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C Zeng
0

Question about assigning value in a matrix.

Asked by C Zeng
on 6 Jun 2013

Hi, all, again, I want to do the work quickly: I have a large matrix A, and I want to assign value in some entry in each row.

For example:
A=zeros(4);
a=[1,2;
 2,3;
 3,2;
 4,1];

I want (1,2), (2,3),(3,2), (4,1) of matrix A be 1. How shall I make it work in no more than 3 commands?

What I am doing now is using loops:

for i=1:4
A(a(i,1),a(i,2)=1;
end

However, as i is very large like 3^14, the program is very slow. So I wonder is there a way to speed the programming?

Thanks.

  0 Comments

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3 Answers

Answer by Walter Roberson
on 6 Jun 2013
 Accepted answer
idx = sub2ind(size(A), [1 2 3 4], [2 3 2 1]);
A(idx) = 1;

  3 Comments

Thanks, Walter! So sub2ind can find the index of A quickly by loops, right? I will definitely try this code. I like it!

I've always found for-loops to be significantly faster than sub2ind and ind2sub. Especially as the number of indices gets large.

It's been a few years since I was timing it for my own work though...

Oh, really? I always found that MATLAB is not efficient for large loops. That is why I tried to avoid large loops.


Answer by Azzi Abdelmalek
on 6 Jun 2013
A=zeros(4);
a=[1,2;
2,3;
3,2;
4,1];
idx= sub2ind(size(A),a(:,1),a(:,2));
A(idx)=1

  1 Comment

Yes, Azzi! Thanks.


Answer by Sean de Wolski
on 6 Jun 2013

I've always found for-loops to be significantly faster than sub2ind and ind2sub. Especially as the number of indices gets large.

It's been a few years since I was timing it for my own work though...

  12 Comments

Well a few things real quick:

prMatrix = diag(pr);

Skips the first for-loop and call to zeros()

Second, you migh twant to consider taking the outer for-loop and converting it to a parfor with the Parallel Computing Toolbox. This would allow this to work in Parallel, over a few workers and speed things up that way.

Alternatively, just run it over the weekend or overnight, at two times total and 556 seconds, that's still only 20 minutes...

Thanks, Sean!

Let me try it again. I have to run large data on server, because 3^12 is memory out here.

C Zeng
on 13 Jun 2013

Sean, thanks for your advice. My program got speedup. sub2ind will be faster if the size is large.


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