⍨ — Self / Commute

Monadic operator. Keys: Alt-1 Backtick. Hanzi: 己/换 (己/換)

f⍨Y is Y f Y.

×⍨3                ⍝ 9

X f⍨ Y is Y f X.

2-⍨10              ⍝ 8

A subject operand makes a constant function: A⍨Y and X A⍨Y return A.

42⍨0               ⍝ 42

Inverse

With one argument fixed, f⍨⁻ inverts f with that argument on the other side. +⍨⁻Y halves Y, ×⍨⁻Y takes its square root, and ⌊⍨⁻Y, ⌈⍨⁻Y and ⊢⍨⁻Y give back Y. For an array k, k⍨⁻Y gives back Y, which must match k.

+⍨⁻10   ⍝ 5
×⍨⁻9    ⍝ 3