Expand 1-D array to 2-D matrix using 'ones' function and colon ':'

Example:

matrixA = 6:10;
matrixA = matrixA(ones(1,5), :);

or:

matrixB = (6:10)';
matrixB = matrixB(:, ones(1,5));

My Question:

What's going on here? I just don't get it. Is there any reference on this "exotic" usage? Many thanks for any inputs!

Answer 1

[Daniel Shub]

It is not that crazy. It is basically

matrixA = 6:10;
for ii = 1:5
  for jj = 1:5
    temp(ii, jj) = matrixA(1, jj);
  end
end
matrixA = temp;

but lump together into a vectorized version. You can get rid of the inner loop with:

matrixA = 6:10;
for ii = 1:5
  temp(ii, :) = matrixA(1, :);
end
matrixA = temp;

and you can finally get rid of all loops with

matrixA = 6:10;
temp = matrixA(ones(1,5), :);
matrixA = temp;

Answer 2

[Oleg Komarov]

matrixA = matrixA(ones(1,5), :)

select all elements from row one 5 times.

matrixB = matrixB(:, ones(1,5));

select all elements from column one 5 times.

Answer 3

[Edwin]

Thanks to Daniel and Oleg! I was mostly confused with the role of ones(1,5) here, but came to realize how it works when I read the example below from Matlab's help documents:

B = magic(4)
% Swap the two middle columns:
A = B(:,[1 3 2 4])