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Question about assigning value in a matrix.

Asked by C Zeng

C Zeng

on 6 Jun 2013
Accepted Answer by Walter Roberson

Walter Roberson

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

C Zeng

C Zeng

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3 Answers

Answer by Walter Roberson

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

C Zeng

C Zeng

on 6 Jun 2013

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

Sean de Wolski

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...

C Zeng

C Zeng

on 6 Jun 2013

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

Walter Roberson

Walter Roberson

Answer by Azzi Abdelmalek

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

C Zeng

C Zeng

on 6 Jun 2013

Yes, Azzi! Thanks.

Azzi Abdelmalek

Azzi Abdelmalek

Answer by Sean de Wolski

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

Sean de Wolski

Sean de Wolski

on 7 Jun 2013

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...

C Zeng

C Zeng

on 7 Jun 2013

Thanks, Sean!

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

C Zeng

C Zeng

on 13 Jun 2013

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

Sean de Wolski

Sean de Wolski

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