Expand a column vector to a matrix without using for loop

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bugatti79 2013-10-1

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回答： Jan 2013-10-1

Hi Folks,

Is there a built in command to expand a (n*1) to an (n*n) matrix without filling the matrix using a for loop? All entries are of the same scaler value. Thanks in advanced.

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Answer 1

ES 2013-10-1

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在 MATLAB Online 中打开

ResultMatrix=repmat(a,1,n)

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Answer 2

Jan 2013-10-1

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在 MATLAB Online 中打开

Or:

a = rand(10, 1);
A = a(:, ones(1, n));

Under some circumstances this is not created explicitly, e.g. in:

a = rand(10, 1);
b = rand(1, 10);
C = a(:, ones(1, 10)) + b(ones(1, 10), :);
D = bsxfun(@plus, a, b);

While bsxfun is smart enough not to create the temporarily expanded arrays, the calculation of C can profit from the same trick, when the JIT recognizes this. While this happened in all of my test cases until R2011a, since 2011b the JIT seems to handle this expanding less efficient.

Please note, that the JIT is not documented and my opinion about what happens inside is based only on the observed runtime and the used memory.