Editing APL (programming language) (section)

==== Simple statistics ====
Suppose that {{code|X}} is an array of numbers. Then {{code|(+/X)÷⍴X}} gives its average. Reading ''right-to-left'', {{code|⍴X}} gives the number of elements in X, and since {{code|÷}} is a dyadic operator, the term to its left is required as well. It is surrounded by parentheses since otherwise X would be taken (so that the summation would be of {{code|X÷⍴X}}—each element of X divided by the number of elements in X), and {{code|+/X}} gives the sum of the elements of X. Building on this, the following expression computes [[standard deviation]]: {{sxhl|2=apl|((+/((X - (+/X)÷⍴X)*2))÷⍴X)*0.5}}

Naturally, one would define this expression as a function for repeated use rather than rewriting it each time. Further, since assignment is an operator, it can appear within an expression, so the following would place suitable values into T, AV and SD: {{sxhl|2=apl|SD←((+/((X - AV←(T←+/X)÷⍴X)*2))÷⍴X)*0.5}}