64 lines
2.3 KiB
HTML
64 lines
2.3 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> Logical Operators: and and or</TITLE>
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<style type="text/css">
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tt {font-weight: bold}
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</style>
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</HEAD>
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<BODY>
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<meta name="description" value=" Logical Operators: And and Or">
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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="node28.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node24.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node26.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="node28.html"> Exercises</A>
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<B>Up:</B>
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<A HREF="node24.html"> More Predicates and </A>
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<B> Previous:</B>
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<A HREF="node26.html"> Checking for NIL</A>
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<BR> <HR> <P>
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<H2> Logical Operators: <tt>and</tt> and <tt>or</tt></H2>
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<P>
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<tt>and</tt> and <tt>or</tt> are functions but not predicates since they may return values other than t or nil. Each evaluates its arguments in the order they appear, but only enough arguments to provide a definitive result are evaluated. So, some arguments to <tt>and</tt> and to <tt>or</tt> may not be evaluated at all.
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<P>
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<tt>and</tt> returns nil as soon as it finds an argument which evaluates to nil; otherwise it returns the value of its last argument. For example:
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<BLOCKQUOTE>
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<PRE>> (and 1 2 3 4)
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4
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> (and 1 (cons 'a '(b)) (rest '(a)) (setf y 'hello))
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NIL
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</PRE>
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</BLOCKQUOTE>
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In the example immediately above, the expression <tt>(setf y 'hello)</tt> is never evaluated since <tt>(rest '(a))</tt> returns nil. You can check this out by evaluating y directly:
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<BLOCKQUOTE>
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<PRE>> y
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49
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</PRE>
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</BLOCKQUOTE>
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<tt>or</tt> returns the result of its first non-nil argument, and does not evaluate the rest of its arguments. If all the arguments evaluate to nil, then <tt>or</tt> returns nil. Examples:
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<BLOCKQUOTE>
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<PRE>> (or nil nil 2 (setf y 'goodbye))
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2
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> (or (rest '(a)) (equal 3 4))
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NIL
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</PRE>
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</BLOCKQUOTE>
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Once again, you will see that y's value is unchanged by these examples.
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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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