In the almost current development version (2009-05-22 r48594) and also in https://svn.r-project.org/R/trunk/src/library/base/man/factor.Rd ?factor contains (compare the formulations marked by ^^^^^^)
\section{Warning}{ The interpretation of a factor depends on both the codes and the \code{"levels"} attribute. Be careful only to compare factors with the same set of levels (in the same order). ^^^^^^^^^^^^^^^^^ \section{Comparison operators and group generic methods}{ ... Only \code{==} and \code{!=} can be used for factors: a factor can only be compared to another factor with an identical set of levels (not necessarily in the same ordering) or to a character vector. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ In the development version 2009-05-22 r48594, the latter formulation "not necessarily in the same ordering" is correct. f1 <- factor(c("a", "b", "c", "c", "b", "a", "a"), levels=c("a", "b", "c")) f2 <- factor(c("a", "b", "c", "c", "b", "a", "c"), levels=c("c", "b", "a")) f1 == f2 # [1] TRUE TRUE TRUE TRUE TRUE TRUE FALSE The first formulation "Be careful to compare ... levels in the same order" may be just a warning against a potential problem if the levels have different order, however this is not clear. Petr. ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel