MATLAB: How to delete specific columns from a matrix

matrix manipulation

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
and it doesn't work. Please help. Here is the matrix I'm currently working with:
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

Best Answer

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) = [];
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