Remove for loop and reduce time

Question

Nagesh A P 2018-6-29

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评论： Nagesh A P 2018-6-29

I want to reduce the running time of my code. I found that most of the time in my code is spent in this loop, as this loop is invoked multiple times in my code. Is there anyway to reduce time?? Code explanation: ac1 is an acceleration matrix having values ranging from -10 to 10. I am using discretize function to group different acceleration values into different bins, represented by edges_a. Now, I have to find how many values are there in each bin and store those values in the fourth column of acc_dis matrix.

Here is the code:

if true
  edges_a=min(ac1):1.25:max(ac1);
  grp_ac=discretize(ac1,edges_a);
    for i=1:length(edges_a)
    acc_dis(i,4)=sum(grp_ac==i);
    end
  end

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Nagesh A P 2018-6-29

grp_ac is an 18000x1 array. It's not fixed, it will change in each iteration. But always between 17500 to 18500. But that won't matter to you. U can use even a 100x1 array as grp_ac for example I guess.

Jan 2018-6-29

@Nagesh A P: Please post a relevant set for the input data.

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Answer 1

Guillaume 2018-6-29

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在 MATLAB Online 中打开

Points for using discretize instead of writing your own loop for that. The thing is, what you really want is the histogram and there's also a function for that: histcounts:

edges_a = min(ac1):1.25:max(ac1);
h = histcounts(ac1, edges_a);
acc_dis(1:numel(h), 4) = h;

Note: I don't know how many rows acc_dis has. Hopefully at least enough. If it already has the correct number of rows, that last line can be simplified to:

acc_dis(:, 4) = h;

Note 2: If you still need grp_ac, then:

[h, ~, grp_ac] = histcounts(ac1, edges_a);

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Nagesh A P 2018-6-29

That was what I was looking for. Thanks.

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Answer 2

Nithin Banka 2018-6-29

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/408009-remove-for-loop-and-reduce-time#answer_326801

编辑：Nithin Banka 2018-6-29

在 MATLAB Online 中打开

if true
    edges_a=min(ac1):1.25:max(ac1);
    grp_ac=discretize(ac1,edges_a);
    len = length(edges_a);
    auxArr = zeros(len, 1));
    len1 = length(grp_ac);
    for i=1:len1
        if(grp_ac(i)<=len&grp_ac(i)>=1)
            auxArr(grp_ac(i)) = auxArr(grp_ac(i)) + 1;
        end
    end
    acc_dis(1:len, 4) = auxArr;
end