Matlab equivalent of Numpy broadcasting?

Question

Hi,

I'm trying to find some way to substract a size 3 vector from each column of a 3*(a big number) matrix in Matlab. Of course I could use a loop, but I'm trying to find some more efficient solution, a bit like numpy broadcasting. Oh, and I can't use repmat because I just don't have enough memory to use it (as it creates yet another 3*(a big number) matrix)...

Is this possible?

Thanks in advance,

Antony

Answer 1

Loops aren't bad in MATLAB anymore thanks to compiler optimizations like just-in-time acceleration (JITA). etc. Most of the time, I've noticed that a solution with loops in current MATLAB versions is much faster than complicated (albeit, cool :D) one-liners.

bsxfun might do the trick but in my experience, it tends to have memory issues as well but less so than repmat.

So the syntax would be:

AA = bsxfun(@minus,A,b) where b is the vector and A is your big matrix

But I urge you to profile the loopy version and then decide! Most probably, due to memory constraints, you might not have a choice :)

Answer 2

I don't know if this will speed up the code, but subtraction of a scalar from a vector doesn't have memory issues. Since your matrix size is so asymmetrical, the overhead from a for-loop on the short dimension is negligible.

So maybe

matout = matin;
for j = 1:size(matin, 1) %3 in this case
     matout(j,:) = matin(j,:) - vec_to_subtract(j);
end

of course, you could do this in place, but I didn't know if you wanted to preserve the original matrix.

Answer 3

Actually, it seems that http://www.frontiernet.net/~dmschwarz/genops.html (operator overloading with mex files) does the trick too, even though I haven't tested it yet.