list {base} | R Documentation

## Lists – Generic and Dotted Pairs

### Description

Functions to construct, coerce and check for all 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

`...` | objects.
`x` | object to be coerced or tested.
`all.names` | a logical indicating whether to copy all values or (default) only those whose names do not begin with a dot.

### 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 `\list{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 list type. For functions, this returns the concatenation of the list of formal 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. `is.pairlist` returns `TRUE` iff the argument is a pairlist or `NULL` (see below).

`is.list` and `is.pairlist` are generic: you can write methods to handle specific classes of objects, see InternalMethods.

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 environmnent 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`.

### 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)

[Package base version 2.3.1 Index]