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Hook

A train is a sequence of two or three words in a row that is given a special meaning. A train of two verbs is a hook and is evaluated as follows:

  (f g) y	evaluates as	y f g y
x (f g) y	evaluates as	x f g y

Suppose you wanted to scale a list of numbers such that the result was each number divided by the maximum number in the list. The maximum over a list is given by the verb derived by applying the adverb / to the verb >. .

   a =. 3 5 8 2 7
   maxover =. >./
   maxover a
8

To divide an argument by the maximum over the argument you can use:

   a % maxover a
0.375 0.625 1 0.25 0.875

The above can be written as a hook.

   (% maxover) a
0.375 0.625 1 0.25 0.875

You can now define scale as a hook with % >./ .

   scale =. % >./
   scale a
0.375 0.625 1 0.25 0.875

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