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142 lines
7.3 KiB
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>6.4. Logical Operators</TITLE>
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<BODY>
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<meta name="description" value=" Logical Operators">
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<meta name="keywords" value="clm">
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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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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html2444 HREF="node76.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2442 HREF="node69.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2438 HREF="node74.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2446 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2447 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2445 HREF="node76.html"> Control Structure</A>
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<B>Up:</B> <A NAME=tex2html2443 HREF="node69.html"> Predicates</A>
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<B> Previous:</B> <A NAME=tex2html2439 HREF="node74.html"> Equality Predicates</A>
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<HR> <P>
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<H1><A NAME=SECTION001040000000000000000>6.4. Logical Operators</A></H1>
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<P>
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Common Lisp provides three operators on Boolean values: <tt>and</tt>, <tt>or</tt>,
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and <tt>not</tt>. Of these, <tt>and</tt> and <tt>or</tt>
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are also control structures because their arguments are evaluated
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conditionally.
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The function <tt>not</tt> necessarily examines its single argument, and so
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is a simple function.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>not</tt> <tt><i>x</i></tt><P><tt>not</tt> returns <tt>t</tt> if <i>x</i> is <tt>nil</tt>, and otherwise returns <tt>nil</tt>.
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It therefore inverts its argument considered as a Boolean value.
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<P>
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<tt>null</tt> is the same as <tt>not</tt>; both functions are included for the sake
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of clarity. As a matter of style,
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it is customary to use <tt>null</tt> to check whether something is the empty list
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and to use <tt>not</tt> to invert the sense of a logical value.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>and</tt> <tt>{<i>form</i>}*</tt><P><tt>(and <i>form1</i> <i>form2</i> ... )</tt> evaluates each <i>form</i>, one at a time,
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from left to right. If any <i>form</i> evaluates to <tt>nil</tt>, the value <tt>nil</tt>
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is immediately returned without evaluating the remaining
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<i>form</i>s. If every <i>form</i> but the last evaluates to a non-<tt>nil</tt> value,
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<tt>and</tt> returns whatever the last <i>form</i> returns.
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Therefore in general <tt>and</tt> can be used both for logical operations,
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where <tt>nil</tt> stands for <i>false</i> and non-<tt>nil</tt> values stand for <i>true</i>,
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and as a conditional expression.
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An example follows.
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<P><pre>
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(if (and (>= n 0)
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(< n (length a-simple-vector))
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(eq (elt a-simple-vector n) 'foo))
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(princ "Foo!"))
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</pre><P>
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The above expression prints <tt>Foo!</tt> if element <tt>n</tt> of <tt>a-simple-vector</tt>
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is the symbol <tt>foo</tt>, provided also that <tt>n</tt> is indeed a valid index
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for <tt>a-simple-vector</tt>. Because <tt>and</tt> guarantees left-to-right testing
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of its parts, <tt>elt</tt> is not called if <tt>n</tt> is out of range.
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<P>
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To put it another way,
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the <tt>and</tt> special form does <i>short-circuit</i> Boolean evaluation,
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like the <b>and then</b> operator in Ada
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and what in some Pascal-like languages is called <b>cand</b> (for ``conditional
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and''); the Lisp <tt>and</tt> special form is
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unlike the Pascal or Ada <b>and</b> operator,
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which always evaluates both arguments.
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<P>
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In the previous example writing
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<P><pre>
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(and (>= n 0)
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(< n (length a-simple-vector))
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(eq (elt a-simple-vector n) 'foo)
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(princ "Foo!"))
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</pre><P>
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would accomplish the same thing. The difference is purely stylistic.
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Some programmers never use expressions containing side effects
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within <tt>and</tt>, preferring to use <tt>if</tt> or <tt>when</tt> for that purpose.
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<P>
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From the general definition, one can deduce that
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<tt>(and <i>x</i>)</tt> == <i>x</i>. Also,
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<tt>(and)</tt> evaluates to <tt>t</tt>, which is an identity for this operation.
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<P>
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One can define <tt>and</tt> in terms of <tt>cond</tt> in this way:
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<P><pre>
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(and <i>x</i> <i>y</i> <i>z</i> ... <i>w</i>) == (cond ((not <i>x</i>) <tt>nil</tt>)
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((not <i>y</i>) <tt>nil</tt>)
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((not <i>z</i>) <tt>nil</tt>)
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...
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(<tt>t</tt> <i>w</i>))
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</pre><P>
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<P>
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See <tt>if</tt> and <tt>when</tt>, which are sometimes stylistically
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more appropriate than <tt>and</tt> for conditional purposes.
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If it is necessary to test whether a predicate is true
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of all elements of a list or vector (element 0 <i>and</i> element 1 <i>and</i>
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element 2 <i>and</i> <b>...</b>), then the function <tt>every</tt> may be useful.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>or</tt> <tt>{<i>form</i>}*</tt><P><tt>(or <i>form1</i> <i>form2</i> ... )</tt> evaluates each <i>form</i>, one at a time,
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from left to right. If any <i>form</i> other than the last
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evaluates to something other than <tt>nil</tt>,
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<tt>or</tt>
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immediately returns that non-<tt>nil</tt> value without evaluating the remaining
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<i>form</i>s. If every <i>form</i> but the last evaluates to <tt>nil</tt>,
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<tt>or</tt> returns whatever evaluation of the last of the <i>form</i>s returns.
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Therefore in general <tt>or</tt> can be used both for logical operations,
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where <tt>nil</tt> stands for <i>false</i> and non-<tt>nil</tt> values stand for <i>true</i>,
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and as a conditional expression.
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<P>
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To put it another way,
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the <tt>or</tt> special form does <i>short-circuit</i> Boolean evaluation,
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like the <b>or else</b> operator in Ada
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and what in some Pascal-like languages is called <b>cor</b> (for ``conditional
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or''); the Lisp <tt>or</tt> special form is
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unlike the Pascal or Ada <b>or</b> operator,
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which always evaluates both arguments.
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<P>
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From the general definition, one can deduce that
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<tt>(or <i>x</i>)</tt> == <i>x</i>. Also,
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<tt>(or)</tt> evaluates to <tt>nil</tt>, which is the identity for this operation.
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<P>
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One can define <tt>or</tt> in terms of <tt>cond</tt> in this way:
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<P><pre>
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(or <i>x</i> <i>y</i> <i>z</i> ... <i>w</i>) == (cond (<i>x</i>) (<i>y</i>) (<i>z</i>) ... (<tt>t</tt> <i>w</i>))
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</pre><P>
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<P>
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See <tt>if</tt> and <tt>unless</tt>, which are sometimes
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stylistically more appropriate than <tt>or</tt> for conditional purposes.
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If it is necessary to test whether a predicate is true of
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one or more elements of a list or vector (element 0 <i>or</i> element 1 <i>or</i>
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element 2 <i>or</i> <b>...</b>), then the function <tt>some</tt> may be useful.
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<P>
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<P>
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<BR> <HR><A NAME=tex2html2444 HREF="node76.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2442 HREF="node69.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2438 HREF="node74.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2446 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2447 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2445 HREF="node76.html"> Control Structure</A>
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<B>Up:</B> <A NAME=tex2html2443 HREF="node69.html"> Predicates</A>
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<B> Previous:</B> <A NAME=tex2html2439 HREF="node74.html"> Equality Predicates</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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