Re: Even-Odd sorting
On May 23, 3:41 pm, Ney Andr=E9 de Mello Zunino <zun...@inf.ufsc.br>
wrote:
[snip]
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."
[snip]
The program is not producing the expected behavior and I suspect
it has to do with my predicate.
[snip]
class EvenOddSorting {
public:
bool operator()(const int i, const int j) const {
return i % 2 == 0 && j % 2 != 0;
}
};
[snip]
std::set<int, EvenOddSorting> intSet;
[snip]
Your main problem is your predicate along with your use of an
std::set. Note that a set can only hold unique items. That is, no two
elements a and b exist in a set such that a == b.
Your predicate tests if a < b. If a < b OR b < a then a != b. If (NOT
a < b) AND NOT (b < a) then a == b. The set uses your predicate to
determine uniqueness. Because your predicate only compares based on
parity and not values, in effect you have defined all even numbers to
be equivalent, and all odd numbers to be equivalent. Therefore your
set, which must only contain unique values, will contain one even
number, and one odd number.
If you want the value of the integers to be a criteria for uniqueness
your predicate must take that into account. You can also, then, have
sorting "within" the even and odd numbers, if you do something like
this instead:
class EvenOddValueSorting {
public:
bool operator()(const int i, const int j) const {
bool i_is_even = !(i % 2);
bool j_is_even = !(j % 2);
if (i_is_even == j_is_even) // if parity is same...
return i < j; // ... compare by value only
else
return i_is_even; // ... otherwise even always < odd.
}
};
Alternatively, you can use the predicate you have now for sorting, but
do not use a container that requires value uniqueness. For example,
use a vector with your current predicate:
std::vector<int> v;
v.push_back(10);
v.push_back(7);
v.push_back(5);
v.push_back(18);
v.push_back(3);
std::sort(v.begin(), v.end(), EvenOddSorting());
Doing that with EvenOddSorting will place all the even numbers before
all the odd ones. Doing it with EvenOddValueSorting will sort by
numeric value also. Note that std::sort is not stable; but you can use
std::stable_sort if that is a requirement. See:
http://www.sgi.com/tech/stl/table_of_contents.html
Jason