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
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 Expression Syntax. 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.
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.
#tif x is a pair; otherwise return
The two parts of a pair are traditionally called car and
cdr. They can be retrieved with procedures of the same name
cdr), and can be modified with the procedures
set-cdr!. Since a very common operation in
Scheme programs is to access the car of a car of a pair, or the car of
the cdr of a pair, etc., the procedures called
cadr and so on are also predefined.
Return the car or the cdr of pair, respectively.
These two macros are the fastest way to access the car or cdr of a pair; they can be thought of as compiling into a single memory reference.
These macros do no checking at all. The argument pair must be a valid pair.
These procedures are compositions of
cdr, where for example
caddrcould be defined by(define caddr (lambda (x) (car (cdr (cdr x)))))
cadddrpick out the second, third or fourth elements of a list, respectively. SRFI-1 provides the same under the names
fourth(see SRFI-1 Selectors).
Stores value in the car field of pair. The value returned by