Re: Even-Odd sorting

Victor Bazarov <>
Fri, 23 May 2008 16:02:31 -0400
Ney Andr? de Mello Zunino wrote:

So I decided [..] "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 {
    bool operator()(const int i, const int j) const {
        return i % 2 == 0 && j % 2 != 0;

What if they are both odd? What if they are both even? Does the
predicate satisfy the requirement of strict weak ordering?

The predicate returns 'true' if the first is even and the second is odd.
  For the rest it would return 'false', right? That means that when
comparing 5 and 7 it would determine that 5 is not less than 7. And
when comparing 7 and 5 it would determine that 7 is not less than 5.
That seems to me that 5 is the same as 7. That is, 5 and 7 cannot
coexist in the set that has your 'EvenOddSorting' as the predicate.

Do the same with two evens.


int main() {
    std::set<int, EvenOddSorting> intSet;

After this the collection should be {10, 7}. Is it?


After this the collection _could_ be 10, 5, 7, or it could be 10, 7, 5,
if the predicate allowed placing both 5 and 7 next to each other. It
does not, AFAICT. So, here the collection will be {10, 7}, still.

And so on.

    std::cout << "Even-Odd ordered set: ";
              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

OK, that's what it *should* read (you believe). What *does* it read?

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?

Not sure what kind of help you are seeking. You've not specified all
the requirements for your predicate to implement it correctly. Have I
given you enough hints to go on?

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