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74 lines
4.2 KiB
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Special Operator IF</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="m_cond.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_data_a.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="m_or.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<HR>
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<A NAME="if"><I>Special Operator</I> <B>IF</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>if</B> <I><I>test-form</I> <I>then-form</I> [<I>else-form</I>]</I> => <I><I>result</I><B>*</B></I><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>Test-form</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
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<I>Then-form</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
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<I>Else-form</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. The default is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<I>results</I>---if the <I>test-form</I> <I>yielded</I> <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> returned by the <I>then-form</I>; otherwise, the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> returned by the <I>else-form</I>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#if"><B>if</B></A> allows the execution of a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> to be dependent on a single <I>test-form</I>. <P>
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First <I>test-form</I> is evaluated. If the result is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>, then <I>then-form</I> is selected; otherwise <I>else-form</I> is selected. Whichever form is selected is then evaluated. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(if t 1) => 1
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(if nil 1 2) => 2
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(defun test ()
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(dolist (truth-value '(t nil 1 (a b c)))
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(if truth-value (print 'true) (print 'false))
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(prin1 truth-value))) => TEST
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(test)
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>> TRUE T
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>> FALSE NIL
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>> TRUE 1
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>> TRUE (A B C)
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=> NIL
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</PRE>
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</TT> <P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>Exceptional Situations:</B> None.
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<P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="m_cond.htm#cond"><B>cond</B></A>, <A REL=DEFINITION HREF="m_when_.htm#unless"><B>unless</B></A>, <A REL=DEFINITION HREF="m_when_.htm#when"><B>when</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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<PRE>
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(if test-form then-form else-form)
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== (cond (test-form then-form) (t else-form))
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</PRE>
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</TT> <P>
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<HR>
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