Lists – Generic and Dotted Pairs

Description

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

Usage

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

as.list(x, ...)
## S3 method for class 'environment':
as.list(x, all.names = FALSE, ...)
as.pairlist(x)

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

alist(...)

Arguments

Details

Most lists in R internally are Generic Vectors, whereas traditional dotted pair lists (as in LISP) are available but rarely seen by users (except as formals of functions).

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

alist handles its arguments as if they described function arguments. So the values are not evaluated, and tagged arguments with no value are allowed whereas list simply ignores them. alist is most often used in conjunction with formals.

as.list attempts to coerce its argument to a list. For functions, this returns the concatenation of the list of formal arguments and the function body. For expressions, the list of constituent elements is returned. as.list is generic, and as the default method calls as.vector(mode="list") methods for as.vector may be invoked. As from R 2.4.0, as.list turns a factor into a list of one-element factors. All attributes will be dropped unless the argument already is a list. (This is inconsistent with functions such as as.character, and is for efficiency since lists can be expensive to copy.)

is.list returns TRUE if and only if its argument is a list or a pairlist of length > 0. is.pairlist returns TRUE if and only if the argument is a pairlist or NULL (see below).

The "environment" method for as.list copies the name-value pairs (for names not beginning with a dot) from an environment to a named list. The user can request that all named objects are copied. The list is in no particular order (the order depends on the order of creation of objects and whether the environment is hashed). No parent environments are searched. (Objects copied are duplicated so this can be an expensive operation.)

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

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

vector("list", length) for creation of a list with empty components; c, for concatenation; formals. unlist is an approximate inverse to as.list().

Examples

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

is.pairlist(.Options)  # a user-level pairlist

## "pre-allocate" an empty list of length 5
vector("list", 5)

# Argument lists
f <- function() x
# Note the specification of a "..." argument:
formals(f) <- al <- alist(x=, y=2+3, ...=)
f
al

## environment->list coercion

e1 <- new.env()
e1$a <- 10
e1$b <- 20
as.list(e1)