Lists - Generic and Dotted Pairs

Description

Functions to construct, coerce and check for all kinds of R lists.

Usage

list(...)
pairlist(...)

as.list(x)
as.list.default(x)
as.pairlist(x)

is.list(x)
is.pairlist(x)

alist(...)

Details

Since version 0.63, most lists in R internally are Generic Vectors, whereas traditional dotted pair lists (as in LISP) are still available.

The arguments to list or pairlist are of the form value or tag=value. The functions return a list composed of its arguments with each value either tagged or untagged, depending on how the argument was specified.

alist is like list, except in the handling of tagged arguments with no value. These are handled as if they described function arguments with no default (cf. formals), whereas list simply ignores them.

as.list attempts to coerce its argument to list type. For functions, this returns the concatenation of the list of formals arguments and the function body. For expressions, the list of constituent calls is returned.

is.list returns TRUE iff its argument is a list or a pairlist of length> 0, whereas is.pairlist only returns TRUE in the latter case.

An empty pairlist, pairlist() is the same as NULL. This is different from list().

See Also

vector(., mode="list"), c, for concatenation; formals.

Examples

data(cars)
# create a plotting structure
pts <- list(x=cars[,1], y=cars[,2])
plot(pts)

# Argument lists
f <- function()x
# Note the specification of a "..." argument:
formals(f) <- al <- alist(x=, y=2, ...=)
f
str(al)

str(pl <- as.pairlist(ps.options()))

## These are all TRUE:
is.list(pl) && is.pairlist(pl)
!is.null(list())
is.null(pairlist())
!is.list(NULL)
is.pairlist(pairlist())
is.null(as.pairlist(list()))
is.null(as.pairlist(NULL))