need repmat help

Hello,

I have a 4D array with two for loops that is very slow. I know repmat is a good tool to speed things up but I am not very good at using it yet. Is there a way to use repmat to replace the follow set of for loops:

%th_diff3_1 has the following dimensions (621,1405,12,116)

for a = 1:621

for b = 1:1405

    x = sum(th_diff3_1(a,b,:,:));
    x = sum(x(1,1,:,:));
    percent3_1(a,b) = (x/1390) * 100;

end

end

Please help me understand repmat!

Thanks,

Dan

Answer 1

[Fangjun Jiang]

It's not about repmat. I think you can do this:

x=sum(th_diff3_1,4);
x=sum(x,3);
percent3_1=(x/1390) * 100

[Jan]

It is faster to sum at first over the 3rd dimension and then over the 4th, because this allows a better usage of the cache.

(x/1390)*100 is slowetr than x*(100/1390). In the first case two matrix operations are needed, in the second only one.

[Fangjun Jiang]

Jan, you are amazing! How do you know all of that? I admire you!

[Jan]

@Fangjun: Thanks, but I do not _know_ such details. I just try it taking into account that the calculations are performed by a processor. I've learned programming when efficiency was really important: on a ZX81 with 1kB RAM. It was possible to read the complete operating system in two afternoons.

Although Matlab has a higher complexity, the fundamental rules for efficiency did not change: Keep the data compact to avoid unneded operations.

[Dan]

Fangjun and Jan, thanks so much. I realize now this is not a repmat problem, I just have for loops engrained in my head and need to think differently!