How can I index a matrix using an array?

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Yizhuang Alden Cheng
编辑： Yizhuang Alden Cheng 2019-7-31
Hi,
I want to create a very large logical matrix, and I have an array which has the indices of the matrix which I want to be true (with the remaining entries being false).
For example, suppose I had the following array 8x2 array:
idx = [1 1;
1 2;
3 1;
3 3;
4 2;
5 1;
5 2;
5 3];
where each row represents index for a 5x3 matrix, and I want to use this array to create the following index:
A = [1 1 0;
0 0 0;
1 0 1;
0 1 0;
1 1 1];
Is there an efficient way to do this without using loops (since in my actual application, A may have something like 1,000,000 rows and 10,000 columns)? Thanks!

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采纳的回答

madhan ravi
madhan ravi 2019-7-31
A = accumarray(idx,1,[],[],0)
  2 个评论
Yizhuang Alden Cheng
Yizhuang Alden Cheng 2019-7-31
Thank you! I had to turn this into a logical matrix after using this code, but nonetheless this a very elegant answer.

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更多回答（1 个）

Andrei Bobrov
Andrei Bobrov 2019-7-31
s = max(idx);
A = false(s);
A(sub2ind(s,idx(:,1),idx(:,2))) = true;
  1 个评论
Yizhuang Alden Cheng
Yizhuang Alden Cheng 2019-7-31
编辑：Yizhuang Alden Cheng 2019-7-31
Thank you! This works very well, and is in fact faster. I would have accepted this as well if I could accept multiple answers.

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