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Change the value of a matrix according to the indexes stored in another one.

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Hey, goodies, let's pretend I have the next matrix.
Z = zeros(5,6);
And in another matrix called idx I have the information of which rows and columns of Z I want to change the value, for example:
idx = [1 1; 2 5; 4 6]
Being the first column the position of the columns I want to change and the value of the second idx column the position of the rows I want to change, and if the value i want to obtein in Z in idx position is 5, what i expect to have is:
This is just an example, in the realization I have many values that I want to change, therefore the option:
Z(1,1) = 5;
Z(2,5) = 5;
Z(4,6) = 5;
Is not an option
I'm looking for something that allows me to do it automatically, without the need for loops, if anyone knows it would help me a lot, thanks.

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Accepted Answer

KSSV
KSSV on 2 Jun 2020
Edited: KSSV on 2 Jun 2020
Read about sub2ind
Z =zeros(5,6) ;
idx = [1 1; 2 5; 4 6] ;
idx = sub2ind(size(Z),idx(:,1),idx(:,2)) ;
Z(idx) = 5

More Answers (1)

Chris Angeloni
Chris Angeloni on 2 Jun 2020
Edited: Chris Angeloni on 2 Jun 2020
You probably want to get the index as a linear subscript instead of row,column, then index a vectorized version of your original matrix.
Z = zeros(5,6);
idx = [1 1; 2 5; 4 6];
% save size of Z
sz = size(Z);
% vectorize Z
Z = Z(:);
% make row,col index to linear index
lInd = sub2ind(sz,idx(:,1),idx(:,2))
Z(lInd) = 5;
% resize Z to original size
Z = reshape(Z,sz(1),sz(2));

  2 Comments

Alejandro Fernández
Alejandro Fernández on 2 Jun 2020
Thank you very much for the answer, it works, however KSSV's answer is much smaller in terms of code.
Chris Angeloni
Chris Angeloni on 2 Jun 2020
Yes, I saw it after! I forgot you can use the linear index to index the original matrix

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