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I'm just reading the book "R in a Nutshell". And it seems as if I skipped the part where the "." as in "sample.formula" was explained.

> sample.formula <- as.formula(y~x1+x2)

Is sample an object with a field formula as in other languages? And if so, how can I find out, what other fields/functions this object has? (Type declaration)

EDIT: I just found another confusing use of the ".":

> svm(formula = is_spam~., data = spambase.training)

(the dot between ~., )

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The dot you see with the is_spam~. command means that there are no explanatory variables. Typically with model formulas, you will see y~x, but if you have no x variable, y~. says to guess at the value of y using no other variables. This is the same as the model $y=\beta_0$ –  Christopher Aden May 12 '11 at 20:06
11  
@Christopher On the contrary, the . in the formula tells R to use all variables in the dataframe spambase.training (except is_spam) as predictors. The model $y = \beta_{0}$ is fit with y ~ 1. –  caracal May 12 '11 at 20:25
    
I didn't check sources beforehand. Thank you for the correction! –  Christopher Aden May 12 '11 at 23:31
    
@caracal (+1) Wow I was just wondering how to do this. Thanks! –  Thomas Levine May 13 '11 at 4:37

3 Answers 3

up vote 21 down vote accepted

The dot can be used as in normal name. It has however additional special interpretation. Suppose we have an object with specific class:

 a <- list(b=1)
 class(a) <- "myclass"

Now declare myfunction as standard generic in the following way:

 myfunction <- function(x,...) UseMethod("myfunction")

Now declare the function

 myfunction.myclass <- function(x,...) x$b+1

Then the dot has special meaning. For all objects with class myclass calling

 myfunction(a)

will actualy call function myfunction.myclass:

 > myfunction(a)
  [1] 2

This is used widely in R, the most apropriate example is function summary. Each class has its own summary function, so when you fit some model for example (which usually returns object with specific class), you need to invoke summary and it will call appropriate summary function for that specific model.

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Look at the help page for ?formula with regard to . Here's the relevant bits:

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’.

Alternatively, the reshape and reshape2 packages use . and ... a bit differently (from ?cast):

There are a couple of special variables: "..." represents all other variables not used in the formula and "." represents no variable

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There are some exceptions (S3 method dispatch), but generally it is simply used as legibility aid, and as such has no special meaning.

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2  
I would say the opposite - it has special meaning (the S3 dispatch you mention), but some old naming conventions caused function names that are not S3 generics to have names that include a .. That pertains to names of functions. As for names of (non-function) objects, then yes, there is no special meaning. –  Gavin Simpson May 12 '11 at 15:54
    
At the beginning of a name, a . makes the object invisible to ls() though. –  caracal May 12 '11 at 20:48

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