Numarray: Using sum() within functions

[Christopher T King]

On Sat, 14 Aug 2004, Jim Cser wrote:

> I have a function to generate a multi-dimensional array, which then
> gets summed over one axis.  The problem is that the dimensions
> are large, and I run out of memory when I create the entire array,
> so I'm trying to do the sum *within* the function.
> 
> Example-- variables x,y,z,t; dimensions numX, numY, numZ, numT;
> functions f1(x,y,z,t), f2(y,z,t);  want to calculate f1*f2 and
> sum over t to get out[x,y,z].

Would something like the following work?

 def f3(x, y, z, t=arange(numT)):
     return sum(f1(x,y,z,t)*f2(y,z,t))

This assumes that f1() and f2() can operate on array objects.  If not, the 
following (much slower) method should work:

 def f3(x, y, z):
     return sum(fromfunction(lambda t: f1(x,y,z,t)*f2(y,z,t),(numT,)))

To make the output matrix, you could use either this:

 out = fromfunction(f3, (numX, numY, numZ))

which will save memory, but will be slow, or (assuming f1 and f2 support 
arrays):

 out = f3(arange(numX), arange(numY), arange(numZ))

which will eat lots of memory (more that you say you have, if you use the
first f3()), but will be very fast.

Hope this helps.