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## something like a modulo function (but not)?

**From**: |
E. Joshua Rigler |

**Subject**: |
something like a modulo function (but not)? |

**Date**: |
Wed, 27 Oct 2004 11:15:09 -0600 |

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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**something like a modulo function (but not)?**,
*E. Joshua Rigler* **<=**