Just from staring at the result, it looks like the result of ⍺⍳⍵ has the same shape as ⍵, and for each item of ⍵ it tells you either the coordinates where it was found in ⍺, or ⍬ if it was not found. (Dunno what it does if ⍺ is a scalar, where ⍬ would be a valid coordinate.)
N.B. in Dyalog APL dyadic iota has been extended to high rank arrays in a completely different way: https://help.dyalog.com/17.1/#Language/Primitive%20Functions/Index%20Of.htm Jay. On Tue, 14 Apr 2020 at 09:15, Elias Mårtenson <loke...@gmail.com> wrote: > The ISO spec doesn't address what the index-of function should do when the > left argument is not a scalar or a one-dimensional array. > > GNU APL extends this, but I don't really understand in what way. How am I > to interpret the output from this? > > * (2 2 ⍴ 104 105 106 107) ⍳ (3 4 ⍴ 100+⍳100)* > ┏→━━━━━━━━━━━━━━━━━━━━━━┓ > ↓┏⊖┓ ┏⊖┓ ┏⊖┓ ┏⊖┓ ┃ > ┃┃0┃ ┃0┃ ┃0┃ ┃0┃ ┃ > ┃┗━┛ ┗━┛ ┗━┛ ┗━┛ ┃ > ┃┏→━━┓ ┏→━━┓ ┏→━━┓ ┏→━━┓┃ > ┃┃0 0┃ ┃0 1┃ ┃1 0┃ ┃1 1┃┃ > ┃┗━━━┛ ┗━━━┛ ┗━━━┛ ┗━━━┛┃ > ┃┏⊖┓ ┏⊖┓ ┏⊖┓ ┏⊖┓ ┃ > ┃┃0┃ ┃0┃ ┃0┃ ┃0┃ ┃ > ┃┗━┛ ┗━┛ ┗━┛ ┗━┛ ┃ > ┗∊━━━━━━━━━━━━━━━━━━━━━━┛ > > Regards, > Elias >
