Question

The following two expressions are equivalent:

(third (list 1 2 3 4))

(first (nthcdr 2 (list 1 2 3 4)))

However, using “third,” “fourth,” “fifth,” etc. isn’t always practical and (first (nthcdr n list)) seems a little verbose. Is there a way to say something like (item 2 (list 1 2 3 4)) to get the nth item in a list?

Answer 1

(nth 3 (list 1 2 3 4))

returns 4th item (zero based!)

According to the HyperSpec:

Accessor NTH

Description:

nth locates the nth element of list, where the car of the list is the “zeroth” element. Specifically,

(nth n list) ==  (car (nthcdr n list))

Examples:

(nth 0 '(foo bar baz)) =>  FOO
(nth 1 '(foo bar baz)) =>  BAR
(nth 3 '(foo bar baz)) =>  NIL
(setq 0-to-3 (list 0 1 2 3)) =>  (0 1 2 3)
(setf (nth 2 0-to-3) "two") =>  "two"
0-to-3 =>  (0 1 "two" 3)