Simple examples

This page contains examples that show APL's strengths. The examples require minimal background and have no special dependencies.

Text processing
APL represents text as character lists (vectors), making many text operations trivial.

Indices of multiple elements
gives us a mask for elements (characters) in the left argument that are members of the right argument: 'mississippi'∊'sp' 0 0 1 1 0 1 1 0 1 1 0 gives us the indices where true (1): ⍸'mississippi'∊'sp' 3 4 6 7 9 10 We can combine this into an anonymous infix (dyadic) function: 'mississippi' (⍸∊) 'sp' 3 4 6 7 9 10

Parenthesis nesting level
First we compare all characters to the opening and closing characters; ∘.='plus(square(a),plus(square(b),times(2,plus(a,b)))' 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 An opening increases the current level, while a closing decreases, so we convert this to changes (or deltas) by subtracting the bottom row from the top row:     -⌿∘.='plus(square(a),plus(square(b),times(2,plus(a,b)))' 0 0 0 0 1 0 0 0 0 0 0 1 0 ¯1 0 0 0 0 0 1 0 0 0 0 0 0 1 0 ¯1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 ¯1 ¯1 ¯1 The running sum is what we're looking for: +\-⌿''∘.='plus(square(a),plus(square(b),times(2,plus(a,b)))' 0 0 0 0 1 1 1 1 1 1 1 2 2 1 1 1 1 1 1 2 2 2 2 2 2 2 3 3 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 3 2 1