Constructing a permutation matrix

[stevengj]

@cnaak

Approaching the problem from a slightly different angle, one can first transform a permutation vector p into a permutation matrix, for instance:

Using an explicit multiplication PA of dense matrices to permute the rows of A transforms the cost from O(n^2) to O(n^3) for an n \times n matrix.

But if you do want a permutation matrix, it’s much easier (and faster) to simply do

using LinearAlgebra
MP(p) = I(length(p))[p,:]