Re: something like a modulo function (but not)?

[Stefan van der Walt]

The ind2sub function should do what you want:

octave:7> [x,y] = ind2sub([5 5], [3 5 8 10 13 15 18 20 23 25])
x =

  3  5  3  5  3  5  3  5  3  5

y =

  1  1  2  2  3  3  4  4  5  5

Regards
Stefan

On Wed, Oct 27, 2004 at 11:15:09AM -0600, E. Joshua Rigler wrote:
> I feel a little silly even asking this question, but since when has that
> stopped me :^)?  I have a vector of sequential indices to a 2-D matrix
> that I want to convert to a set of column-wise vector indices.  For
> example a particular 5x5 matrix would be fortran indexed as:
> 
>         mtrx = [ 1   6  11  16  21 ]
>                  [ 2   7  12  17  22 ]
>                  [ 3   8  13  18  23 ]
>                  [ 4   9  14  19  24 ]
>                  [ 5  10  15  20  25 ]
>         
> I extract a subset of this matrix that gives me the following vector of
> indices:
> 
>         idx1 = [ 3   5   8  10  13  15  18  20  23  25 ]
> 
> and wish to convert it to something like:
> 
>         idx2 = [ 3   5   3   5    3   5   3    5   3   5 ]
> 
> The first thing that popped into my head was that I would need to use
> the modulo function in Octave (i.e., idx2=mod(idx1,5)), but of course
> that would give me:
> 
>         idx2 = [ 3   0   3   0    3   0   3    0   3   0 ]
> 
> I guess what I really want is something similar to modulo, except where 
> mod(n,I*n)=n,  not zero ("I" is an integer).  The best solution I could
> come up with on my own is:
> 
>         octave:44> idx2 = mod(idx1,5)+(mod(idx1,5)==0)*5
>         idx2 =
>          
>           3  5  3  5  3  5  3  5  3  5
>         
> Can anyone out there suggest something a little more elegant that would
> have less impact on a long and already computationally intensive
> iterative function?  Am I just being dense?
> 
> -EJR



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