Lisp - Lists

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Lists had been the most important and the primary composite data structure in traditional LISP. Present day's Common LISP provides other data structures like, vector, hash table, classes or structures.

Lists are single linked lists. In LISP, lists are constructed as a chain of a simple record structure named cons linked together.

The cons Record Structure

A cons is a record structure containing two components called the car and the cdr.

Cons cells or cons are objects are pairs of values that are created using the function cons.

The cons function takes two arguments and returns a new cons cell containing the two values. These values can be references to any kind of object.

If the second value is not nil, or another cons cell, then the values are printed as a dotted pair enclosed by parentheses.

The two values in a cons cell are called the car and the cdr. The car function is used to access the first value and the cdr function is used to access the second value.

Example

Create a new source code file named main.lisp and type the following code in it.

main.lisp

; create and print a cons of car 1, cdr 2
(write (cons 1 2))
; terminate printing
(terpri)
; create and print a cons of car a, cdr b
(write (cons 'a 'b))
; terminate printing
(terpri)
; create and print a cons of car 1, cdr nil
(write (cons 1 nil))
; terminate printing
(terpri)
; create and print a cons of car 1, cdr as another cons
(write (cons 1 (cons 2 nil)))
; terminate printing
(terpri)
; create and print multiple cons
(write (cons 1 (cons 2 (cons 3 nil))))
; terminate printing
(terpri)
; create and print multiple cons
(write (cons 'a (cons 'b (cons 'c nil))))
; terminate printing
(terpri)
; create and print multiple cons
(write ( car (cons 'a (cons 'b (cons 'c nil)))))
; terminate printing
(terpri)
; create and print multiple cons
(write ( cdr (cons 'a (cons 'b (cons 'c nil)))))

Output

When you execute the code, it returns the following result −

(1 . 2)
(A . B)
(1)
(1 2)
(1 2 3)
(A B C)
A
(B C)

The above example shows how the cons structures could be used to create a single linked list, e.g., the list (A B C) consists of three cons cells linked together by their cdrs.

Lists in LISP

Although cons cells can be used to create lists, however, constructing a list out of nested cons function calls can't be the best solution. The list function is rather used for creating lists in LISP.

The list function can take any number of arguments and as it is a function, it evaluates its arguments.

The first and rest functions give the first element and the rest part of a list. The following examples demonstrate the concepts.

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; create and print a list 
(write (list 1 2))
; terminate printing
(terpri)
; create and print a list 
(write (list 'a 'b))
; terminate printing
(terpri)
; create and print a list 
(write (list 1 nil))
; terminate printing
(terpri)
; create and print a list 
(write (list 1 2 3))
; terminate printing
(terpri)
; create and print a list 
(write (list 'a 'b 'c))
; terminate printing
(terpri)
; create and print a list an car
(write (list 3 4 'a (car '(b . c)) (* 4 -2)))
; terminate printing
(terpri)
; create and print a list of lists
(write (list (list 'a 'b) (list 'c 'd 'e)))

Output

When you execute the code, it returns the following result −

(1 2)
(A B)
(1 NIL)
(1 2 3)
(A B C)
(3 4 A B -8)
((A B) (C D E))

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; define a function to return the list
(defun my-library (title author rating availability)
   (list :title title :author author :rating rating :availabilty availability)
)

; call the function and print the result
(write (getf (my-library "Hunger Game" "Collins" 9 t) :title))

Output

When you execute the code, it returns the following result −

"Hunger Game"

List Manipulating Functions

The following table provides some commonly used list manipulating functions.

Please note that all sequence functions are applicable to lists.

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; create a car of a sequence
(write (car '(a b c d e f)))
; terminate printing
(terpri)
; create a cdr of a sequence
(write (cdr '(a b c d e f)))
; terminate printing
(terpri)
; create a cons
(write (cons 'a '(b c)))
; terminate printing
(terpri)
; create a list of lists
(write (list 'a '(b c) '(e f)))
; terminate printing
(terpri)
; create a list by appending lists
(write (append '(b c) '(e f) '(p q) '() '(g)))
; terminate printing
(terpri)
; get last car
(write (last '(a b c d (e f))))
; terminate printing
(terpri)
; get reverse of the list
(write (reverse '(a b c d (e f))))

Output

When you execute the code, it returns the following result −

A
(B C D E F)
(A B C)
(A (B C) (E F))
(B C E F P Q G)
((E F))
((E F) D C B A)

Concatenation of car and cdr Functions

The car and cdr functions and their combination allows extracting any particular element/ member of a list.

However, sequences of car and cdr functions could be abbreviated by concatenating the letter a for car and d for cdr within the letters c and r.

For example we can write cadadr to abbreviate the sequence of function calls - car cdr car cdr.

Thus, (cadadr '(a (c d) (e f g))) will return d

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; create a cons from sequences
(write (cadadr '(a (c d) (e f g))))
; terminate printing
(terpri)
; create a cons from a list
(write (caar (list (list 'a 'b) 'c)))   
; terminate printing
(terpri)
; create a cons from a list
(write (cadr (list (list 1 2) (list 3 4))))
; terminate printing
(terpri)

Output

When you execute the code, it returns the following result −

D
A
(3 4)