What's the most efficient way to erase duplica

Question

I need to take a C++ vector with potentially a lot of elements, erase duplicates, and sort it.

I currently have the below code, but it doesn't work.

vec.erase(
      std::unique(vec.begin(), vec.end()),
      vec.end());
std::sort(vec.begin(), vec.end());

How can I correctly do this?

Additionally, is it faster to erase the duplicates first (similar to coded above) or perform the sort first? If I do perform the sort first, is it guaranteed to remain sorted after std::unique is executed?

Or is there another (perhaps more efficient) way to do all this?

Answer 1

More understandable code from: https://en.cppreference.com/w/cpp/algorithm/unique

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <cctype>

int main() 
{
    // remove duplicate elements
    std::vector<int> v{1,2,3,1,2,3,3,4,5,4,5,6,7};
    std::sort(v.begin(), v.end()); // 1 1 2 2 3 3 3 4 4 5 5 6 7 
    auto last = std::unique(v.begin(), v.end());
    // v now holds {1 2 3 4 5 6 7 x x x x x x}, where 'x' is indeterminate
    v.erase(last, v.end()); 
    for (int i : v)
      std::cout << i << " ";
    std::cout << "\n";
}

ouput:

1 2 3 4 5 6 7

Answer 2

std::set<int> s;
std::for_each(v.cbegin(), v.cend(), [&s](int val){s.insert(val);});
v.clear();
std::copy(s.cbegin(), s.cend(), v.cbegin());