How can I index a matrix using an array?

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!

Answer 1

[madhan ravi]

A = accumarray(idx,1,[],[],0)

[Alex Mcaulley]

nice solution +1

[Yizhuang Alden Cheng]

Thank you! I had to turn this into a logical matrix after using this code, but nonetheless this a very elegant answer.

Answer 2

[Andrei Bobrov]

s = max(idx);
A = false(s);
A(sub2ind(s,idx(:,1),idx(:,2))) = true;

[Yizhuang Alden Cheng]

Thank you! This works very well, and is in fact faster. I would have accepted this as well if I could accept multiple answers.