Subtracting within a Matrix

Question

Dan Lynn 2015-10-13

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https://ww2.mathworks.cn/matlabcentral/answers/248254-subtracting-within-a-matrix

编辑： Star Strider 2015-10-13

I have this matrix:

In the second column, I need to find the highest and lowest number that are associated with the same number in the first column, and then subtract the two. In row 1, the numbers 1 and 0 exist. In row 3, the numbers 1 and 3 exists. I need to subtract the 3 and the 0 and get 3 as my output.

The output should be a matrix that looks like this:

3
1
5

Also if a number in the first column only occurs once

Then the number in the second column associated with it in the output matrix should be a 0.

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Answer 1

Joseph Cheng 2015-10-13

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you can use the built in function unique to determine this:

   A =[ 1 0; 1 2; 1 3; 2 3; 2 4; 3 1; 3 6; 4 5]
  [c1,ia1,ic1] = unique(A(:,1));
  for ind = 1:length(c1)
      tempMIN = min(A(ind==A(:,1),2));
      tempMAX = max(A(ind==A(:,1),2));
      tostore = tempMAX-tempMIN    
  end

also you maybe able to do this simpler with accumarray().

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Answer 2

Star Strider 2015-10-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/248254-subtracting-within-a-matrix#answer_195753

编辑：Star Strider 2015-10-13

在 MATLAB Online 中打开

The accumarray approach:

M = [ 1 0
     1 2
     1 3
     2 3
     2 4
     3 1
     3 6
     4 5];
Mu = unique(M(:,1));
R = accumarray(Mu, Mu, [], @(x)[max(M(M(:,1)==Mu(x),2))-min(M(M(:,1)==Mu(x),2))]);
Result = [Mu R]
Result =
       1     3
       2     1
       3     5
       4     0