How can I delete specific columns from a matrix?

Question

Josh Halff 2017-7-3

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https://ww2.mathworks.cn/matlabcentral/answers/347170-how-can-i-delete-specific-columns-from-a-matrix

回答： Arezoo Samiei 2019-4-2

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
Columns 16 through 30
   0     0     0     0     0     0     9     0     0     0    10     0    11    11
   0     0     0     0     0     0     2     0     0     0     6     0     1     6
   0     0     0     0     0     0     3     0     0     0     8     0     7     9
Columns 31 through 36
   0    12    12     0     0
   0     1     2     0     0
   0     4     8     0     0

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Answer 1

Guillaume 2017-7-3

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编辑：Guillaume 2017-7-3

在 MATLAB Online 中打开

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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Guillaume 2017-7-3

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 2017-7-3

Thanks so much! It works perfectly!

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Answer 2

Andrei Bobrov 2017-7-3

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https://ww2.mathworks.cn/matlabcentral/answers/347170-how-can-i-delete-specific-columns-from-a-matrix#answer_272748

在 MATLAB Online 中打开

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));