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PORandomly permute rows/columns of a matrix with eigen
 

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  1. PORandomly permute rows/columns of a matrix with eigen
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    <p>I'm using Eigen and I've got a matrix:</p> <pre><code>MatrixXi x = MatrixXi::Random(5); </code></pre> <p>I'd like to randomly permute the rows and columns using a randomly drawn permutation (just one permutation for both the rows and columns), i.e. if I have a permutation that sends indices [0,1,2,3,4] -> [3,4,2,1,0] than I want to reorder the rows and the columns with the same permutation. </p> <p>Part 1: I can't find an example of PermutationMatrix online, and I'm having trouble figuring out the syntax.</p> <p>Part 2: how do I obtain a randomly permuted vector of indices to pass to it? Maybe std::random_shuffle?</p> <p><strong>update:</strong></p> <p>Here is a (probably inefficient) way to get a shuffled set of indices:</p> <pre><code>std::vector&lt;int&gt; perm; for (int i=0; i&lt;5; ++i) { perm.push_back(i); } std::random_shuffle(perm.begin(), perm.end()); </code></pre> <p>So now the question is how I do reorder my matrix x so that it's rows/columns are ordered by perm? </p> <p><strong>update 2:</strong></p> <p>Getting closer, this works (source for ideas: cplusplus.com):</p> <pre><code>int myrandom (int i) { return std::rand()%i;} PermutationMatrix&lt;Dynamic,Dynamic&gt; perm(5); perm.setIdentity(); for (int i=dim-1; i&gt;0; --i) { swap (perm.indices()[i],perm.indices()[myrandom(i+1)]); } cout &lt;&lt; "original x" &lt;&lt; x &lt;&lt; endl &lt;&lt; endl; cout &lt;&lt; "permuted x" &lt;&lt; perm * x * perm &lt;&lt; endl &lt;&lt; endl; </code></pre> <p>Anyone know how to do this with random_shuffle? (see attempt that didn't work below.)</p> <p>(Bonus: any thoughts on whether perm * x * perm will be efficient if perm is a 1e4 x 1e4 matrix?)</p>
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