The two function calls do NOT give the same answer because std::sort
is not a stable algorithm, i.e. it does not keep identical elements in their relative ordenings. Below an example where elements of std::pair<int, int>
are sorted on their first element. Sorting and sorting in reverse order with the reversed comparison function does not yield identical sequences. Doing the same with std::stable_sort
does yield identical results.
#include <algorithm>
#include <iostream>
#include <ios>
#include <vector>
int main()
{
typedef std::pair<int, int> Element;
std::vector<Element> v;
v.push_back( Element(1,1) );
v.push_back( Element(-1,1) );
v.push_back( Element(1,2) );
v.push_back( Element(-1,2) );
v.push_back( Element(1,3) );
v.push_back( Element(-1,3) );
v.push_back( Element(1,4) );
v.push_back( Element(-1,4) );
v.push_back( Element(1,5) );
v.push_back( Element(-1,5) );
v.push_back( Element(1,6) );
v.push_back( Element(-1,6) );
v.push_back( Element(1,16) );
v.push_back( Element(-1,16) );
v.push_back( Element(1,22) );
v.push_back( Element(-1,22) );
v.push_back( Element(1,33) );
v.push_back( Element(-1,33) );
v.push_back( Element(1,44) );
v.push_back( Element(-1,44) );
v.push_back( Element(1,55) );
v.push_back( Element(-1,55) );
v.push_back( Element(1,66) );
v.push_back( Element(-1,66) );
for (auto it = v.begin(); it != v.end(); ++it) {
std::cout << "(" << it->first << "," << it->second << ")" << " ";
}
std::cout << "\n";
auto w1 = v;
std::sort(w1.begin(), w1.end(), [](Element const& e1, Element const& e2){
return e1.first < e2. first;
});
auto w2 = v;
std::sort(w2.rbegin(), w2.rend(), [](Element const& e1, Element const& e2) {
return e1.first > e2.first;
});
std::cout << std::boolalpha << std::equal(w1.begin(), w1.end(), w2.begin()) << "\n";
auto w3 = v;
std::stable_sort(w3.begin(), w3.end(), [](Element const& e1, Element const& e2){
return e1.first < e2. first;
});
auto w4 = v;
std::stable_sort(w4.rbegin(), w4.rend(), [](Element const& e1, Element const& e2) {
return e1.first > e2.first;
});
std::cout << std::boolalpha << std::equal(w3.begin(), w3.end(), w4.begin()) << "\n";
}
Output on LiveWorkSpace