Is there an efficient (i.e. fast) way to permute the columns of an Eigen::MatrixXd?

I currently have a loop that fills in the columns of a new Matrix by copying from the old one with:
newmatrix.col(newindex) = oldmatrix.col(oldindex);

This seems to take a bit of time (more than I would expect), so I was wondering whether there is a better method. This is a 3 row x N column matrix where N would be in the range of 20 to 80 typically.

Thanks,
Noel

If your permutation is just a transposition (aka 'swap two columns') then you should use swap(), it is faster.

If you have a more general kind of permutation, there's no very easy answer as making your permutation efficient means have better memory locality, which is very hard with a general permutation.

If you're using Eigen 3, the best you could do, though, is use Eigen's high-level API for permutations, so if we ever make that faster, you'll benefit automatically. This requires your permutation indices to be stored in a PermutationMatrix object, and then you can simply use operator* to apply it. But for now, it won't be faster than your trivial code.

Thanks Benoit.