[R] effective matrix subset

[jgarcia at ija.csic.es]

Hi;
If we have a matrix A, and a vector X, where length(X)=nrow(A), and X
contains a wanted column for each row in A, in row ascending order. How
would be the most effective way to extract the desired vector V (with
length(V)=nrow(A))?

Wishes,
Javier