Conditional calculations on a numeric matrix

Question

Wah On Ho 2019-11-18

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https://ww2.mathworks.cn/matlabcentral/answers/491670-conditional-calculations-on-a-numeric-matrix

评论： Walter Roberson 2019-11-18

Hi All,

I'm relatively new to Matlab and I find myself slightly stuck. I have a numeric matrix of numbers comprising of floating point values. Depending on whether the value in each cell is less than or greater than some threshold value, either one or another calculation is made.

Below is a much simplified expression of my problem but it illustrates the form of what I'm trying to do with millions of cells of values which takes a rather longer time than the illustration below.

I realise that I can make a logic matrix from the input matrix but then I'm stuck on what follows to complete the calculations.

Any advice is gratefully received.

input = magic(10);
thresholdvalue = 50;
for col = 1 : 10
    for row = 1 : 10
        if input(row,col) > thresholdvalue
            result(row,col) = input(row,col)*2;
        else
            result(row,col) = input(row,col)/2;
        end
    end
end

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Answer 1

Walter Roberson 2019-11-18

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mask = input > thresholdvalue;
result(mask) = input(mask) * 2;
result(~mask) = input(mask) / 2;

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Wah On Ho 2019-11-18

Dear Walter,

That's excellent - thank you! So simple yet so effective.

The result from the 3rd line initially confused me as it didn't output a result that was divided by 2 until I realised its missing the tilde for the inverse mask for input(~mask) / 2. Anyway I'm now unstuck and can progress.