100 lines
2.9 KiB
HTML
100 lines
2.9 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Originally converted to HTML using LaTeX2HTML 95 (Thu Jan 19 1995) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE> Association Lists</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Association Lists">
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<meta name="keywords" value="lp">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<BR> <HR>
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<A HREF="node52.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node50.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node50.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <BR>
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<A HREF="lp.html"><B>Contents</B></A>
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<B> Next:</B>
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<A HREF="node52.html"> Property Lists</A>
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<B>Up:</B>
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<A HREF="node50.html"> Simple Data Structures </A>
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<B> Previous:</B>
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<A HREF="node50.html"> Simple Data Structures </A>
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<BR> <HR> <P>
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<H1> Association Lists</H1>
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<P>
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An association list is any list of the following form:
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<BLOCKQUOTE>
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<PRE>((<key1> ...<expressions> )
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(<key2> ...<expressions> )....)
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</PRE>
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</BLOCKQUOTE>
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The keys should be atoms. Following each key, you can put any sequence of Lisp expressions.
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<P>
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Use the interpreter to enter this example of an association list:
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<P>
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<BLOCKQUOTE>
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<PRE>> (setf person1 '((first-name john)
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(last-name smith)
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(age 23)
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(children jane jim)))
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((FIRST-NAME JOHN) (LAST-NAME SMITH) (AGE 23)
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(CHILDREN JANE JIM))
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</PRE>
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</BLOCKQUOTE>
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Lisp provides a function, assoc, to retrieve information easily from association lists given a retrieval key.
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<P>
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For example:
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<P>
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<BLOCKQUOTE>
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<PRE>> (assoc 'age person1)
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(AGE 23)
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> (assoc 'children person1)
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(CHILDREN JANE JIM)
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</PRE>
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</BLOCKQUOTE>
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Notice that assoc returns the entire key-expression sublist. It does not matter to assoc what order the keys appear in the association list or how many expressions are associated with each key.
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<P>
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Setf can be used to change particular values. For example, here is a function that can be used on a birthday to update a person's age automatically.
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<P>
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<BLOCKQUOTE>
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<PRE>(defun make-older (person)
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(setf (second (assoc 'age person))
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(1+ (second (assoc 'age person)))))
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</PRE>
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</BLOCKQUOTE>
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Have your Lisp interpreter evaluate this definition, then see that it works:
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<P>
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<BLOCKQUOTE>
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<PRE>> (make-older person1)
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24
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> (assoc 'age person1)
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(AGE 24)
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</PRE>
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</BLOCKQUOTE>
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Assoc will return nil if the key is not found.
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<P>
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<BLOCKQUOTE>
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<PRE>> (assoc 'sex person1)
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NIL
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</PRE>
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</BLOCKQUOTE>
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But it is very easy to add new key-expression sublists, again using setf.
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<P>
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<BLOCKQUOTE>
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<PRE>> (setf person1 (cons '(sex male) person1))
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((SEX MALE) (FIRST-NAME JOHN) (LAST-NAME SMITH) (AGE 24)
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(CHILDREN JANE JIM))
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</PRE>
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</BLOCKQUOTE>
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<P>
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<BR> <HR>
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<P>
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<ADDRESS>
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<I>© Colin Allen & Maneesh Dhagat <BR>
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March 2007 </I>
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</ADDRESS>
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</BODY>
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