Re: trying to optimize my octave program

[Judd Storrs]

2010/3/19 Tim Rueth <address@hidden>:
>  If ndgrid makes a 4-D matrix which has, say, 256 elements
> (4 in each dim), then I'll be creating 256 copies of several vectors inside
> analyze(), each which has a length of 4288.  I started implementing this but
> hit two snags:
>
> 1) How can I initialize a multidimensional array with, say "foo" in each
> element, without a for loop?  Exactly like zeros(...) but filling each
> element with "foo" instead of 0.

repmat() is the fastest way to do this e.g.

a = repmat(123.456,[7 8 9]) ; # Numeric matrix
a = repmat("foo", [7 8 9]) ; # Character matrix
a = repmat({"foo"},[7 8 9]) ; # Cell matrix

> 2) How do I do element-wise comparisons and conditional string assignments
> on matrices without stepping through them?  For example, let's say I have
> two 4-D matrices, a_mat and b_mat with numeric values.  I'd like to create a
> new 4-D string matrix, c_mat, where
>
> for each i in a_mat
>  if a_mat(i) < b_mat(i)
>    c_mat(i) = "foo";
>    cntr++;
>  endif
> endfor

The trick is to use logical arrays for indexing in the assignment.
Assuming c_mat is a cell array:

a_mat = rand([4 5 6 7]) ;
b_mat = rand([4 5 6 7]) ;
c_mat = repmat({""},[4 5 6 7]) ;

c_mat(a_mat < b_mat) = "foo" ;

Another option is:

idx = find(a_mat < b_mat) ;
c_mat(idx) = "foo" ;
cntr = length(idx) ;



--judd