For loops are too slow

Question

Joseph 2013-8-5

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/84047-for-loops-are-too-slow

I have some MATLAB code that was given to me by a coworker. This code contains a three layered for loop. So the code has the basic structure:

for
   for
     for

The code works but it takes an incredibly long time to execute as the three layered for loop is performing operations on a 40000x4 matrix. I need a way to either bypass the for loops or make them run faster. Someone mentioned to me that I could call Perl in MATLAB to do this but I am unsure how this works. Dose anyone have any ideas. Thank you.

3 个评论
显示 1更早的评论隐藏 1更早的评论

Joseph 2013-8-7

在 MATLAB Online 中打开

OK my loops look something like this:

for i = 1:1:N
    for j = 1:1:norigin
        jstart = (j-1)*N + i;
        for k = nmin:1:nmax
            kend = jstart + k*N;
            xmsd(k,1:3) = xmsd(k,1:3) + (md_msd(kend,1:3) - md_msd(jstart,1:3) ).^2;
        end
    end
end

This is part of a larger m file but the rest of the code works fine. Any thoughts? Thank you for your help.

Matt Kindig 2013-8-7

在 MATLAB Online 中打开

I think the pre-allocating xmsd would help streamline the code, while still retaining the for loops. Something like this:

xmsd = NaN(nmax, 3);
  for i = 1:1:N
      for j = 1:1:norigin
          jstart = (j-1)*N + i;
          for k = nmin:1:nmax
              kend = jstart + k*N;
              xmsd(k,1:3) = xmsd(k,1:3) + (md_msd(kend,1:3) - md_msd(jstart,1:3) ).^2;
          end
      end
  end

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

dpb 2013-8-7

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/84047-for-loops-are-too-slow#answer_93834

编辑：dpb 2013-8-8

在 MATLAB Online 中打开

for i = 1:1:N
  for j = 1:1:norigin
    jstart = (j-1)*N + i;
    for k = nmin:1:nmax
      kend = jstart + k*N;
      xmsd(k,1:3) = xmsd(k,1:3) + (md_msd(kend,1:3)-md_msd(jstart,1:3)).^2;
    end
  end
end

Start in the inner loop...first off

md_msd(jstart,1:3)

is invariant w/ k so replace the indexed expression w/ the equivalent

...
  for j = 1:1:norigin
    jstart = (j-1)*N + i;
    mdj=md_msd(jstart,1:3);
    for k = nmin:nmax
      kend = jstart + k*N;
      xmsd(k,1:3) = xmsd(k,1:3) + (md_msd(kend,1:3)-mdj).^2;
    end
  end

Then, kend takes on values of jstart+N, jstart+2N, jstart+3N, ... so define an index vector as

kdx=jstart+N:N:jstart+nmax*N;

and then the loop on k can be written as

xmsd(nmin:nmax,1:3) = xmsd(nmin:nmax,1:3) + (md_msd(kdx,1:3)-mdj).^2;

So, after the first loop reduction you're left with

for i = 1:1:N
  for j = 1:1:norigin
    jstart = (j-1)*N + i;
    kdx=jstart+nmin*N:N:jstart+nmax*N;
    mdj=md_msd(jstart,1:3);
    xmsd(nmin:nmax,1:3) = xmsd(nmin:nmax,1:3) + (md_msd(kdx,1:3)-mdj).^2;
  end
end