Difference between revisions of "Index Of"

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<p>For example, in origin-1</p>
 
<p>For example, in origin-1</p>
  
<apll>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;M←2 3⍴'abcdef'<br />
+
<apll><pre>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;M⍳'afg'<br />
+
      M←2 3⍴'abcdef'
&nbsp;1 1&nbsp;&nbsp;2 3&nbsp;&nbsp;3 4<br >
+
      M⍳'afg'
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;M[M⍳'af']<br />
+
1 1 2 3 3 4
af<br />
+
      M[M⍳'af']
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;L←2 ⋄ ⎕fmt L⍳⍳3<br />
+
af
┌3──────────┐<br />
+
      L←2 ⋄ ⎕FMT L⍳⍳3
│┌0┐ ┌0┐ ┌0┐│<br />
+
┌3──────────┐
││0│ │0│ │0││<br />
+
│┌0┐ ┌0┐ ┌0┐│
│└~┘ └~┘ └~┘2<br />
+
││0│ │0│ │0││
└∊──────────┘</apll>
+
│└~┘ └~┘ └~┘2
 +
└∊──────────┘</pre></apll>
  
 
<p>Note that this extension preserves the identity <apll>R≡L[L⍳R]</apll> for all <apll>R⊆L</apll>.</p>
 
<p>Note that this extension preserves the identity <apll>R≡L[L⍳R]</apll> for all <apll>R⊆L</apll>.</p>
Line 47: Line 48:
 
<p>This extension is implemented via an internal magic function:</p>
 
<p>This extension is implemented via an internal magic function:</p>
  
<apll>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Z←L #DydIota R;⎕IO;O</apll><br />
+
<apll><pre>
<apll>[1]&nbsp;&nbsp;&nbsp;O←⎕IO ⋄ ⎕IO←0</apll><br />
+
    ∇ Z←L #DydIota R;⎕IO;O
<apll>[2]&nbsp;&nbsp;&nbsp;Z←⊂[0] O+(1+⍴L)⊤(¯1↓,(1+⍴L)↑L)⍳R</apll><br />
+
[1]   O←⎕IO ⋄ ⎕IO←0
<apll>&nbsp;&nbsp;&nbsp;&nbsp;∇</apll>
+
[2]   Z←⊂[0] O+(1+⍴L)⊤(¯1↓,(1+⍴L)↑L)⍳R
 +
    ∇</pre></apll>

Latest revision as of 22:33, 15 April 2018

Z←L⍳R returns a nested array of indices suitable to indexing L.
L is an array of rank not equal 1.
R is an arbitrary array.
Z is a nested array of shape ⍴R whose items are each integer vectors of length ⍴⍴L, suitable for use as indices to L, except for where the item in R is not found in L, in which case the corresponding item in Z is ⎕IO+⍴L.
This feature extends dyadic iota to non-vector left arguments.
This function is sensitive to ⎕IO and ⎕CT.


For example, in origin-1

      M←2 3⍴'abcdef'
      M⍳'afg'
 1 1  2 3  3 4
      M[M⍳'af']
af
      L←2 ⋄ ⎕FMT L⍳⍳3
┌3──────────┐
│┌0┐ ┌0┐ ┌0┐│
││0│ │0│ │0││
│└~┘ └~┘ └~┘2
└∊──────────┘

Note that this extension preserves the identity R≡L[L⍳R] for all R⊆L.

This extension is implemented via an internal magic function:

    ∇ Z←L #DydIota R;⎕IO;O
[1]   O←⎕IO ⋄ ⎕IO←0
[2]   Z←⊂[0] O+(1+⍴L)⊤(¯1↓,(1+⍴L)↑L)⍳R
    ∇