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

Question

Alejandro Fernández 2020-6-2

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https://ww2.mathworks.cn/matlabcentral/answers/540583-change-the-value-of-a-matrix-according-to-the-indexes-stored-in-another-one

评论： Chris Angeloni 2020-6-2

采纳的回答： KSSV

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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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Answer 1

KSSV 2020-6-2

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https://ww2.mathworks.cn/matlabcentral/answers/540583-change-the-value-of-a-matrix-according-to-the-indexes-stored-in-another-one#answer_444999

编辑：KSSV 2020-6-2

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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

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Alejandro Fernández 2020-6-2

Thank you so much, it works

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Answer 2

Chris Angeloni 2020-6-2

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https://ww2.mathworks.cn/matlabcentral/answers/540583-change-the-value-of-a-matrix-according-to-the-indexes-stored-in-another-one#answer_445003

编辑：Chris Angeloni 2020-6-2

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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));