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);