⌸
Key r←[x](F⌸)y
If x
is provided, let k
be the major cells of x
and v
be the major cells of y
; if x
is not provided, let k
be the major cells of y
and v
be ⍳≢y
.
r
is the result of applying F
to each unique value of k
and the corresponding entries in v
.