri  JfC  LJ  Phr  Dic  Voc  !:  Help  Learning J

# Chapter 8: Composing Verbs

This chapter is concerned with operators which combine two verbs to produce new composite verbs.

Recall from Chapter 03 the composition conjunction @: (at colon, called "At"). Given verbs sum and square we can define a composite verb, sum-of-the-squares.
```   sum    =: +/
square =: *:
```

 sumsq =: sum @: square sumsq 3 4 sum@:square 25

The general scheme is that if f and g are monads then

```               (f @: g) y    means   f (g y)
```
Note in particular that f is applied to the whole result (g y). To illustrate, suppose g applies separately to each row of a table, so we have:
```   g =: sum " 1
f =: <
```

 y =: 2 2 \$ 1 2 3 4 g y f g y (f @: g) y 1 2 3 4 3 7 +---+ |3 7| +---+ +---+ |3 7| +---+

We have just seen the most basic of kind of composition. Now we look at some variations.

If f is a monad and g is a dyad, then (f @: g) is a dyadic verb such that
```           x (f @: g) y    means    f (x g y)
```
For example, the sum of the product of two vectors x and y is called the "scalar product".
```   sp =: +/ @: *
```

 x =: 1 2 y =: 2 3 x * y +/(x * y) x sp y 1 2 2 3 2 6 8 8

The last example showed that, in the expression (x (f @: g) y) the verb f is applied once to the whole of (x g y)

The conjunction &: (ampersand colon, called "Appose") will compose dyad f and monad g. The scheme is:
```               x (f &: g) y   means   (g x) f (g y)
```
For example, we can test whether two lists are equal in length, with the verb (= &: #)
```   eqlen =: = &: #
```

 x y #x #y (#x) = (#y) x eqlen y 1 2 2 3 2 2 1 1

Here f is applied once to the whole of (g x) and (g y).

## 8.4 Ambivalent Compositions

To review, we have seen three different schemes for composition. These are:
```              (f @: g) y    =    f (g y)
```
```            x (f @: g) y    =    f (x g y)
```
```            x (f &: g) y    =    (g x) f (g y)
```
There is a fourth scheme,
```              (f &: g) y    =    f (g y)
```
which is, evidently, the same as the first. This apparent duplication may be useful if we are interested in writing an ambivalent definition, that is, with both a monadic and a dyadic case.

Notice that from the first and second schemes it follows that if verb g is ambivalent then the composition f @: g is also ambivalent. For example, suppose g is the ambivalent built-in verb |. with |. y being the reverse of y and x |. y being the rotation of y by x places.

 y =: 'abcdef' (< @: |.) y 1 (< @: |.) y abcdef +------+ |fedcba| +------+ +------+ |bcdefa| +------+

From the third and fourth schemes above it follows that if verb f is ambivalent, then (f &: g) is ambivalent. For example, suppose that f is the verb % (reciprocal or divide). and g is *: (square).

 % *: 2 (% &: *:) 2 (*: 3) % (*: 2) 3 (% &: *:) 2 0.25 0.25 2.25 2.25

There is a conjunction @ (at, called "Atop"). It is a variation of the @: conjunction. Here is an example to show the difference between (f @: g) and (f @ g).
```   y =: 2 2 \$ 0 1 2 3
```

 y f g (f @: g) y (f @ g) y 0 1 2 3 < sum"1 +---+ |1 5| +---+ +-+-+ |1|5| +-+-+

We see that with (f @: g) verb f is applied once. However, with (f@g), for each separate application of g there is a corresponding application of f. We could say that applications of f track the applications of g.

Recall from Chapter 07 that a verb has in general three ranks, monadic, left and right, and for a verb f these ranks are yielded by the expression f b. 0. For example

 g g b. 0 sum"1 1 1 1

Suppose that the monadic rank of g is G.

```   G =: 0 { (g b. 0)
```
Then (f @ g) means (f @: g) applied separately to each G-cell, that is, (f @: g)"G.

 (f @ g) y (f @: g)"G y +-+-+ |1|5| +-+-+ +-+-+ |1|5| +-+-+

and so the general scheme is:

```             (f @ g) y    means     (f @: g) " G   y
```

Next we look at the composition (f @ g) for a dyadic g. Suppose f and g are defined by:
```   f =: <
g =: |. " 0 1  NB. dyadic
```
Here x g y means: rotate vectors in y by corresponding scalars in x. For example:

 x=: 1 2 y=: 2 3 \$ 'abcdef' x g y 1 2 abc def bca fde

Here now is an example to show the difference between f @: g and f @ g

 f (x g y) x (f @: g) y x (f @ g) y +---+ |bca| |fde| +---+ +---+ |bca| |fde| +---+ +---+---+ |bca|fde| +---+---+

We see that with (f @: g) verb f is applied once. With (f@g), for each separate application of g there is a corresponding application of f.

Suppose that the left and right ranks of dyad g are L and R. Then (f @ g) means (f @: g) applied separately to each pair of an L-cell from x and corresponding R-cell from y. That is, (f@g) means (f @: g)"G where G = L,R.

 G =: 1 2 { (g b. 0) x (f @:g)" G y x (f @ g) y 0 1 +---+---+ |bca|fde| +---+---+ +---+---+ |bca|fde| +---+---+

The scheme is:

```              x (f@g) y =  x (f@:g) " G y
```

Recall that in Chapter 03 we met the conjunction & as a bonding operator. With one argument a noun and the other argument a dyadic verb the result is a monad. For example +&6 is a monad which adds 6 to its argument.

If both arguments of & are verbs then & has a different interpretation. In this case it is is a composition operator, called "Compose". Now we look at the composition f & g for dyadic f.

Suppose g is the "Square" function, and f is the "comma" function which joins two lists.

```   f =: ,
g =: *:
```

 x =: 1 2 y =: 3 4 g x g y 1 2 3 4 1 4 9 16

Here now is an example to show the difference between (f &: g) and (f & g)

 (g x) f (g y) x (f &: g) y x (f & g) y 1 4 9 16 1 4 9 16 1  9 4 16

We see that in (f &: g) the verb f is applied just once, to give 1 4 , 9 16. By contrast, in (f & g) there are two separate applications of f, giving firstly 1,9 and secondly 4,16.

The scheme is that

```              x (f & g) y  means  (g x) (f " G,G) (g y)
```
where G is the monadic rank of g. Here f is applied separately to each combination of a G-cell from x and a corresponding G-cell from y. To illustrate:

 G =: 0 { (g b. 0) (g x)(f " (G,G))(g y) x (f & g) y 0 1  9 4 16 1  9 4 16

## 8.8 Ambivalence Again

The composition f&g can be ambivalent. The dyadic case, x f&g y, we saw above. For the monadic case, f&g y means the same as f@g y.
```   f =: <
g =: *:
```

 f&g 1 2 3 f@g 1 2 3 +-+-+-+ |1|4|9| +-+-+-+ +-+-+-+ |1|4|9| +-+-+-+

## 8.9 Summary

Here is a summary of the 8 cases we have looked at so far.
```         @:       (f @: g) y  =  f (g y)
```
```         @:     x (f @: g) y  =  f (x g y)
```
```
```
```         &:       (f &: g) y  =  f (g y)
```
```         &:     x (f &: g) y  =  (g x) f (g y)
```
```
```
```         @        (f @ g)  y  =  (f @: g) " G  y
```
```         @      x (f @ g)  y  =  x (f @: g) " LR y
```
```
```
```         &        (f & g)  y  =  (f @: g) " G  y
```
```         &      x (f & g)  y  =  (g x) (f " (G,G)) (g y)
```
where G is the monadic rank of g and LR is the vector of left and right ranks of g.

## 8.10 Inverses

The "Square" verb, (*:), is said to be the inverse of the "Square-root" verb (%:). The reciprocal verb is its own inverse.

 *: 2 %: 4 % 4 % 0.25 4 2 0.25 4

Many verbs in J have inverses. There is a built-in conjunction ^: (caret colon, called "Power") such that the expression f ^: _1 is the inverse of verb f. (This is like writing f-1 in conventional notation.)

For example, the inverse of square is square root:

 sqrt =: *: ^: _1 sqrt 16 *:^:_1 4

^: can automatically find inverses, not only of built-in verbs, but of user-defined verbs such as compositions. For example, the inverse of "twice the square-root of" is "the square of half of"

```   foo    =: (2&*) @: %:
fooINV =: foo ^: _1

```

 foo 16 fooINV 8 foo fooINV 36 8 16 36

## 8.11 Composition: Verb Under Verb

We now look at composition with the conjunction &. (ampersand dot, called "Under"). The idea is that the composition "f Under g" means: apply g, then f, then the inverse of g.

For an example, the square root of a number can be found by taking the logarithm, halving and taking the antilog, that is, halving under logarithm. Recall that halve is -: and logarithm is ^.

 SQRT =: -: &. ^. SQRT 16 -:&.^. 4

The general scheme is that

```             (f &. g) y   means  (g ^: _1) f g y
```
This is the end of Chapter 8.

The examples in this chapter were executed using J version 701. This chapter last updated 29 Jul 2012
Copyright © Roger Stokes 2012. This material may be freely reproduced, provided that this copyright notice is also reproduced.

>>  <<  Usr  Pri  JfC  LJ  Phr  Dic  Voc  !:  Help  Learning J