Function Default Given as Vector

Question

In the help file for some functions, the defaults are sometimes given as vectors. An example is:

?base::rank

This opens a help file in which the usage is shown.

Usage:

 rank(x, na.last = TRUE,
      ties.method = c("average", "first", "last", "random", "max", "min"))

In this example the default for na.last is TRUE. But, the default for ties.method is given as a vector. What exactly does this mean in terms of which is chosen by default? And, more importantly, why is it written this way in the first place?

Thanks

Answer 1

The default is the first value. It is written that way so that you can see at a glance all of the possible options.

Internally, the function will use match.arg to evaluate user input and match it to the vector used. This matching is done using pmatch (p for partial matching), so that the argument can be abbreviated. For example, rank(x, "first") can be abbreviated to rank(x, "f"). See ?match.arg for more details. Quoting the ?match.arg Description:

match.arg matches arg against a table of candidate values as specified by choices, where NULL means to take the first one.

match.arg is commonly used when there are a small to medium number of possible options for an argument.