2.4.8 Lists

List functions can also be found in the sections on Association Lists, Sets, and Sequences.

(cons x y)

Creates a new pair containing x and y.

Examples:

> (cons 1 nil)

(1)

> (cons 2 (cons 3 nil))

(2 3)

> (cons 4 5)

(4 . 5)

(consp x)

Returns true when x is a pair.

Examples:

> (consp nil)

()

> (consp 5)

()

> (consp "string")

()

> (consp (cons 1 nil))

t

> (consp '(1 2 3))

t

(car x) → t

x : (or (consp x) (equal x nil))

Returns the first element of a cons-pair, or nil for nil.

Examples:

> (car '(1 2 4 6))

1

> (car nil)

()

> (car "violates guard")

Dracula program broke the contract (-> (or/c "nil"

(cons/c any/c any/c)) any) on here; expected <(or/c nil

(cons/c any/c any/c))>, given: "violates guard"

(cdr x) → t

x : (or (consp x) (equal x nil))

Returns the second element of a cons-pair, or nil for nil.

Examples:

> (cdr '(1 2 4 6))

(2 4 6)

> (cdr nil)

()

> (cdr "violates guard")

Dracula program broke the contract (-> (or/c "nil"

(cons/c any/c any/c)) any) on here; expected <(or/c nil

(cons/c any/c any/c))>, given: "violates guard"

(append lst ...)

Concatenates all the elements in the given lists into one list. This a macro that expands into calls of the function binary-append.

Examples:

> (append nil nil)

()

> (append nil (list 1 2))

(1 2)

> (append (list 1 2) (list 3 4))

(1 2 3 4)

> (append 1 nil)

append: expected argument of type <proper list>; given 1

(binary-append x y) → t

x : (true-listp x)

y : t

Concatenates two lists. Returns a new list containing all the items from x followed by all the items from y.

Examples:

> (binary-append (list 1 2 3) (list 4 5 6))

(1 2 3 4 5 6)

> (binary-append (list 1 2 3) 4)

(1 2 3 . 4)

> (binary-append 5 "<-error")

append: expected argument of type <proper list>; given 5

(mv x ...)

Form for returning multiple values. This is like list, but with the restriction that if a function returns using mv, then it must always return the same number of values.

Examples:

> (mv 1 2 3 4)

(1 2 3 4)

(revappend x y) → t

x : (true-listp x)

y : t

append the reverse of x to y.

Examples:

> (revappend nil nil)

()

> (revappend nil (list 1 2))

(1 2)

> (revappend (list 1 2) (list 3 4))

(2 1 3 4)

> (revappend (list 1 2) 3)

(2 1 . 3)

> (revappend "not a list" nil)

Dracula program broke the contract (-> (listof any/c) any/c

any) on here; expected <(listof any/c)>, given: "not a list"

(list elem ...)

Creates a new true list containing all the given elements.

Examples:

> (list)

()

> (list 1 2 3)

(1 2 3)

(list* elem ... tail)

Uses cons to add each elem to tail.

Examples:

> (list* 1 2 nil)

(1 2)

> (list* 5 6 '(7 8 9))

(5 6 7 8 9)

> (list* 1 2 3)

(1 2 . 3)

> (list*)

list*: expects at least 1 argument, given 0

(atom-listp x) → t

x : t

Reports whether x is a list of atoms.

Examples:

> (atom-listp (list 2 3 4))

t

> (atom-listp (list (list 23 34) 45))

()

(character-listp x) → t

x : t

Reports whether x is a list of characterp elements.

Examples:

> (character-listp (list #\a #\b))

t

> (character-listp (list #\a "b"))

()

(endp x) → t

x : (or (consp lst) (equal x nil))

Same as atom, but with the guard that x is either a consp or is nil.

Examples:

> (endp nil)

t

> (endp (cons 2 nil))

()

> (endp "violates guard")

Dracula program broke the contract (-> (or/c "nil" pair?)

any) on here; expected <(or/c nil pair?)>, given: "violates

guard"

(eqlable-listp x) → t

x : t

Reports whether x is a list of eqlablep elements.

Examples:

> (eqlable-listp nil)

t

> (eqlable-listp (cons 4 nil))

t

> (eqlable-listp t)

()

(integer-listp x) → t

x : t

Reports whether x is a list of integerp elements.

Examples:

> (integer-listp nil)

t

> (integer-listp (list 24 -21 95))

t

> (integer-listp (list 53 44 "number"))

()

(keyword-value-listp x) → t

x : t

Reports whether x is of even length and every other element in the list satisfies keywordp.

Examples:

> (keyword-value-listp (list :a 1 :b 2 :c 3))

t

> (keyword-value-listp (list 'a 1 'b 'c 3))

()

(listp x) → t

x : t

Reports whether x is either a consp or nil.

Examples:

> (listp nil)

t

> (listp (cons 4 nil))

t

> (listp t)

()

(null x) → t

x : t

Returns true if x equals nil (using eq).

Examples:

> (null nil)

t

> (null (list 1 2 3))

()

(proper-consp x) → t

x : t

Reports whether x is a proper (nil-terminated) nonempty list

Examples:

> (proper-consp nil)

()

> (proper-consp (list 1 2 3))

t

> (proper-consp (cons 1 2))

()

(rational-listp x) → t

x : t

Reports whether x is a list of rationalp elements.

Examples:

> (rational-listp (list 1 2/5 3))

t

> (rational-listp (list 1 2/5 "number"))

()

(standard-char-listp x) → t

x : t

Reports whether x is a list of standard-char-p elements.

Examples:

> (standard-char-listp (list #\a #\b #\c))

t

> (standard-char-listp (list 1 2 3))

()

(string-listp x) → t

x : t

Reports whether x is a list of stringp elements.

Examples:

> (string-listp (list "ab" "cd" "ef"))

t

> (string-listp (list 1 2 3))

()

(symbol-listp x) → t

x : t

Reports whether x is a list of symbolp elements.

Examples:

> (symbol-listp (list 'ab 'cd 'ef))

t

> (symbol-listp (list 1 2 3))

()

(true-list-listp x) → t

x : t

Reports whether x is a list of true-listp elements.

Examples:

> (true-list-listp (list 1 2 3 4 5))

()

> (true-list-listp '((1) (2) (3) (4) (5)))

t

(true-listp x) → t

x : t

Reports whether x is a proper nil-terminated list.

Examples:

> (true-listp (list 1 2 3 4 5))

t

> (true-listp "list")

()

(butlast l n) → t

l : (true-listp l)

n : (and (integerp n) (<= 0 n))

(butlast l n) is the list obtained by removing the last n elements from the true list l.

Examples:

> (butlast nil 1)

()

> (butlast (list 1 2 3) 1)

(1 2)

> (butlast (list 1 2 3) 3)

()

> (butlast (list 1 2 3) 10)

()

> (butlast (list 1 2 3) -1)

drop-right: expected argument of type <non-negative exact

integer>; given -1

(fix-true-list x) → t

x : t

Coerces x to a true list by replacing the final cdr with nil.

Examples:
> (fix-true-list (list 1 2 3))
(1 2 3)
> (fix-true-list (cons 1 (cons 2 3)))
(1 2)
> (fix-true-list "abc")
()
> (fix-true-list 5)
()

(len lst) → t

lst : t

Finds the length of the given list. Returns 0 if lst is not a list.

Examples:
> (len nil)
0
> (len (list 1 2 3 4 5))
5
> (len "string")
0
> (len t)
0

(make-list n) → true-listp

n : (and (integerp n) (<= 0 n))

Make a list of the given size.

Examples:

> (make-list 0)

()

> (make-list 3)

(() () ())

> (make-list -1)

Dracula program broke the contract (case-> (->

natural-number/c any) (-> natural-number/c

(symbols ’:initial-element) any/c any)) on here;

expected <natural-number/c>, given: -1

(nth n lst) → t

n : (and (integerp n) (>= n 0))

lst : (true-listp lst)

Gets the nth element of lst

Examples:

> (nth 2 (list 1 2 3))

3

> (nth 4 (list 1 2 1))

()

> (nth "string" (list 1 2 3))

Dracula program broke the contract (-> natural-number/c

(listof any/c) any) on here; expected <natural-number/c>,

given: "string"

> (nth 2 "string")

Dracula program broke the contract (-> natural-number/c

(listof any/c) any) on here; expected <(listof any/c)>,

given: "string"

(nthcdr n lst) → t

n : (and (integerp n) (>= n 0))

lst : (true-listp lst)

Gets the nth cdr of the given list

Examples:

> (nthcdr 2 (list 1 2 3))

(3)

> (nthcdr 3 (list 1 2 1))

()

(reverse x) → t

x : (or (true-listp x) (stringp x))

Reverse the given list or string

Examples:

> (reverse (list 1 2 3 4))

(4 3 2 1)

> (reverse "abcd")

"dcba"

> (reverse 12345)

Dracula program broke the contract (-> (or/c string?

(listof any/c)) any) on here; expected <(or/c string?

(listof any/c))>, given: 12345

(take n l) → t

n : (and (integerp n) (not (< n 0)))

l : (true-listp l)

Examples:

> (take 3 (list 1 2 3 4 5))

(1 2 3)

> (take 0 (list 1 2 3 4 5))

()

> (take -1 (list 1 2 3 4 5))

Dracula program broke the contract (-> natural-number/c

any) on here; expected <natural-number/c>, given: -1

> (take 2 (cons 1 2))

Dracula program broke the contract (-> (or/c "nil"

(cons/c any/c any/c)) any) on here; expected <(or/c nil

(cons/c any/c any/c))>, given: 2

(update-nth n v lst) → t

n : (and (integerp n) (<= 0 n))

v : t

lst : (true-listp l)

Replaces the nth (0-based) position in lst with the value v. If n is greater than the length of lst, then update-nth will add padding to the end of the list consisting of nil values.

Examples:

> (update-nth 0 'z '(a b c d e))

(z b c d e)

> (update-nth 8 'z '(a b c d e))

(a b c d e () () () z)

> (update-nth -1 'z '(a b c d e))

Dracula program broke the contract (-> natural-number/c

any) on here; expected <natural-number/c>, given: -1

> (update-nth 1 'z "not a list")

Dracula program broke the contract (-> (or/c "nil"

(cons/c any/c any/c)) any) on here; expected <(or/c nil

(cons/c any/c any/c))>, given: "not a list"

(first lst)

first member of the list

Examples:

> (first (list 1 2 3 4 5 6 7 8))

1

(second lst)

Examples:

> (second (list 1 2 3 4 5))

2

(third lst)

Examples:

> (third (list 1 2 3 4 5 6 7 8 9 10))

3

(fourth lst)

fourth member of the list

Examples:

> (fourth (list 1 2 3 4 5 6 7 8))

4

(fifth lst)

fifth member of the list

Examples:

> (fifth (list 1 2 3 4 5 6 7 8))

5

(sixth lst)

Sixth member of the list

Examples:

> (sixth (list 1 2 3 4 5 6 7 8 9 10))

6

(seventh lst)

Examples:

> (seventh (list 1 2 3 4 5 6 7 8 9 10))

7

(eighth lst)

eighth member of the list

Examples:

> (eighth (list 1 2 3 4 5 6 7 8))

8

(ninth x)

Examples:

> (ninth (list 1 2 3 4 5 6 7 8 9 10))

9

(tenth lst)

Examples:

> (tenth (list 1 2 3 4 5 6 7 8 9 10))

10

(last lst)

last member of the list

Examples:

> (last (list 1 2 3 4 5))

(5)

(rest lst)

The same as cdr

Examples:

> (rest (list 'w 'x 'y 'z))

(x y z)