⍳ — Index generator / Index of⍳N generates 1…N. Dimensions are nonnegative integers; exact input gives exact coordinates.
⍳4 ⍝ 1 2 3 4
⍳3x ⍝ 1x 2x 3x
A shape vector generates an array of coordinate vectors.
⍳2 2 ⍝ 2 2⍴(1 1)(1 2)(2 1)(2 2)
X⍳Y finds the first matching major cell in X for each cell of Y. Not found: 1+≢X. Uses tolerant matching and returns exact positions.
'abc'⍳'cabz' ⍝ 3x 1x 2x 4x