From sources-request at octave dot org Sat May 29 19:47:40 2004
Subject: sub2ind() convenience patch
From: Etienne Grossmann <etienne at cs dot uky dot edu>
To: octave-sources at octave dot org
Cc: etienne at cs dot uky dot edu
Date: Sat, 29 May 2004 20:39:34 -0400



  Hi all,

I propose the following patch so that does not gripe about the size of
its index arguments, as long as the number of elements is right.

  With this patch, code to reshape index lists passed to sub2ind()
becomes unnecessary. Advantage: shorter code.

  For example,

    octave:34> sub2ind ([3,4,2],[1 3 2 3],[1 2 4 4],[2 1 2 2])
    ans =

    13   6  23  24

    octave:35> sub2ind ([3,4,2],[1 3 2 3],[1 2 4 4],[2 1 2 2]')
    error: sub2ind: all index arguments must be the same size
    error: evaluating if command near line 49, column 6
    error: evaluating if command near line 42, column 4
    error: evaluating if command near line 41, column 2
    error: evaluating for command near line 39, column 7
    error: evaluating if command near line 33, column 5
    error: evaluating if command near line 32, column 3
    error: called from `sub2ind' in file `/usr/share/octave/2.1.57/m/general/sub2ind.m'
    octave:35> mysub2ind ([3,4,2],[1 3 2 3],[1 2 4 4],[2 1 2 2]')
    ans =

    13   6  23  24

  The overhead seems reasonable or negative (!) (timing of mysub2ind
is on the right).

======================================================================
     octave:57> T = 100; szs = [3 5 4]; 
     for N=[5 10 50 100 1000, 10000],
       N
       i1 = 1+floor(szs(1)*rand(1,N));
       i2 = 1+floor(szs(2)*rand(1,N));
       i3 = 1+floor(szs(3)*rand(1,N)); 
       tic; 
       for i = 1:T, 
	 sub2ind (szs,i1,i2,i3);
       end; tt=toc/T; tic; 
       for i = 1:T,
	  mysub2ind (szs,i1,i2,i3); 
       end; 
       [tt, toc/T]
     end

     N = 5
     ans =

       0.0016043  0.0015547

     N = 10
     ans =

       0.0015739  0.0013806

     N = 50
     ans =

       0.0016303  0.0014407

     N = 100
     ans =

       0.0017061  0.0016547

     N = 1000
     ans =

       0.0026176  0.0024954

     N = 10000
     ans =

       0.012621  0.013198
====================================================================== 

  Just my 2c contribution,

  Etienne 


======================================================================
--- /usr/share/octave/2.1.57/m/general/sub2ind.m        2004-04-11 23:29:44.000000000 -0400
+++ ./mysub2ind.m       2004-05-29 20:10:50.000000000 -0400
 at @ -46,9 +46,9 @@
              error ("sub2ind: index out of range");
            endif
          else
-           if (all (size (first_arg) == size (arg)))
+           if (prod (size (first_arg)) == prod (size (arg)))
              if ((i > nd && arg == 1) || (arg > 0 & arg <= dims(i)))
-               ind += scale(i-1) * (arg - 1);
+               ind(:) += scale(i-1) * (arg(:) - 1);
              else
                error ("sub2ind: index out of range");
              endif
======================================================================
-- 
Etienne Grossmann ------ http://www.cs.uky.edu/~etienne