Lisp - Accessing Element of a List

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In LISP, List is implmented as single linked list. LISP provides several methods to access element of a list. In this chapter, we're uncovering commonly used methods with examples.

car and cdr, Fundamental Accessors

A list element can be accessed via car, cdr functions.

• car− can be treated as "Content of Address Register". car method returns the first element of the list.

• cdr− can be treated as "Content of Decrement Register". car method returns the rest of the list without first element.

Using car

car function returns the first element of the list.

; print A
(write(car '(a b c)))

Output

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

A

Using cdr

cdr function returns rest of the elements after removing first element of the list.

; print B C
(write(cdr '(a b c)))

Output

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

(B C)

cadr and caddr etc., Composite Accessors

cadr, caddr are shorthand for nested car and cdr calls. cadr is equivalent to (car(cdr ...)),caddr is equivalent to (car(cdr (cdr...))) and so.

a is used to represent car and d is used to represent cdr in nested calls. We can chain upto four a or d between c and r. For higher value access, we can use nth or recursive calls.

Using cadr, caddr

Following code shows the usage of cadr and caddr functions.

; print B
(write(cadr '(a b c)))
(terpri)
; print C
(write(caddr '(a b c)))

Output

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

B 
C

Using nth and elt, Positional Accessors

nth function returns element at particular index. Index starts from 0. Whereas elt also returns element at particular index starting from 0 but it can work with vectors.

; print C
(write(nth 2 '(a b c)))
(terpri)
; print C
(write(elt '(a b c) 2))

Output

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

C

Using Recursion

Following code shows the recursive way to print all elements of a list.

(defun print-list (lst)
   ; if list is not empty
   (when lst 
      ; print first element
      (print (car lst))  
   ; print sub-list recursively
   (print-list (cdr lst)))) 

(print-list '(a b c d e)) 

Output

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

A 
B 
C 
D 
E