need repmat help
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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
采纳的回答
Fangjun Jiang
2011-9-19
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
4 个评论
Jan
2011-9-19
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
2011-9-19
Jan, you are amazing! How do you know all of that? I admire you!
Jan
2011-9-19
@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
2011-9-20
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