Repeat matrix columns in pairs.

Hello,

I'm trying to use the repelem function to repeat some columns of a matrix A like:

A = [1 2 3 4; 5 6 7 8];

If I use the repelem function I'll get the following output:

out = repelem(A,1,2);
out =
 [1 1 2 2 3 3 4 4 ; 5 5 6 6 7 7 8 8]

However I'm trying to repeat two columns at the same time instead of doing it individually. So that I get an output like:

 Output = [1 2 1 2 3 4 3 4; 5 6 5 6 7 8 7 8]

Anyone can come around a solution to this problem?

Thank you,

Santos

Answer 1

[David Hill]

out=[repmat(A(:,1:2),1,2),repmat(A(:,3:4),1,2)];

[Santos García Rosado]

Thank's for your quick answer @David Hill. But what if I have a very long matrix? Isn't there a more automated way of doing it? Thank you again. Santos.

[David Hill]

out=[];
for k=2:2:size(A,2)
  out=[out,repmat(A(:,k-1:k),1,2)];
end

[Santos García Rosado]

So it seems like a loop is inevitable then. I'll try to find a way around that loop. Thank you David!

[David Hill]

You could reshape, not sure it will be any faster.

out=reshape(repelem(reshape(A',2,[]),1,2),2*size(A,2),[])';

[Santos García Rosado]

I'll take a good look at it. Thank you David.

Answer 2

[Wolfgang]

Here is a more generalized solution which duplicates pairs of columns and doesn't depend on the number of columns, however the number of columns in A has to be even.

B = reshape(kron(ones(2,1), reshape(A, 2*size(A,1), [])), size(A,1), []);

This also allows to triplicate pairs of columns by using:

B = reshape(kron(ones(3,1), reshape(A, 2*size(A,1), [])), size(A,1), []);

Or you can triplicate triples of columns (number of columns in A must be a multiple of 3):

B = reshape(kron(ones(3,1), reshape(A, 3*size(A,1), [])), size(A,1), []);

Also possible: duplicate triples of columns:

B = reshape(kron(ones(2,1), reshape(A, 3*size(A,1), [])), size(A,1), []);

And so on ...