Deleting rows in a matrix

Hi,

I have two matrices which are generated in a loop for n years:

A (m-rows x n-columns) B (k-rows)

Matrix B includes values which represent row numbers of matrix A that should be deleted. So, I want for every value in matrix B, the corresponding row in matrix A to be deleted.

For example,

A=[1 2; 2 2; 3 4], B= [2]

The result should be a matrix [1 2;3 4]. I.e. the middle row is deleted as B includes value 2.

For some reason I'm not able to find the correct coding for this. I tried several things, but inside the loop it doesn't work. This is what I tried, but this does't work:

    for j= 1:n
      B;
         for i= 1:length(B)
              indices = find(A(:,1)==B(i));
              A(indices,:) = [];
         end
     end

Can anyone help me with this?

[James Tursa]

Does B really contain the row numbers, or does it contain values that must match values in the first column of A? Your words say the former but the code looks like the latter.

[Bas]

Sorry, B includes 0s and values that should match values in the first column of A..

Answer 1

[Brendan Hamm]

If you want to delete the second row because B has the value 2 you can use this vector to index into A as such:

A(B,:) = [];

That is how I interpret the question you pose, but the code seems to approach a different problem, outlined below.

If you want to delete the second row because the first element of A is 2, then you would find if each element of the first column of A is in the vector B and delete those locations:

locA = ismember(A(:,1),B); % Logical vector (true if A(i,1) is in B, false otherwise)
% use the logical vector to index into A and delete
A(locA,:) = [];

[Bas]

Thanks for the reply. It's not working yet. As you can see below, matrix UitDienst contains zero values and some non zero values. These non zero values correspond to crew IDs which are defined in a(:,1). I want the rows in matrix a(i,:) corresponding to the non zero values from matrix UitDienst for each forecasting year j to be deleted. So, I want to have matrix a to be saved in excel after each forecasting year in the loop.

Unfortunately, script below is not working. Can you help?

[Brendan Hamm]

I am a bit confused with your question here still, what is not working specifically? Does this work fine on the first iteration of the loop? Is it that you are overwriting the excel file at every iteration of the loop? Are you receiving any errors?

Taking a stab at it:

If UitDienst is a matrix and not a vector as in the example you gave, then you likely do not want to perform a linear indexing as you do with

UitDienst(j)

Matlab is a column major language which means that elements of a matrix are stored column by column:

A = [1 2 3; 4 5 6]

is stored in memory as [1 4 2 5 3 6] (column-by-column) This means that A(2) will return the value 4, A(3) will return the value 2, etc. It seems that maybe you need to change that to read:

UitDienst(:,j)

Answer 2

[James Tursa]

"Matrix B includes values which represent row numbers of matrix A that should be deleted"

Assuming this is exactly what you want, then this will do the job:

A(B,:) = [];

Answer 3

[Jon]

Your code produces the result you expect. You'll need to better specify your problem. You can lose the loop completely, though, with a simple indexing:

A(B,:) = [];

This is assuming that B is unique and contains no elements larger than the length of A.