50 lines
2 KiB
HTML
50 lines
2 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> If</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" If">
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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="node23.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node21.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node21.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="node23.html"> Cond</A>
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<B>Up:</B>
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<A HREF="node21.html"> Conditional Control</A>
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<B> Previous:</B>
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<A HREF="node21.html"> Conditional Control</A>
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<BR> <HR> <P>
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<H2> If</H2>
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<P>
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An if statement has the form:
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<BLOCKQUOTE>
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<PRE> (if <test> <then> <else> )
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</PRE>
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</BLOCKQUOTE>
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The test, then, and else expressions can be any evaluable Lisp expressions -- e.g., symbols or lists. If the evaluation of the test expression returns anything other than nil, e.g. T, 3, FOO, (A S D F), the interpreter evaluates the then expression and returns its value, otherwise it returns the result of evaluating the else expression.
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<P>
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We can use if to define a function to return the absolute difference between two numbers, by making use of the predicate <b>> </b> (greater than). Here it is:
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<BLOCKQUOTE>
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<PRE>(defun absdiff (x y)
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(if (> x y)
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(- x y)
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(- y x)))
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</PRE>
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</BLOCKQUOTE>
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If x is greater than y, then the test, i.e. (<b>> </b> x y), returns T, so the then clause is evaluated, in this case (- x y), which gives the positive difference. If x is less than or equal to y, then the expression (- y x) gets evaluated, which will return 0 or a positive difference.
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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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