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`match.call` Argument Matching

Description

match.call returns a call in which all of the specified arguments are specified by their full names.

Usage

match.call(definition = sys.function(sys.parent()),
           call = sys.call(sys.parent()),
           expand.dots = TRUE,
           envir = parent.frame(2L))

Arguments

`definition`	a function, by default the function from which `match.call` is called. See details.
`call`	an unevaluated call to the function specified by `definition`, as generated by `call`.
`expand.dots`	logical. Should arguments matching `...` in the call be included or left as a `...` argument?
`envir`	an environment, from which the `...` in `call` are retrieved, if any.

Details

‘function’ on this help page means an interpreted function (also known as a ‘closure’): match.call does not support primitive functions (where argument matching is normally positional).

match.call is most commonly used in two circumstances:

To record the call for later re-use: for example most model-fitting functions record the call as element call of the list they return. Here the default expand.dots = TRUE is appropriate.
To pass most of the call to another function, often model.frame. Here the common idiom is that expand.dots = FALSE is used, and the ... element of the matched call is removed. An alternative is to explicitly select the arguments to be passed on, as is done in lm.

Calling match.call outside a function without specifying definition is an error.

Value

An object of class call.

References

Chambers, J. M. (1998) Programming with Data. A Guide to the S Language. Springer.

Examples

match.call(get, call("get", "abc", i = FALSE, p = 3))
## -> get(x = "abc", pos = 3, inherits = FALSE)
fun <- function(x, lower = 0, upper = 1) {
  structure((x - lower) / (upper - lower), CALL = match.call())
}
fun(4 * atan(1), u = pi)

match.call Argument Matching