Y must be a simple scalar or vector array of non-negative numbers. R is a numeric array composed of the set of all possible coordinates of an array of shape Y. The shape of R is Y and each element of R occurs in its self-indexing position in R. In particular, the following identity holds:

      ⍳Y ←→ (⍳Y)[⍳Y]

⎕IO is an implicit argument of Index Generator. This function is also known as Interval.

      ⎕IO
1
      ⍴⍳0
0
      ⍳5
1 2 3 4 5
 
      ⍳2 3
 1 1  1 2  1 3
 2 1  2 2  2 3
 
      ⊢A←2 4⍴'MAINEXIT'
MAIN
EXIT
      A[⍳⍴A]
MAIN
EXIT
      

      ⎕IO←0
      ⍳5
0 1 2 3 4
 
      ⍳2 3
 0 0  0 1  0 2
 1 0  1 1  1 2
 
      A[⍳⍴A]
MAIN
EXIT