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cl:functions:car_cdr_and_their_compositions [2019/11/11 05:00]
cl:functions:car_cdr_and_their_compositions [2020/06/07 01:00] (current)
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 +====== Accessors CAR, CDR and their compositions ======
 +  * **car** //x// → //object//
 +  * **cdr** //x// → //object//
 +  * (**setf** (**car** //x//) //​new-object//​)
 +  * (**setf** (**cdr** //x//) //​new-object//​)
 +Click [[CL:​Functions:​car,​ cdr and their compositions:​Full List|here]] for the syntax and equivalence chart of all **car** and **cdr** compositions.
 +====Arguments and values==== ​
 +  * //x// - a //​[[CL:​Glossary:​list]]//​.
 +  * //object// - an //​[[CL:​Glossary:​object]]//​.
 +  * //​new-object//​ - an //​[[CL:​Glossary:​object]]//​.
 +If //x// is a //​[[CL:​Glossary:​cons]]//,​ **car** returns the //​[[CL:​Glossary:​car]]//​ of that //​[[CL:​Glossary:​cons]]//​. If //x// is **[[CL:​Constant Variable:​nil]]**,​ **car** returns **[[CL:​Constant Variable:​nil]].**
 +If //x// is a //​[[CL:​Glossary:​cons]]//,​ **cdr** returns the //​[[CL:​Glossary:​cdr]]//​ of that //​[[CL:​Glossary:​cons]]//​. If //x// is **[[CL:​Constant Variable:​nil]]**,​ **cdr** returns **[[CL:​Constant Variable:​nil]]**.
 +//​[[CL:​Glossary:​function|Functions]]//​ are provided which perform compositions up to four **car** and **cdr** operations. Their //​[[CL:​Glossary:​name|names]]//​ consist of a ''​C'',​ followed by two, three or four occurrences of ''​A''​ or ''​D'',​ and finally a ''​R''​. The series of ''​A''​s and ''​D''​s in each //​[[CL:​Glossary:​function|function'​s]]//​ //​[[CL:​Glossary:​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.
 +**[[CL:​Macros:​setf]]** can also be used with any of these functions to change an existing component of //x//, but **[[CL:​Macros:​setf]]** will not make new components. So, for example, the //​[[CL:​Glossary:​car]]//​ of a //​[[CL:​Glossary:​cons]]//​ can be assigned with **[[CL:​Macros:​setf]]** of **car**, but the //​[[CL:​Glossary:​car]]//​ of **[[CL:​Constant Variable:​nil]]** cannot be assigned with **[[CL:​Macros:​setf]]** of **car**. ​
 +Similarly, the //​[[CL:​Glossary:​car]]//​ of the //​[[CL:​Glossary:​car]]//​ of a //​[[CL:​Glossary:​cons]]//​ whose //​[[CL:​Glossary:​car]]//​ is a //​[[CL:​Glossary:​cons]]//​ can be assigned with **[[CL:​Macros:​setf]]** of **caar**, but neither **[[CL:​Constant Variable:​nil]]** nor a //​[[CL:​Glossary:​cons]]//​ whose //​[[CL:​Glossary:​car]]//​ is **[[CL:​Constant Variable:​nil]]** can be assigned with **[[CL:​Macros:​setf]]** of **caar**.
 +The argument //x// is permitted to be a //​[[CL:​Glossary:​dotted list]]// or a //​[[CL:​Glossary:​circular list]]//.
 +(car [[CL:​Constant Variable:​nil]]) <​r>​[[CL:​Constant Variable:​nil|NIL]]</​r>​
 +(cdr '(1 . 2)) <​r>​2</​r>​
 +(cdr '(1 2)) <​r>​(2)</​r>​
 +(cadr '(1 2)) <​r>​2</​r>​
 +(car '(a b c)) <​r>​A</​r>​
 +(cdr '(a b c)) <r>(B C)</​r>​
 +====Exceptional situations====
 +The functions **car** and **cdr** should signal **[[CL:​Errors:​type-error]]** if they receive an argument which is not a //​[[CL:​Glossary:​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**.
 +====See also====
 +**[[CL:​Functions:​rplaca|Function RPLACA]]**, **[[CL:​Functions:​rplacd|Function RPLACD]]**, **[[CL:​Functions:​first|Function FIRST]]**, **[[CL:​Functions:​rest|Function REST]]**
 +====Example Implementation====
 +To be done.
 +The //​[[CL:​Glossary:​car]]//​ of a //​[[CL:​Glossary:​cons]]//​ can also be altered by using **[[CL:​Functions:​rplaca]]**,​ and the //​[[CL:​Glossary:​cdr]]//​ of a //​[[CL:​Glossary:​cons]]//​ can be altered by using **[[CL:​Functions:​rplacd]]**.
 +  * ''​(**car** //​x//​)''​ is equivalent to ''​(**[[CL:​Functions:​first]]** //​x//​)''​.
 +  * ''​(**cadr** //​x//​)''​ is equivalent to ''​(**[[CL:​Functions:​second]]** //​x//​)''​.
 +  * ''​(**caddr** //​x//​)''​ is equivalent to ''​(**[[CL:​Functions:​third]]** //​x//​)''​.
 +  * ''​(**cadddr** //​x//​)''​ is equivalent to ''​(**[[CL:​Functions:​fourth]]** //​x//​)''​.