# Index Of

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Revision as of 20:20, 10 April 2008 by Sudleyplace (talk | contribs) (New page: <p>Dyadic iota (<apll>L⍳R</apll>) is extended to allow <apll>1<⍴⍴L</apll>, returning a nested array of shape <apll>⍴R</apll> whose items are each integer vectors of length <apll...)

Dyadic iota (L⍳R) is extended to allow 1<⍴⍴L, returning a nested array of shape ⍴R whose items are each integer vectors of length ⍴⍴R, suitable for use as indices to L.

For example, in origin-1

M←2 3⍴'abcdef'

M⍳'af'

1 1 2 3

M[M⍳'af']

af

This extension is implemented via an internal magic function:

∇ Z←L F R;⎕IO;O

[1] O←⎕IO ⋄ ⎕IO←0

[2] Z←⊂[0] O+(1+⍴L)⊤(¯1↓,(1+⍴L)↑L)⍳R

∇