matrix multiplication without declaring the destination matrix first

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Elinor Ginzburg
Elinor Ginzburg 2019-10-29
评论： Elinor Ginzburg 2019-10-30
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!
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dpb
dpb 2019-10-29
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
Elinor Ginzburg 2019-10-30
thank you very much! the problem was that without the ';' matlab printed the matrix, and it took some time.

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回答（1 个）

Matt J
Matt J 2019-10-29
编辑：Matt J 2019-10-29
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.

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