Classic Edition: the symbol ⍸ (Iota Underbar) is not available in Classic Edition, and Where is instead represented by ⎕U2378.
Y must be a simple Boolean or numeric array of non-negative integers.
The model for Where can be expressed as {(,⍵)/,⍳⍴⍵}.
If Y is Boolean, R is a vector of the indices of all the 1s in Y. If Y is all zeros, R is an empty vector.
⎕IO is an implicit argument of Where.
Examples
⎕IO
1
⍸ 1 0 1 0 0 0 0 1 0
1 3 8
⍸'e'='Pete'
2 4
3 4⍴0 1 1
0 1 1 0
1 1 0 1
1 0 1 1
⍸ 3 4⍴0 1 1
┌───┬───┬───┬───┬───┬───┬───┬───┐
│1 2│1 3│2 1│2 2│2 4│3 1│3 3│3 4│
└───┴───┴───┴───┴───┴───┴───┴───┘
⍸2 3 4⍴0 0 0 0 1
┌─────┬─────┬─────┬─────┐
│1 2 1│1 3 2│2 1 3│2 2 4│
└─────┴─────┴─────┴─────┘
⍸ 0 1 0 2
2 4 4
{⍵/⍥,⍳⍴⍵} 0 1 0 2
2 4 4
⍸2 2⍴0 1 2 3
┌───┬───┬───┬───┬───┬───┐
│1 2│2 1│2 1│2 2│2 2│2 2│
└───┴───┴───┴───┴───┴───┘