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Lists in LISP

  • Last Updated : 12 Oct, 2021

Lists in common LISP is simply a single Linked list. In LISP, Lists are designed as a chain of records. While talking about record structures in LISP, the concept of Cons is vital. Cons in LISP is a record structure with 2  primary components. A cons function takes in 2 arguments and returns a new cons cell with car and dir.

  1. car: It is used to access the first value in a cons function.
  2. cdr: It is used to access the second value in the cons function.

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

Example:

Lisp




; cons with 2 string object reference
(write (cons 'Geeksforgeeks 'Is_Best))
(terpri)
 
; cons with 1 nil value as argument
(write (cons 999 nil))
(terpri)
 
;cons with another cons as argument
(write (cons 'A (cons 'B nil)))
(terpri)
 
;cons with otyhen nested cons as argument
(write (cons 'A (cons 'B (cons 'C nil))))

Output:

(GEEKSFORGEEKS . IS_BEST)
(999)
(A B)
(A B C)

Lists in LISP:

The list function in LISP can be used to create a list in LISP.

Syntax:

write( list value1 value 2 ...)

Note: The list function can take any no. of arguments.

Example: 

Lisp




(write (list 1 2))
(terpri)
(write (list 'g 'e 'e'k's))
(terpri)
(write (list 'Geeksforgeeks' nil))
(terpri)
 
;list with a cons as an argument
(write (list 3 4 'geeks (car '(G . F)) (* 99  +78)))
(terpri)
 
; list with another list as an argument
(write (list (list 'Geeksforgeeks 'is) (list 'the 'best 'resource 'for 'DSA)))

Output:

(1 2)
(G E E K S)
(GEEKSFORGEEKS NIL)
(3 4 GEEKS G 7722)
((GEEKSFORGEEKS IS) (THE BEST RESOURCE FOR DSA))

Accessing Elements of a LIST:

The combination of car and cdr functions in common LISP can be used to extract elements from a list. The combination of carr and cdr can be abbreviated as cadadr/caar/cadr and so on.

Example:

Lisp




; here we will extract the string Best
(write (cadadr '(Geeksforgeeks (is best) (for Data Structures))))
(terpri)
 
; here we will extract the string Geeks
(write (caar (list (list 'Geeks 'for) 'geeks)))  
(terpri)
 
; here we will use the abbv. cadr
(write (cadr (list (list 'A 'B) (list 'C'D))))

Output:

BEST
GEEKS
(C D)

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