Nested for loop fit with function

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William 2014-4-22

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https://ww2.mathworks.cn/matlabcentral/answers/126672-nested-for-loop-fit-with-function

评论： William 2014-4-22

I understand for loops are not ideal for Matlab, but not sure how to avoid using them. In this case I have a matrix 'TE_ROI' that contains values at each position which need to be run through a function 'multi_exp_fit'. It fits 2 exponential curves to the data and then puts the coefficients inside another preallocated matrix. Here is the code. Please advise.

T2_gof   = zeros(s_x,s_y,s_z,numel(X));
T2_range1= zeros(s_x,s_y,s_z,numel(X));
T2_range2= zeros(s_x,s_y,s_z,numel(X));
for x = 1:s_x
    for y = 1:s_y
        for z = 1:s_z
            for v = 1:s_v
            [fitresult,gof] = multi_exp_fit(TE,ROI(:,x,y,z,v));
            out = coeffvalues(fitresult);
            T2_gof(x,y,z,v) = gof.rsquare;
            T2_range1(x,y,z,v) = (-1/out(2));
            T2_range2(x,y,z,v) = (-1/out(4));
            end
        end
    end
end

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Answer 1

Jan 2014-4-22

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https://ww2.mathworks.cn/matlabcentral/answers/126672-nested-for-loop-fit-with-function#answer_134166

It is interesting, that you understand, that loops are not ideal for Matlab. I've heard of this rumor in the times of Matlab 6.1 also. But since the year 2002 and release 6.5 the introduction of the JIT acceleration improved the speed of loops substantially.

So my advice is to be happy with the loop, as long as it does not consume more than 25% of the total computing time. And even if it is the bottleneck, it is not worth to spend 1 hour in the vectorization, when you save 0.1 seconds of runtime.

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William 2014-4-22

Thanks for your rapid response Jan. The looping issue is probably because I am new to it. It currently takes over half an hour to run, the results are fine but would be nice to make it faster. Or is this expected?

William 2014-4-22

在 MATLAB Online 中打开

I've isolated the bottleneck to this, if it helps:

for x=1:s_x
    for y=1:s_y
        for z=1:s_z
            for v=1:s_v
            [fitresult, gof] = fit( xData, squeeze(yData(:,x,y,z,v)), ft, opts );
            out = coeffvalues(fitresult);
            T2_gof(x,y,z,v) = gof.rsquare;
            T2_range1(x,y,z,v) = (-1/out(2));
            T2_range2(x,y,z,v) = (-1/out(4));
            end
        end
    end
end