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<HTML><HEAD><TITLE>Predicates</TITLE></HEAD>
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<P>&copy;Copyright 1996 Rogue Wave Software</P>
<H2>Predicates</H2>
<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&#60;int>::iterator firstLeap = 
   find_if(aList.begin(), aList.end(), isLeapYear);</PRE>

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