23.6 Selectors

Function: first pair

Function: second pair

Function: third pair

Function: fourth pair

Function: fifth pair

Function: sixth pair

Function: seventh pair

Function: eighth pair

Function: ninth pair

Function: tenth pair

Syntax: fifth/stx pair

Syntax: sixth/stx pair

Syntax: seventh/stx pair

Syntax: eighth/stx pair

Syntax: ninth/stx pair

Syntax: tenth/stx pair

Synonyms for car, cadr, caddr, ...

(third '(a b c d e))
⇒ c

Function: car+cdr pair

The fundamental pair deconstructor:

(lambda (p)
  (values (car p) (cdr p)))

Function: take-left dotted i

Function: drop-left dotted i

Syntax: take-left/stx dotted i

Syntax: drop-left/stx dotted i

take-left returns the first i elements of list dotted. drop-left returns all but the first i elements of list dotted.

(take-left '(a b c d e) 2)
⇒ (a b)

(drop-left '(a b c d e) 2)
⇒ (c d e)

dotted may be any value: a proper, circular, or dotted list:

(take-left '(1 2 3 . d) 2)
⇒ (1 2)

(drop-left '(1 2 3 . d) 2)
⇒ (3 . d)

(take-left '(1 2 3 . d) 3)
⇒ (1 2 3)

(drop-left '(1 2 3 . d) 3)
⇒ d

For a legal i, take-left and drop-left partition the list in a manner which can be inverted with append:

(append (take-left x i) (drop-left x i)) = x

drop-left is exactly equivalent to performing i cdr operations on dotted; the returned value shares a common tail with dotted.

If the argument is a list of non–zero length, take-left is guaranteed to return a freshly–allocated list, even in the case where the entire list is taken: (take-left dotted (length+ dotted)).

Function: take-right dotted i

Function: drop-right dotted i

Syntax: take-right/stx dotted i

Syntax: drop-right/stx dotted i

take-right returns the last i elements of dotted. drop-right returns all but the last i elements of dotted.

(take-right '(a b c d e) 2)
⇒ (d e)

(drop-right '(a b c d e) 2)
⇒ (a b c)

The returned list may share a common tail with the argument list.

dotted may be any finite list, either proper or dotted:

(take-right '(1 2 3 . d) 2)
⇒ (2 3 . d)

(drop-right '(1 2 3 . d) 2)
⇒ (1)

(take-right '(1 2 3 . d) 0)
⇒ d

(drop-right '(1 2 3 . d) 0)
⇒ (1 2 3)

For a legal i, take-right and drop-right partition the list in a manner which can be inverted with append:

(append (drop-right flist i) (take-right flist i)) = flist

The return value of take-right is guaranteed to share a common tail with dotted.

If the argument is a list of non–zero length, drop-right is guaranteed to return a freshly–allocated list, even in the case where nothing is dropped, e.g. (drop-right dotted 0).

Function: take-left! dotted i

Function: drop-right! dotted i

Syntax: take-left!/stx dotted i

Syntax: drop-right!/stx dotted i

take! and drop-right! are like take and drop-right, but they are allowed to alter the argument list to produce the result.

An error is raised if the length of the list is less than i.

Function: split-at dotted i

Function: split-at! dotted i

Syntax: split-at/stx dotted i

Syntax: split-at!/stx dotted i

split-at splits the list dotted at index i, returning a list of the first i elements, and the remaining tail. It is equivalent to:

(values (take-left x i) (drop-left x i))

split-at! is allowed to alter the argument list to produce the result.

(split-at '(a b c d e f g h) 3)
⇒ (a b c) (d e f g h)

Function: last pair

Function: last-pair pair

Syntax: last/stx pair

Syntax: last-pair/stx pair

last returns the last element of the non–empty, finite list pair. last-pair returns the last pair in the non–empty, finite list pair.

(last '(a b c)) ⇒ c
(last-pair '(a b c)) ⇒ (c)