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

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.

[Jan]

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

[Leonardo Rodrigues]

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

Answer 1

[Jan]

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.

[Leonardo Rodrigues]

What does the Av stands for?

[Image Analyst]

Av is the name of the array - what you called A.

Since "remove" is a built-in method (function), you might want to call it "rowsToDelete" instead of "remove".

[Jan]

"Av" was a typo, use "A" instead.

@Image Analyst: Thanks for pointing this out. I did not use containers.Map yet, so I was not aware that "remove" is a Matlab command now.

Answer 2

[Marco]

Hello Leonardo, you can use the "removerows" function: https://it.mathworks.com/help/nnet/ref/removerows.html

You can see also this link: https://it.mathworks.com/matlabcentral/answers/41762-delete-row-from-matrix

BR Marco

[Jan]

Note that you need the Neural Network Toolbox for removerows.