Matrix manipulation

Question

Pedro Cavaco 2011-4-12

Hi,

I have a matrix A(1000,6) and i want to write 1's at certain positions say:

 i = [785 58 595 389 450 956];
 j = [4 6 2 3 5 1];

If I do A(i,j) = 1 it places 1's in the positions "i" of all the columns "j".

But what i want is:

 A(i(1),j(1)) = 1
 A(i(2),j(2)) = 1 
 ...
 A(i(end),j(end)) = 1

Does anybody knows how to do this without having to use a for loop?

Teja Muppirala 2011-4-12

Is there a compelling reason not to use a FOR loop? A loop will probably do this faster than any non-loop solution (that I can think of anyways).

Answer 1

Andrei Bobrov 2011-4-12

variant

 i = [785 58 595 389 450 956];
 j = [4 6 2 3 5 1];
 A(sub2ind(size(A), i, j)=1;