Hi all, Suppose I have a formula: a = log(y) ~ x1 + I(x2^2) How can I extract the original variable names 'y', 'x1', 'x2' from this formula? Thanks a lot! Regards, Yihui -- Yihui Xie <xieyihui at gmail.com> Phone: 515-294-6609 Web: http://yihui.name Department of Statistics, Iowa State University 3211 Snedecor Hall, Ames, IA