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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)`.

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.

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