missing_value: Return an empty symbol.

In crowding/vadr: Macros, metaprogramming, and pleasant programming idioms.

Description

The empty symbol (that is, the symbol whose string representation is "" is used to represent missing values in the R language; for instance in the value of formal function arguments when there is no default; in the expression slot of a promise when a missing argument is given; and bound to the value of a variable when it is called with a missing value. When computing on the language, then, you may need to explicitly invoke the "missing" value.

Usage

missing_value(n)

Arguments

n	Optional; a number. If provided, will return a list of missing values with this many elements.

Value

A symbol with empty name, or a list of such.

See Also

list_missing dots_missing

Examples

# These statements are equivalent:
quote(function(x, y=1) x+y)
call("function", pairlist(x=missing_value(), y=1), quote(x+y))

# These statements are also equivalent:
quote(df[,1])
substitute(df[row,col], list(row = missing_value(), col = 1))

# These statements are also equivalent:
quote(function(a, b, c, d, e) print("hello"))
call("function", as.pairlist(put(missing_value(5), names, letters[1:5])),
                 quote(print("hello")))

crowding/vadr documentation built on May 14, 2019, 11:33 a.m.