# How can I insert many columns into a matrix at different locations efficiently?

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Darcy Cordell on 26 Sep 2022
Commented: Chunru on 28 Sep 2022
I have a matrix in which I need to delete several columns of zeros, perform some operation (i.e. matrix inverse), and then re-insert the zero columns back into the matrix at the same position. My matrix is very large, so I don't want to store a duplicate of the matrix in memory. For example:
%Create some "data"
A = rand(100); %some random data
A(:,randsample(100,15))=zeros(100,15); %some random columns of zeros
%Now that we have "data", let's find the zero columns:
col = find(~all(A));
%Delete the zero columns
A(:,col) = [];
%Now do some stuff to A ....
%Now I want to re-add the columns in????
One simple option to solve the problem is:
for i = 1:length(col)
A = [A(:,1:col(i)-1) zeros(100,1) A(:,col(i):end)];
end
But I feel like there must be a more elegant way to do it without the for loop that can essentially "reverse" the deletion. Any help is appreciated.
Darcy Cordell on 27 Sep 2022
I need to invert the non-zero columns. I've updated the answer accordingly.

Chunru on 27 Sep 2022
Edited: Chunru on 27 Sep 2022
%Create some "data"
A = rand(100); %some random data
col = randsample(100,15);
A(:, col)=zeros(100,15); %some random columns of zeros
%Now that we have "data", let's find the zero columns:
col = find(all(A==0))
col = 1×15
1 2 4 5 7 20 33 35 41 49 67 69 70 86 92
% No need to delete the zero columns
% Just re-assign anything back to A
%
% This is a faster approach since array size is kept the same.
%
% For example
A(:, col) = ones(size(A,1), length(col));
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Chunru on 28 Sep 2022
You can refenrence to the sub array. But we dont know what you exactly want to do.