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5.5.1 Cons Cells

A cons cell is an ordered pair of two objects, the car and the cdr.

The read syntax of a cons cell is an opening parenthesis followed by the read syntax of the car, a dot, the read syntax of the cdr and a closing parenthesis. For example a cons cell with a car of 10 and a cdr of the string ‘foo’ would be written as,

(10 . "foo")
Function: cons car cdr

This function creates a new cons cell. It will have a car of car and a cdr of cdr.

(cons 10 "foo")
    ⇒ (10 . "foo")
Function: consp object

This function returns true if object is a cons cell.

(consp '(1 . 2))
    ⇒ t

(consp '())
    ⇒ ()

(consp (cons 1 2))
    ⇒ t

The strange syntax ‘'(1 . 2)’ is known as quoting (see Quoting), it tells the evaluator that the object following the quote-mark is a constant, and therefore should not be evaluated. This will be explained in more detail later.

In Lisp an atom is any object which is not a cons cell (and is, therefore, atomic).

Function: atom object

Returns true if object is an atom (not a cons cell).

Given a cons cell there are a number of operations which can be performed on it.

Function: car cons-cell

This function returns the object which is the car (first element) of the cons cell cons-cell.

(car (cons 1 2))
    ⇒ 1

(car '(1 . 2))
    ⇒ 1
Function: cdr cons-cell

This function returns the cdr (second element) of the cons cell cons-cell.

(cdr (cons 1 2))
    ⇒ 2

(cdr '(1 . 2))
    ⇒ 2

Rep also provides cXXr, cXXXr, and cXXXXr, where each x is either a or d. For example, (cadr z) is equal to (car (cdr z)).

Function: rplaca cons-cell new-car

This function sets the value of the car (first element) in the cons cell cons-cell to new-car. The value returned is cons-cell.

(setq x (cons 1 2))
    ⇒ (1 . 2)
(rplaca x 3)
    ⇒ (3 . 2)
x
    ⇒ (3 . 2)
Function: rplacd cons-cell new-cdr

This function is similar to rplacd except that the cdr slot (second element) of cons-cell is modified.

Function: setcar cons-cell new-car
Function: setcdr cons-cell new-cdr

Same as rplaca and rplacd respectively, but the return value is new-car / new-cdr.


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