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