Reduce (`f/`)¶

Syntax¶

f/ Y

f is a dyadic scalar function. Y is the argument array.

Inserts f between consecutive elements of Y, evaluated right to left.

For a vector a b c d, f/ a b c d computes a f (b f (c f d)).

      +/ 1 2 3 4 5
15
      ×/ ⍳5
120
      ⌈/ 3 1 4 1 5
5
      ⌊/ 3 1 4 1 5
1

Right-to-left evaluation matters for non-commutative functions:

      -/ 1 2 3 4 5
3

This computes 1-(2-(3-(4-5))) = 1-2+3-4+5 = 3.

On matrices and higher-rank arrays, reduce works along the last axis. The result drops the last dimension.

      +/ 3 4⍴⍳12
10 26 42

Each row is summed independently.

Reducing an empty array raises DOMAIN ERROR.