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<HTML><HEAD><TITLE>3.2 Predicates</TITLE></HEAD><BODY><A HREF="ug1.htm"><IMG SRC="images/banner.gif"></A><BR><A HREF="fun_5137.htm"><IMG SRC="images/prev.gif"></A><A HREF="booktoc1.htm"><IMG SRC="images/toc.gif"></A><A HREF="tindex1.htm"><IMG SRC="images/tindex.gif"></A><A HREF="fun_0476.htm"><IMG SRC="images/next.gif"></A><BR><STRONG>Click on the banner to return to the user guide home page.</STRONG><H2>3.2 Predicates</H2><A NAME="idx28"><!></A><P>A <I>predicate</I> is simply a function that returns either a boolean (true/false) value or an integer value. Following the normal C convention, an integer value is assumed to be true if non-zero, and false otherwise. An example function might be the following, which takes as argument an integer and returns <SAMP>true</SAMP> if the number represents a leap year, and <SAMP>false</SAMP> otherwise:</P><PRE>bool isLeapYear (unsigned int year) // return true if year is leap year{ // millennia are leap years if (0 == year % 1000) return true; // every fourth century is if (0 == year % 400) return true; // every fourth year is if (0 == year % 4) return true; // otherwise not return false;}</PRE><P>A predicate is used as an argument, for example, in the generic algorithm named <SAMP>find_if()</SAMP>. This algorithm returns the first value that satisfies the predicate, returning the end-of-range value if no such element is found. Using this algorithm, the following locates the first leap year in a list of years:</P><PRE>list<int>::iterator firstLeap = find_if(aList.begin(), aList.end(), isLeapYear);</PRE><HR><A HREF="fun_5137.htm"><IMG SRC="images/prev.gif"></A> <A HREF="booktoc1.htm"><IMG SRC="images/toc.gif"></A><A HREF="tindex1.htm"><IMG SRC="images/tindex.gif"></A><A HREF="fun_0476.htm"><IMG SRC="images/next.gif"></A><P>©Copyright 1996, Rogue Wave Software, Inc.</P></BODY></HTML>?? 快捷鍵說明
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