# How can I delete specific columns from a matrix?

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Josh Halff on 3 Jul 2017
Answered: Arezoo Samiei on 2 Apr 2019
I have a 3xn matrix of data, and anywhere that data is repeated I've converted it to a column of zeros. Now, I want to clear the zero columns and leave only the relevant data. How can I do this? I've tried:
for i = length(matrix):1
if matrix(1,i) == 0
matrix(:,i) = [];
end
end
Columns 1 through 15
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Columns 16 through 30
0 0 0 0 0 0 0 9 0 0 0 10 0 11 11
0 0 0 0 0 0 0 2 0 0 0 6 0 1 6
0 0 0 0 0 0 0 3 0 0 0 8 0 7 9
Columns 31 through 36
0 0 12 12 0 0
0 0 1 2 0 0
0 0 4 8 0 0

Guillaume on 3 Jul 2017
Edited: Guillaume on 3 Jul 2017
Of course, it does not work:
for i = length(matrix):1
I assume you're trying to run the loop in reverse so that you don't shift rows yet to process as you delete rows. That's not how you tell matlab to run a loop in reverse. You have to specify the increment/decrement. Also, never use length on a matrix. If your n is less than 3, it'll return the number of columns instead and leads to interesting bugs. Always use size with an explicit dimension. So:
for i = size(matrix, 2):-1:1
would work. However, a loop is just a waste of time when it can all be accomplished in just one line with:
matrix(:, matrix(1, :) == 0) = [];
##### 3 CommentsShow 1 older commentHide 1 older comment
Guillaume on 3 Jul 2017
Sorry, an extra closing bracket got in there by mistake. Fixed now (and also edited to delete columns instead of rows as I misread your post.
Josh Halff on 3 Jul 2017
Thanks so much! It works perfectly!

Andrei Bobrov on 3 Jul 2017
your_mtx = [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 9 0 0 0 10 0 11 11 0 0 12 12 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 0 0 6 0 1 6 0 0 1 2 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 0 0 0 8 0 7 9 0 0 4 8 0 0];
out = your_mtx(:,any(your_mtx));

Arezoo Samiei on 2 Apr 2019
Use the following command is the other way
C = setdiff(A,B)