How to use for loop ?

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Sudharsan Srinivasan 2017-9-20

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评论： Jan 2017-9-22

Let's say I have a matrix A = zeros(1000,500). I have two another matrices of size 2 X 50 each for example B = [1 2 3 .... 50 ; 2 4 6 .... 100] and C = [3 6 9 .... 150 ; 4 8 12 ....200]. Let's say I have another matrix D of same size 2 X 50 with some numbers for example [2 4 2 ... 67 ; 6 9 3 .... 54]. Now I have add up all these random numbers in matrix D into the matrix A only at the positions dictated by the matrix B and C. For example the first row in matrix B and C as in [1 2 3 .... 50] and [3 6 9 .... 150] are the x and y positions/co-ordinate where the first row of random numbers as in [2 4 2 .... 67] are to be added with matrix A. The same thing goes for the second row also. Clearly, I have to add [2 4 2 .... 67] to the matrix A at the positions [(1,3) (2,6) (3,9) .... (50,150)] and similarly add up [6 9 3 .... 54] to the matrix A at the positions [(2,4) (4,8) (6,12) .... (100,200)].

How can I use for loop to code this ?

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Answer 1

Jan 2017-9-20

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It does not matter, if B and C are matrices, but you care about the value of the elements only - correctly?

for k = 1:numel(B)
  A(B(k), C(K)) = A(B(k), C(K)) + D(k);
end

This seems to be very easy. Did you try anything else?

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Sudharsan Srinivasan 2017-9-20

Hi Jan, I used the same as technique as you have mentioned. But what does 'nume1' in your code mean ?

Rik 2017-9-20

That isn't the number 1, it is the lowercase L. The function numel counts the number of elements of a matrix. It is therefore equivalent to prod(size(B)) or length(B(:))

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Answer 2

Rik 2017-9-20

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编辑：Rik 2017-9-21

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Why insist on a loop?

A(sub2ind(size(A), B(:), C(:))) = D(:);

That should do the same thing.

Edit: the difference between my solution and Jan's is that mine doesn't work if the indices in B an C are non-unique combinations, which you could check with this:

length(unique([B(:) C(:)],'rows'))==numel(B)

If that is false, you can't use my solution.

if length(unique([B(:) C(:)],'rows'))==numel(B)
    %A(B(:),C(:))=D(:);
    A(sub2ind(size(A), B(:), C(:))) = D(:);
else
    for k = 1:numel(B)
        A(B(k), C(K)) = A(B(k), C(K)) + D(k);
    end
end