MATLAB: What is the reverse function of dmperm()

Question

I'm trying to solve Ax = b using Jacobi iterations.

The function dmperm() is used to permute the rows of A so that the diagonal has no zeros. b is also permuted by the same order of row permutation of A. Then I get x with rows permuted. How can I "unpermute" x to get the original order?

Thanks a lot!

Answer 1

As long as you use the single-output version of 'dmperm'

 p = dmperm(A);

and only permute the rows of matrix A, getting A(p,:), then the elements of x are not permuted.

A permutation of both the rows and columns of A, A(p,q), will permute the elements of x by q. To get x back to the original order is accomplished by

 x(q) = x;

which amounts to the inverse permutation of q.