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⎕IO determines the index of the first element of a non-empty vector.

⎕IO may be assigned the value 0 or 1.  The value in a clear workspace is 1.

⎕IO is an implicit argument of any function derived from the Axis operator ([K]), of the monadic functions Fix (⎕FX), Grade Down (⍒), Grade Up (⍋), Index Generator (⍳), Roll (?), and of the dyadic functions Deal (?), Grade Down (⍒), Grade Up (⍋), Index (⌷), Index Of (⍳), Indexed Assignment, Indexing, Pick (⊃) and Transpose (⍉).

Examples

        ⎕IO←1
        ⍳5
1 2 3 4 5
 
        ⎕IO←0
        ⍳5
0 1 2 3 4
 
        +/[0]2 3⍴⍳6
3 5 7
 
        'ABC',[¯.5]'='
ABC
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