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How to change elements in several arbitrary positions at the same time

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Ruini Qu
Ruini Qu 2018-11-1
关闭： MATLAB Answer Bot 2021-8-20
Hello, I have a matrix A=[0 0 0 0; 0 0 0 0; 0 0 0 0], which I want to change the value of some elements. I have another matrix B=[1 2; 1 3; 2 4], which shows the position of the elements I want to change in A. For example, for the first row in A, I want to change the 1st and 2nd elements from 0 to 1; for the second row in A, I want to change the 1st and 3rd elements to 1; and so on. The final result I am looking for should be C=[1 1 0 0; 1 0 1 0; 0 1 0 1]. I know I can do it with a loop. But I want faster solution. Please help. Thank you!

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回答（2 个）

Rik
Rik 2018-11-1
编辑：Rik 2018-11-1
You can use the sub2ind function (I used the bsxfun function to generate the row numbers)
A=[0 0 0 0; 0 0 0 0; 0 0 0 0];
B=[1 2; 1 3; 2 4];
ind=sub2ind(size(A),bsxfun(@times,(1:size(B,1))',ones(1,size(B,2))),B);
C=A;C(ind)=1;
isequal(C,[1 1 0 0; 1 0 1 0; 0 1 0 1])
Matt J
Matt J 2018-11-1
编辑：Matt J 2018-11-1
[ma,na]=size(A);
C = A + sparse([1:ma;1:ma].',B,1,ma,na)

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