Node:Pairs, Next:Lists, Up:Compound Data Types
Pairs are used to combine two Scheme objects into one compound object. Hence the name: A pair stores a pair of objects.
The data type pair is extremely important in Scheme, just like in any other Lisp dialect. The reason is that pairs are not only used to make two values available as one object, but that pairs are used for constructing lists of values. Because lists are so important in Scheme, they are described in a section of their own (see Lists).
Pairs can literally get entered in source code or at the REPL, in the
so-called dotted list syntax. This syntax consists of an opening
parentheses, the first element of the pair, a dot, the second element
and a closing parentheses. The following example shows how a pair
consisting of the two numbers 1 and 2, and a pair containing the symbols
foo and bar can be entered. It is very important to write
the whitespace before and after the dot, because otherwise the Scheme
parser would not be able to figure out where to split the tokens.
(1 . 2) (foo . bar)
But beware, if you want to try out these examples, you have to
quote the expressions. More information about quotation is
available in the section (REFFIXME). The correct way to try these
examples is as follows.
'(1 . 2) => (1 . 2) '(foo . bar) => (foo . bar)
A new pair is made by calling the procedure cons with two
arguments. Then the argument values are stored into a newly allocated
pair, and the pair is returned. The name cons stands for
"construct". Use the procedure pair? to test whether a
given Scheme object is a pair or not.
| cons x y | Scheme Procedure |
| scm_cons (x, y) | C Function |
Return a newly allocated pair whose car is x and whose
cdr is y. The pair is guaranteed to be different (in the
sense of eq?) from every previously existing object.
|
| pair? x | Scheme Procedure |
| scm_pair_p (x) | C Function |
Return #t if x is a pair; otherwise return
#f.
|
The two parts of a pair are traditionally called car and
cdr. They can be retrieved with procedures of the same name
(car and cdr), and can be modified with the procedures
set-car! and set-cdr!. Since a very common operation in
Scheme programs is to access the car of a pair, or the car of the cdr of
a pair, etc., the procedures called caar, cadr and so on
are also predefined.
| car pair | Scheme Procedure |
| cdr pair | Scheme Procedure |
| Return the car or the cdr of pair, respectively. |
| caar pair | Scheme Procedure |
| cadr pair ... | Scheme Procedure |
| cdddar pair | Scheme Procedure |
| cddddr pair | Scheme Procedure |
These procedures are compositions of car and cdr, where
for example caddr could be defined by
(define caddr (lambda (x) (car (cdr (cdr x))))) |
| set-car! pair value | Scheme Procedure |
| scm_set_car_x (pair, value) | C Function |
Stores value in the car field of pair. The value returned
by set-car! is unspecified.
|
| set-cdr! pair value | Scheme Procedure |
| scm_set_cdr_x (pair, value) | C Function |
Stores value in the cdr field of pair. The value returned
by set-cdr! is unspecified.
|