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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 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.
Return #t if x is a pair; otherwise return
#f.
Return 1 when x is a pair; otherwise return 0.
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 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. However,
using these procedures is often detrimental to readability, and
error-prone. Thus, accessing the contents of a list is usually better
achieved using pattern matching techniques (see Pattern Matching).
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 car and cdr, where
for example caddr could be defined by
(define caddr (lambda (x) (car (cdr (cdr x)))))
cadr, caddr and cadddr pick out the second, third
or fourth elements of a list, respectively. SRFI-1 provides the same
under the names second, third and fourth
(see SRFI-1 Selectors).
Stores value in the car field of pair. The value returned
by set-car! is unspecified.
Stores value in the cdr field of pair. The value returned
by set-cdr! is unspecified.
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