[R] poly objects as data frame columns

[Ulrike Grömping]

Dear UseRs,

I just learnt that the number of columns of a data frame is not always what
I thought it to be, and I wonder where I should have learnt about this.
Consider the following example:

dat <- data.frame(X1=1:10, X2=LETTERS[1:10])
ncol(dat)          ## evaluates to 2 (of course)
dat$X1poly <- poly(dat$X1,3)
dat                  ## five columns displayed
ncol(dat)          ## evaluates to 3
colnames(dat)   ## three names (third is X1poly)  
colnames(dat)[3] <- "newname"
dat                 ## all three previous X1poly columns renamed

This appears intentional, as it treats the column names reasonably. Where is
it documented ? Are there any other scenarios for which the number of
columns displayed when printing a data frame does not coincide with ncol ? 

Regards, Ulrike
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