Array Lookup: Difference between revisions

Revision as of 15:15, 16 June 2013

Z←L⍸R

returns a simple integer vector identical to (⊂⍤¯1 L)⍳⊂⍤(¯1+⍴⍴L) R.

L and R are arbitrary arrays.

Z is an array of rank 0⌈1+(⍴⍴R)-⍴⍴L and shape (1-⍴⍴L)↓⍴R.

For matrices, the result is equivalent to (⊂[2] L)⍳⊂[2] R (lookup the rows of one array in another) which encapsulates a common idiom in one symbol.

For higher rank arrays, the arguments are enclosed to become vectors of subarrays and then the usual lookup produces the final result.

This function is sensitive to both ⎕IO and ⎕CT.

For example, in origin-1

      ⎕←L←6 4⍴'DinaDickJohnJaneSue Mary'
Dina
Dick
John
Jane
Sue
Mary
      ⎕←R←3 4⍴'JaneBob Sue '
Jane
Bob
Sue
      L⍸R
4 7 5

This function was suggested by the language designers of Dyalog APL.

@@ Line 7: / Line 7: @@
        <td></td>
        <td></td>
-       <td>returns a simple integer vector identical to <apll>(⊂⍤(¯1+⍴⍴L) L)⍳⊂⍤(¯1+⍴⍴L) R</apll>.</td>
+       <td>returns a simple integer vector identical to <apll>(⊂⍤¯1 L)⍳⊂⍤(¯1+⍴⍴L) R</apll>.</td>
      </tr>
      </table>
@@ Line 16: / Line 16: @@
 </tr>
 <tr>
-   <td><apll>Z</apll> is an integer vector of length <apll>1↑⍴R</apll>.</td>
+   <td><apll>Z</apll> is an array of rank <apll>0⌈1+(⍴⍴R)-⍴⍴L</apll> and shape <apll>(1-⍴⍴L)↓⍴R</apll>.</td>
 </tr>
 <tr>