while loop, index out of bounds

Question

nomad nomad 2011-5-7

1
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https://ww2.mathworks.cn/matlabcentral/answers/6977-while-loop-index-out-of-bounds

I want to sum the matrix elements classified if the sum> 0, remove the max, and the sum <0 delete min, until the sum be equal to zero.

I tried this loop:

matrice=sort(Deltaf,'descend') ;
while sum(matrice)~=0
  if    sum(matrice)>0
      matrice = matrice(2:end, : );
    else  sum(matrice)<0
        matrice = matrice( :,end-1 );
    end
  end

With this error:

??? Attempted to access matrice(:,0); index must be a positive integer or logical.

Error in ==> test at 105 matrice = matrice( :,end-1 );

[EDITED, Jan, code formatted]

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Answer 1

Daniel Shub 2011-5-7

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You have removed all the elements from your matrix. There is no gaurentee that an arbitraty matrix will sum to 0. Think about [1, 2, 3] or [-3, -1, 2, 4] or [-1, 1+eps].

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Answer 2

Teja Muppirala 2011-5-7

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在 MATLAB Online 中打开

If Deltaf is really a vector, not a 2D matrix, then what you are doing will work fine once you take out the unnecessary colons.

As Daniel pointed out, depending on your data, you might end up with an empty matrix. But at least you shouldn't get an error.

matrice=sort(Deltaf,'descend') ;
while sum(matrice)~=0 
    if sum(matrice)>0 
        matrice = matrice(2:end);
      else 
          matrice = matrice(1:end-1);
      end;
  end;
matrice

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nomad nomad 2011-5-7

Hello,

To make a loop that stops when the sum of positive and negative elements is zero

But I find myself with an empty matrix....

Where is the problem

Matt Fig 2011-5-7

The problem has already been shown to you. Stop for a minute and think about what you are doing. There is no guarantee that an arbitrary matrix will have a zero sum by successively removing the largest and smallest element. Instead of asking why your method isn't working over and over, think for a minute. Do an example:

A = [1 2 3 4]

Now use your method on A. You will NEVER get to zero by summing any subset of the elements of A!

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