61 lines
1.9 KiB
HTML
61 lines
1.9 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> Changing Variable Values</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Changing Variable Values">
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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="node12.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node2.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node10.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="node12.html"> More Functions and </A>
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<B>Up:</B>
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<A HREF="node2.html"> LISt Processing</A>
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<B> Previous:</B>
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<A HREF="node10.html"> Selectors: First and </A>
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<BR> <HR> <P>
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<H1> Changing Variable Values</H1>
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<P>
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What happens to the value of a, after saying (cons 'a a)? Nothing.
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That is, it looks like this:
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<BLOCKQUOTE>
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<PRE>> (cons 'a a)
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(a a s d f)
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> a
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(a s d f)
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</PRE>
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</BLOCKQUOTE>
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Obviously, it would be useful to make these changes stick sometimes.
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To do that you can use setq as follows:
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<P>
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<BLOCKQUOTE>
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<PRE>> (setq a (cons 'a a))
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(a a s d f)
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> a
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(a a s d f)
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</PRE>
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</BLOCKQUOTE>
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and henceforth, that is the new value of a.
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<P>
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We'll let you play with the possibilities here, but using setq with
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just the three functions first, rest, and cons you can do <em> anything</em>
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you want to with lists. These primitives are sufficient. Append and
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list are strictly superfluous -- although they are very convenient.
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For practice, try to achieve the same effects using just first, rest,
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and cons as in the examples that used append and list, above.
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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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