How do I remove rows if the sum of its cells equals to a specific value?

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Leonardo Rodrigues 2017-9-30

1
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评论： Jan 2017-9-30

I have a big matrix with lots of useless rows ( when the sum of the row = -104000 ). How do I remove these rows based on this condition?

I tried this:

 v=1:1:NUMBER_OF_ROWS;
 if sum(A(v,:)) == -104000;
    A(v,:) = [];
 end

but it didn't work.

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Jan 2017-9-30

"Didn't work" is a weak description of the problem. Prefer to explain, what you observe.

Leonardo Rodrigues 2017-9-30

It doesn't remove any row, it just shows the value of each cell in the matrix.

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Answer 1

Jan 2017-9-30

3
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/359133-how-do-i-remove-rows-if-the-sum-of-its-cells-equals-to-a-specific-value#answer_283792

编辑：Jan 2017-9-30

在 MATLAB Online 中打开

This works fluently without a loop:

toDelete       = (sum(A, 2) == -104000);
A(toDelete, :) = [];

Your loop has the problem, that if a v.th row has been removed, processing the (v+1).th row will leave out the new v.th row. In addition you cannot run the loop until NUMBER_OF_ROWS, if you have removed rows already, because then A(NUMBER_OF_ROWS, :) does not exist anymore.

A (bad) solution would be to run the loop in backward direction:

for v = NUMBER_OF_ROWS:-1:1
  if sum(A(v,:)) == -104000
     A(v,:) = [];
  end
end

But then Matlab has to create a new array every time a row is removed and this requires a huge amount of resources. This is equivalent to letting an array grow iteratively. Search for "pre-allocation" in this forum.