Even-Odd sorting

From:
=?ISO-8859-1?Q?Ney_Andr=E9_de_Mello_Zunino?= <zunino@inf.ufsc.br>
Newsgroups:
comp.lang.c++
Date:
Fri, 23 May 2008 16:41:36 -0300
Message-ID:
<g176li$fj0$1@aioe.org>
Hello.

It seems a year is all it takes for one's proficiency in C++ to become
too rusty. Professionally, I've been away from the language (and from
programming in general), but I still preserve an appreciation for it.

So I decided to toy with some idea, just for fun and for evaluating how
rusty I'd become. "Let me write a simple functor for sorting the
elements of a collection! I will start with a simple collection of
integers.", I thought. I then chose an unusual criterion for the
sorting: "I'd like the elements to be ordered so that the even members
should appear before the odd ones." The following is what I came up
with. The program is not producing the expected behavior and I suspect
it has to do with my predicate. It is probably not fully complying to
the requirements for such a function.

#include <iostream>
#include <set>
#include <functional>
#include <algorithm>
#include <iterator>

class EvenOddSorting {
public:
     bool operator()(const int i, const int j) const {
         return i % 2 == 0 && j % 2 != 0;
     }
};

int main() {
     std::set<int, EvenOddSorting> intSet;
     intSet.insert(10);
     intSet.insert(7);
     intSet.insert(5);
     intSet.insert(18);
     intSet.insert(3);
     std::cout << "Even-Odd ordered set: ";
     std::copy(intSet.begin(),
               intSet.end(),
               std::ostream_iterator<int>(std::cout, " "));
     std::cout << "\n";
}

Given the above program, the expected output should read:

Even-Odd ordered set: 10 18 3 5 7

Notice how my proposed output implies a hidden requirement of ordering
among even and odd numbers. That would be ideal, but I'd be satisfied
(at first) with an output like '18 10 5 7 3'. Would anybody care to give
me a hand?

Thank you!

--
Ney Andr? de Mello Zunino

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