How to instead the "for" loop?

Question

Peony Lai 2012-4-24

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/36348-how-to-instead-the-for-loop

Hello, I had a program which use for loop and if, but it take a long time, I want it faster.

How Can I instead of "for" and "if" so that it can make the program become faster?

Here is the program, Thanks

tic
clear all
close all
mov = mmreader('02.avi');
for i = 1: 7111   % number of the frame of the video
      frame1 = read(mov, i);
      if i == 750
         for a = 300 :2: 303;
              for b = 353 :2: 381;
                  for c = 301 :2: 303;
                      for d = 354 :2: 381;
                  frame1(a,b,:) = [0,0,0];
                  frame1(c,d,:) = [0,0,0];
                      end
                  end
              end
          end 
          for a = 300 :2: 303;
              for b = 354 :2: 381;
                  for c = 301 :2: 303;
                      for d = 353 :2: 381;
                  frame1(a,b,:) = [153,153,153];
                  frame1(c,d,:) = [153,153,153];
                      end
                  end
              end
          end            
      end    
      if i == 760
          for a = 300 :2: 303;
              for b = 356 :2: 385;
                  for c = 301 :2: 303;
                      for d = 357 :2: 385;
                      frame1(a,b,:) = [153,153,153];
                      frame1(c,d,:) = [153,153,153];
                      end
                  end
              end
          end 
          for a = 300 :2: 303;
              for b = 357 :2: 385;
                  for c = 301 :2: 303;
                      for d = 356 :2: 385;
                  frame1(a,b,:) = [0,0,0];
                  frame1(c,d,:) = [0,0,0];
                      end
                  end
              end
          end  
      end
          for x = 1 : 478;            
              for y = 1 : 640;
                  if frame1(x,y,1) == frame1(x,y,2) && frame1(x,y,2) == frame1(x,y,3) && frame1(x+1,y,1) == frame1(x+1,y,2) && frame1(x+1,y,2) == frame1(x+1,y,3) && frame1(x+2,y,1) == frame1(x+2,y,2) && frame1(x+2,y,2) == frame1(x+2,y,3)
                      figure; imagesc(frame1);
                      title(['Frame ', num2str(i)])
                      xlabel('x-axis');
                      ylabel('y-axis');
                      coordinate = [x y]
                      frame1 = read(mov, i+1);
                  end
              end
          end
end
toc

Thanks

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Answer 1

Richard Brown 2012-4-24

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https://ww2.mathworks.cn/matlabcentral/answers/36348-how-to-instead-the-for-loop#answer_45489

在 MATLAB Online 中打开

Un-nest your loops - your a,b and c,d loops are independent of each other. What you're doing is running your inner two loops gazillions of times more than you need to. I.e. replace your four-level loops with these:

for a = ...
  for b = ...
    frame1(a, b, :) =...
  end
end
for c = ...
  for d = ...
    frame1(c, d, :) =...
  end
end

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Peony Lai 2012-4-24

Thankyou.

I had try this but the time is also the same. Didn't decrease, any other method? Thanks

Peony Lai 2012-4-24

Dear Walter Roberson,

Thankyou, I try this but have a error,

"??? Subscripted assignment dimension mismatch."

so that the program cannot run

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How to instead the "for" loop?

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How to instead the "for" loop?

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