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

2011 9 5
3 个回答
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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!

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Daniel Shub
Daniel Shub 2011-9-5

0 个投票

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;

更多回答（2 个）

Oleg Komarov
Oleg Komarov 2011-9-5

0 个投票

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.
Edwin
Edwin 2011-9-6

0 个投票

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])

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