What does the period mean when used with ~ (in a formula)?

From the FSelector manual:

data(iris)
subset <- cfs(Species~., iris)
f <- as.simple.formula(subset, "Species")
print(f)

Specifically, I mean the one in "Species~.".

Now, it's awfully tough to Google how a bit of punctua开发者_运维问答tion is used (for me anyway) and I couldn't anything. This code is unclear.

I think you're referring to the period contained in Species~., in which case this is just the standard R formulation of referring to 'all other variables' in the data frame, rather than typing them out one by one, as in Species ~ Variable1 + Variable2 etc.

From the help files of ?formula:

There are two special interpretations of . in a formula. The usual one is in the context of a data argument of model fitting functions and means ‘all columns not otherwise in the formula’: see terms.formula. In the context of update.formula, only, it means ‘what was previously in this part of the formula’.