129 lines
2.8 KiB
HTML
129 lines
2.8 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> Equality Predicates</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Equality Predicates">
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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="node26.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="node24.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="node26.html"> Checking for NIL</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="node24.html"> More Predicates and </A>
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<BR> <HR> <P>
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<H2> Equality Predicates</H2>
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<P>
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Common Lisp contains a number of equality predicates. Here are the four most commonly used:
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<P>
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<DL COMPACT><DT><b>=</b>
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<DD> (= x y) is true if and only x and y are numerically equal.
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<P>
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<DT><b>equal</b>
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<DD> As a rule of thumb, (equal x y) is true if their printed representations are the same (i.e. if they look the same when printed). Strictly, x and y are equal if and only if they are structurally isomorphic, but for present purposes, the rule of thumb is sufficient.
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<P>
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<DT><b>eq</b>
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<DD> (eq x y) is true if and only if they are the same object (in most cases, this means the same object in memory).
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<P>
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<DT><b>eql</b>
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<DD> (eql x y) is true if and only if they are either eq or they are numbers of the same type and value.
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<P>
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</DL>
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<P>
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Generally = and equal are more widely used than eq and eql.
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<P>
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Here are some examples involving numbers:
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<BLOCKQUOTE>
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<PRE>> (= 3 3.0)
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T
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> (= 3/1 6/2)
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T
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> (eq 3 3.0)
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NIL
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> (eq 3 3)
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T or NIL (depending on implementation of Common Lisp)
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> (eq 3 6/2)
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T
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> (eq 3.0 6/2)
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NIL
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> (eql 3.0 3/1)
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NIL
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> (eql 3 6/2)
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T
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> (equal 3 3)
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T
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> (equal 3 3.0)
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NIL
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</PRE>
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</BLOCKQUOTE>
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Suppose now we have the following variable assignments:
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<BLOCKQUOTE>
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<PRE>> (setf a '(1 2 3 4))
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(1 2 3 4)
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> (setf b '(1 2 3 4))
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(1 2 3 4)
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> (setf c b)
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(1 2 3 4)
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</PRE>
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</BLOCKQUOTE>
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Then:
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<BLOCKQUOTE>
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<PRE>> (eq a b)
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NIL
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> (eq b c)
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T
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> (equal a b)
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T
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> (equal b c)
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T
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> (eql a b)
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NIL
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> (eql b c)
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T
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> (= (first a) (first b))
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T
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> (eq (first a) (first b))
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T or NIL (depending on implementation of Common Lisp)
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> (eql (first a) (first b))
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T
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</PRE>
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</BLOCKQUOTE>
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In most cases, you will want to use either = or equal, and fortunately these are the easiest to understand. Next most frequently used is eq. Eql is used by advanced programmers.
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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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