HP Integrity NonStop H-Series - Manual (Page 40)

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©Copyright 1996 Rogue Wave Software
Predicates
A predicate 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 true if the number represents a leap year, and false otherwise:
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;
}
A predicate is used as an argument, for example, in the generic algorithm named find_if(). 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:
list<int>::iterator firstLeap =
find_if(aList.begin(), aList.end(), isLeapYear);