matrix multiplication without declaring the destination matrix first

hey,

I'm trying to multiply two matrices (I don't want to use any shortcuts, I want to do the multiplication by defention). can I do it without declaring first on the matrix?

what I wrote is:

C2 = zeros(N)
for i = 1:N
    for j = 1:N
        for k = 1:N
            C2(i,j) = C2(i,j) + A(i,k)*B(k,j); 
        end
    end
end

but the action of C2 = zeroes(N) is time consuming since N=1000 in this case. anyway to skip this declaration?

Thank you!

[Adam]

It will be even more time consuming if you skip it. The time is spent allocating memory, which is done once here. It will be done multiple times if you remove it. You will also get an M-Lint warning basically telling you this if you get rid of it.

You should terminate it with a ; though. It takes fractions of a seconod on my machine with the ; on the end.

[dpb]

C2(N,N)=0;

may be even a tiny fraction faster...whatever it takes, it's a nit in comparison to not preallocting for larger arrays.

[Elinor Ginzburg]

thank you very much! the problem was that without the ';' matlab printed the matrix, and it took some time.

Answer 1

[Matt J]

You can loop backwards. Then the pre-allocation will be done silently.

clear C2
for i = N:-1:1
    for j = N:-1:1
        for k = 1:N
            C2(i,j) = C2(i,j) + A(i,k)*B(k,j); 
        end
    end
end

However, I don't know why you think the zeros command is the slow part of the code. I also don't know why you care about speed. The only reason it would make sense to do this is as a homework exercise, in which case speed should not be the issue.

[Elinor Ginzburg]

thank you, I've found the problem - missins ';' after the zeros command. it took some time to print the matrix...