Accessor first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth

Syntax:

Arguments and Values:

list—a list, which might be a dotted list or a circular list.

object, new-object—an object.

Description:

The functions 15.2.0 11first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, and tenth 15.2.0 12access the first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, and tenth elements of list, respectively. I changed this to be in terms of car/cdr because I felt this made the error checking requirements more apparent. -kmp 27-Aug-93 If there is no such \term{element} during a \term{read}, \nil\ is returned. The consequences are undefined if there is no such \term{element} during a \term{write}.Specifically,

 (first list)    ≡  (car list)
 (second list)   ≡  (car (cdr list))
 (third list)    ≡  (car (cddr list))
 (fourth list)   ≡  (car (cdddr list))
 (fifth list)    ≡  (car (cddddr list))
 (sixth list)    ≡  (car (cdr (cddddr list)))
 (seventh list)  ≡  (car (cddr (cddddr list)))
 (eighth list)   ≡  (car (cdddr (cddddr list)))
 (ninth list)    ≡  (car (cddddr (cddddr list)))
 (tenth list)    ≡  (car (cdr (cddddr (cddddr list))))

setf can also be used with any of these functions to change an existing component. The same equivalences apply. For example:

 (setf (fifth list) new-object) ≡ (setf (car (cddddr list)) new-object)

Examples:

 (setq lst '(1 2 3 (4 5 6) ((V)) vi 7 8 9 10)) 
→ (1 2 3 (4 5 6) ((V)) VI 7 8 9 10)
 (first lst) → 1
 (tenth lst) → 10
 (fifth lst) → ((V))
 (second (fourth lst)) → 5
 (sixth '(1 2 3)) → NIL
 (setf (fourth lst) "four") → "four"
 lst → (1 2 3 "four" ((V)) VI 7 8 9 10)

Side Effects:

None.

Affected By:

None.

Exceptional Situations:

None.

See Also:

car, nth

Notes:

first is functionally equivalent to car, second is functionally equivalent to cadr, third is functionally equivalent to caddr, and fourth is functionally equivalent to cadddr.

The ordinal numbering used here is one-origin, as opposed to the zero-origin numbering used by nth:

 (fifth x) ≡ (nth 4 x)