something like a modulo function (but not)?

[E. Joshua Rigler]

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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