How can I delete certain rows of a matrix based on specific column values?

I have a matrix that has 6 columns and thousands of rows.

I need to delete the rows based on the following conditions:

1. if column 1 is zero then delete row

2. if column 2,3,4,and 5 is zero, and column 6 is not zero, then delete row

3. if column 2,3,and 4 is zero, and column 5 is not zero, then delete row

4. if column 2,and 3 is zero, and column 4 is not zero, then delete row

5. if column 2 is zero, and column 3 is not zero, then delete row

Someone please help, I feel like I've tried everything!

Answer 1

[Jos (10584)]

This is a job for logical indexing! Note that you do not need to loop over all the lines at all

Assume A is your matrix. Here is a short example (not tested):

A = [0 2 3 4 5 6 ; 11 0 0 0 15 16 ; 21 0 0 0 0 26 ;  31 0 33 34 35 36 ; 41 42 43 0 0 0]

% Specify you conditions
TF1 = A(:,1)==0 
TF2 = all(A(:,2:5)==0,2) & A(:,6) ~= 0 
TF6 = A(:,2) == 0 & A(:,3) A ~= 0 

% combine them
TFall = TF1 & TF2 & TF6

% remove
A(TFall,:) = []

[Leah]

Yes! This worked for me. Thank you very much.

[Pallavi]

I think the OR operator has to be used for TFall instead of AND

TFall = TF1 | TF2 | TF6

will work.

[edfleth]

Yes, use

TFall = TF1 | TF2 | TF6

Also, the TF6 equation has an extra A before the '~='. Should read:

TF6 = A(:,2) == 0 & A(:,3) ~= 0

[Rebecca Park]

After

A(TFall,:) = []

How would you save that as a new matrix under a different name? So that you could then calculate further output from it?

[Jos (10584)]

You can keep a copy of A

B = A ;
B(TfFall,:) = [] ;

and work with B in the remainder of the code.

[William Chamberlain]

Note that Jan's answer on this question gives a few different methods and the time efficiency of each (on Win7_64: your mileage may vary).

[Nathaniel H Werner]

Similar question.

What if you have a N by 3 array "A" and you need to remove M rows, where the length of M can vary? Can I make an M by 1 array of logicals (M by 1 because only need to worry about the row index at this point) and remove them from "A" in a similar fashion as was done above?

[Xhino MELEQI]

What a genius ! thanks

Answer 2

[lis coffey]

To "delete"

 matrix = eye(5);%5x5 identity matrix
 list_o_cols_to_delete = [1 3 5];
 matrix(:,list_o_cols_to_delete) = []

To only use the bits you want:

 matrix = eye(5);%5x5 identity matrix
 list_o_cols_to_use = [1 3 5];
 used = matrix(:,list_o_cols_to_use)

-----------------------------------------------------------

Thanks

[Niha Afzal]

why did you use 5*5 identity matrix?

Answer 3

[Andrei Bobrov]

Let S - your array.

S1 = S(S(:,1) ~= 0,:);
[ii,jj] = find( S1(:,2:end));
ss = sortrows([ii,jj],1);
idx = accumarray(ss(:,1),ss(:,2),[],@(x)x(1));
out = S1(idx < 2,:);

OR

out = S( all(S(:,1:2) ~= 0,2),:)

[Jim]

Most usefully. Thanks.

Answer 4

[Walter Roberson]

    rownum = 5;   %for example
    if columns(rownum, 1) == 0;
      columns(rownum,:) = [];
    end

[Teja Swaroop Naik Mudiki]

A = [0 2 3 4 5 6 ; 11 0 0 0 15 16 ; 21 0 0 0 0 26 ;  31 0 33 34 35 36 ; 41 42 43 0 0 0]
% Specify you conditions
TF1 = A(:,1)==0 
TF2 = all(A(:,2:5)==0,2) & A(:,6) ~= 0 
TF6 = A(:,2) == 0 & A(:,3) ~= 0
% combine them
TFall = TF1 | TF2 | TF6
% remove
A(TFall,:) = [] %#ok<*NOPTS>

Answer 5

[Simon]

Hi!

Suppose you have matrix M:

% logical vector for rows to delete
deleterow = false(size(M, 1), 1);

% loop over all lines
for n = 1:size(M, 1)
  if M(n, 1) == 0
    % mark line for deletion afterwards
    deleterow(n) = true;
  elseif (M(n, [2 3 4]) == [0 0 0]) & (M(n, 5) ~= 0)
    deleterow(n) = true;
  elseif % write every condition in a separate elseif
    deleterow(n) = true;
  end
end

% delete rows
M(deleterows, :) = [];

Be aware that the comparisn to 0 is valid for integers! If you have doubles you should instead write something like

tolerance = 1e-8;
if abs(M(n, 1) < tolerance
  deleterow(n) = true;
end