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

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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)];
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.
James Tursa
James Tursa on 27 Sep 2022
Can you give us details about what operations you are performing on the non-zero columns of A?

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Answers (1)

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));
Darcy Cordell
Darcy Cordell on 27 Sep 2022
The problem is that I need to perform some operations on the non-zero columns of A (i.e. matrix inverse). So I can't replace the columns with ones, I need to delete the columns prior to taking the inverse.
Chunru on 28 Sep 2022
You can refenrence to the sub array. But we dont know what you exactly want to do.

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