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5.3 Accessing Elements of Lists

function car cons-cell​

This function returns the value referred to by the first slot of the cons cell cons-cell. In other words, it returns the CAR of cons-cell.

As a special case, if cons-cell is nil, this function returns nil. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or nil.

(car '(a b c))
     ⇒ a
(car '())
     ⇒ nil

function cdr cons-cell​

This function returns the value referred to by the second slot of the cons cell cons-cell. In other words, it returns the CDR of cons-cell.

As a special case, if cons-cell is nil, this function returns nil; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or nil.

(cdr '(a b c))
     ⇒ (b c)
(cdr '())
     ⇒ nil

function car-safe object​

This function lets you take the CAR of a cons cell while avoiding errors for other data types. It returns the CAR of object if object is a cons cell, nil otherwise. This is in contrast to car, which signals an error if object is not a list.

(car-safe object)
≡
(let ((x object))
  (if (consp x)
      (car x)
    nil))

function cdr-safe object​

This function lets you take the CDR of a cons cell while avoiding errors for other data types. It returns the CDR of object if object is a cons cell, nil otherwise. This is in contrast to cdr, which signals an error if object is not a list.

(cdr-safe object)
≡
(let ((x object))
  (if (consp x)
      (cdr x)
    nil))

macro pop listname​

This macro provides a convenient way to examine the CAR of a list, and take it off the list, all at once. It operates on the list stored in listname. It removes the first element from the list, saves the CDR into listname, then returns the removed element.

In the simplest case, listname is an unquoted symbol naming a list; in that case, this macro is equivalent to (prog1 (car listname) (setq listname (cdr listname))).

x
     ⇒ (a b c)
(pop x)
     ⇒ a
x
     ⇒ (b c)

More generally, listname can be a generalized variable. In that case, this macro saves into listname using setf. See Generalized Variables.

For the push macro, which adds an element to a list, See List Variables.

function nth n list​

This function returns the nth element of list. Elements are numbered starting with zero, so the CAR of list is element number zero. If the length of list is n or less, the value is nil.

(nth 2 '(1 2 3 4))
     ⇒ 3
(nth 10 '(1 2 3 4))
     ⇒ nil

(nth n x) ≡ (car (nthcdr n x))

The function elt is similar, but applies to any kind of sequence. For historical reasons, it takes its arguments in the opposite order. See Sequence Functions.

function nthcdr n list​

This function returns the nth CDR of list. In other words, it skips past the first n links of list and returns what follows.

If n is zero, nthcdr returns all of list. If the length of list is n or less, nthcdr returns nil.

(nthcdr 1 '(1 2 3 4))
     ⇒ (2 3 4)
(nthcdr 10 '(1 2 3 4))
     ⇒ nil
(nthcdr 0 '(1 2 3 4))
     ⇒ (1 2 3 4)

function last list \&optional n​

This function returns the last link of list. The car of this link is the list’s last element. If list is null, nil is returned. If n is non-nil, the nth-to-last link is returned instead, or the whole of list if n is bigger than list’s length.

function safe-length list​

This function returns the length of list, with no risk of either an error or an infinite loop. It generally returns the number of distinct cons cells in the list. However, for circular lists, the value is just an upper bound; it is often too large.

If list is not nil or a cons cell, safe-length returns 0.

The most common way to compute the length of a list, when you are not worried that it may be circular, is with length. See Sequence Functions.

function caar cons-cell​

This is the same as (car (car cons-cell)).

function cadr cons-cell​

This is the same as (car (cdr cons-cell)) or (nth 1 cons-cell).

function cdar cons-cell​

This is the same as (cdr (car cons-cell)).

function cddr cons-cell​

This is the same as (cdr (cdr cons-cell)) or (nthcdr 2 cons-cell).

In addition to the above, 24 additional compositions of car and cdr are defined as cxxxr and cxxxxr, where each x is either a or d. cadr, caddr, and cadddr pick out the second, third or fourth elements of a list, respectively. cl-lib provides the same under the names cl-second, cl-third, and cl-fourth. See List Functions in Common Lisp Extensions.

function butlast x \&optional n​

This function returns the list x with the last element, or the last n elements, removed. If n is greater than zero it makes a copy of the list so as not to damage the original list. In general, (append (butlast x n) (last x n)) will return a list equal to x.

function nbutlast x \&optional n​

This is a version of butlast that works by destructively modifying the cdr of the appropriate element, rather than making a copy of the list.