J combines 2 verbs (either built-in or user-defined) using hooks, and these work differently when called with either 1 or 2 arguments. For example, to convert a list of numbers to proportions of the sum, we can define the hook

prop =: % +/

+/ sums the array to the right (/ inserts the verb to its left (+) between the numbers in the array) and % (divide) takes the list of numbers and divides each of them by the sum.

prop 3 1 10 7 9
0.1 0.0333333 0.333333 0.233333 0.3
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J uses 1 and 0 for true and false values. This allows using summation to count how many true things, and product to see if all items are true. For example, to see if two arrays are equal:
equals =: [: */ <. = >.
a =: b =: 4 5 6 7 7
c =: 4 5 6 6 7
a equals b
1
a equals c
0

<. and >. give the greater and lesser values of each item of the two arrays
= tests for equality, producing 1 when equal
*/ takes the product of the resulting array elements: 1 if all items are identical
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