⍷ — Groups / Find

Function. Keys: Alt-Minus e. Ranks: ∞ monadic, ∞ ∞ dyadic

⍷Y gives the positions of each distinct major cell of Y, in the order of ∪. It matches cells as ∪ does. On a keyed axis, the positions are keys. In Dyalog, this is {⊂⍵}⌸Y.

⍷"abca"             ⍝ (0 3⋄1⋄2)ₓ

X⍷Y marks starting positions where the whole pattern X occurs in Y. The result has Y’s shape. Overlaps count.

"ana"⍷"banana"      ⍝ $f $t $f $t $f $f
1 1⍷1 1 1          ⍝ $t $t $f

Works with multidimensional patterns and tolerant comparison of items.