#     Syntax:

car x => object

cdr x => object

caar x => object

cdar x => object

cddr x => object

caaar x => object

cdaar x => object

cddar x => object

cdddr x => object

caaaar x => object

cdaaar x => object

cddaar x => object

cdddar x => object

cddddr x => object

(setf (car x) new-object)

(setf (cdr x) new-object)

(setf (caar x) new-object)

(setf (cdar x) new-object)

(setf (cddr x) new-object)

(setf (caaar x) new-object)

(setf (cdaar x) new-object)

(setf (cddar x) new-object)

(setf (cdddr x) new-object)

(setf (caaaar x) new-object)

(setf (cdaaar x) new-object)

(setf (cddaar x) new-object)

(setf (cdddar x) new-object)

(setf (cddddr x) new-object)

Pronunciation:

cdr: ['k,duhr]

cddr: ['kduh,duhr] or ['kuh,dduhr]

Arguments and Values:

x---a list.

object---an object.

new-object---an object.

Description:

If x is a cons, car returns the car of that cons. If x is nil, car returns nil.

If x is a cons, cdr returns the cdr of that cons. If x is nil, cdr returns nil.

Functions are provided which perform compositions of up to four car and cdr operations. Their names consist of a C, followed by two, three, or four occurrences of A or D, and finally an R. The series of A's and D's in each function's name is chosen to identify the series of car and cdr operations that is performed by the function. The order in which the A's and D's appear is the inverse of the order in which the corresponding operations are performed. The next figure defines the relationships precisely.

```This place ...  Is equivalent to this place ...
(caar x)        (car (car x))
(cdar x)        (cdr (car x))
(cddr x)        (cdr (cdr x))
(caaar x)       (car (car (car x)))
(caadr x)       (car (car (cdr x)))
(cadar x)       (car (cdr (car x)))
(caddr x)       (car (cdr (cdr x)))
(cdaar x)       (cdr (car (car x)))
(cdadr x)       (cdr (car (cdr x)))
(cddar x)       (cdr (cdr (car x)))
(cdddr x)       (cdr (cdr (cdr x)))
(caaaar x)      (car (car (car (car x))))
(caaadr x)      (car (car (car (cdr x))))
(caadar x)      (car (car (cdr (car x))))
(caaddr x)      (car (car (cdr (cdr x))))
(cadaar x)      (car (cdr (car (car x))))
(caddar x)      (car (cdr (cdr (car x))))
(cadddr x)      (car (cdr (cdr (cdr x))))
(cdaaar x)      (cdr (car (car (car x))))
(cdaadr x)      (cdr (car (car (cdr x))))
(cdadar x)      (cdr (car (cdr (car x))))
(cdaddr x)      (cdr (car (cdr (cdr x))))
(cddaar x)      (cdr (cdr (car (car x))))
(cddadr x)      (cdr (cdr (car (cdr x))))
(cdddar x)      (cdr (cdr (cdr (car x))))
(cddddr x)      (cdr (cdr (cdr (cdr x))))
```

Figure 14-6. CAR and CDR variants

setf can also be used with any of these functions to change an existing component of x, but setf will not make new components. So, for example, the car of a cons can be assigned with setf of car, but the car of nil cannot be assigned with setf of car. Similarly, the car of the car of a cons whose car is a cons can be assigned with setf of caar, but neither nilnor a cons whose car is nil can be assigned with setf of caar.

The argument x is permitted to be a dotted list or a circular list.

Examples:

``` (car nil) =>  NIL
(cdr '(1 . 2)) =>  2
(cdr '(1 2)) =>  (2)
(car '(a b c)) =>  A
(cdr '(a b c)) =>  (B C)
```

Affected By: None.

Exceptional Situations:

The functions car and cdr should signal type-error if they receive an argument which is not a list. The other functions (caar, cadr, ... cddddr) should behave for the purpose of error checking as if defined by appropriate calls to car and cdr.

Notes:

The car of a cons can also be altered by using rplaca, and the cdr of a cons can be altered by using rplacd.

```(car x)    ==  (first x)
(cadr x)   ==  (second x) ==  (car (cdr x))
(caddr x)  ==  (third x)  ==  (car (cdr (cdr x)))
(cadddr x) ==  (fourth x) ==  (car (cdr (cdr (cdr x))))
```

The following X3J13 cleanup issue, not part of the specification, applies to this section:      