replace the b-th row and >30-th column of a by a-30

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Chaoyang Jiang 2018-4-1

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https://ww2.mathworks.cn/matlabcentral/answers/391928-replace-the-b-th-row-and-30-th-column-of-a-by-a-30

编辑： Walter Roberson 2018-4-1

I want to replace the b-th row and >30-th column of a by a-30. How to make the following code more efficient within one line? Thank you!

    a=randi(100,[100,100]);
    b=randi(100,[30,1]);
    for i=1:length(b)
    a(b(i),30:end)=a(b(i),30:end)-30;
    end

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Answer 1

Walter Roberson 2018-4-1

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There is no more efficient version for that.

I am not saying that it cannot be done in vectorized form: it can be done in vectorized form. I am saying that you need to make extra calculations and involve extra memory in such a way that the for loop is likely to be more efficient. Especially since duplicate b values are possible so you are going to be affecting the same location multiple times.