How to remove duplicated items in a sorted vector

Question

I currently have a vector<int> c which contains {1,2,2,4,5,5,6} and I want to remove the duplicated numbers so that c will have {1,4,6}. A lot of solutions on the internet seem to just remove one of the duplicates, but I'm trying to remove all occurrences of the duplicated number.

Assume c is always sorted.

I currently have

#include <iostream>
#include <vector>

int main() {
  vector<int> c{1,2,2,4,5,5,6};
  for (int i = 0; i < c.size()-1; i++) {
    for (int j=1; j<c.size();j++){
      if(c[i] == c[j]){
         // delete i and j? 
      }
    }
  }
}

I tried to use two for-loops so that I can compare the current element and the next element. This is where my doubt kicked in. I'm not sure if I'm approaching the problem correctly.

Could I get help on how to approach my problem?

Answer 1

Generally one has to be very careful when deleting from containers while iterating over them. C++ STL can do this easily and faster (on average) than using nested loops.

#include <vector> 
#include <algorithm>
#include <unordered_set>

int main() {
    std::vector<int> c{1,2,2,4,5,5,6};
    std::unordered_multiset<int> unique( c.begin(), c.end() );
    c.erase(std::remove_if(c.begin(),c.end(),[&](const auto& e){return unique.count(e)>1;}),c.end());

    for(auto e: c){
        std::cout<<e<<' ';
    }
}

//Output: 1 4 6

Alternatively, you could use std::map<int,std::size_t> and count the occurences this way.

Answer 2

Use std::remove_if to move items occurring multiple times to the rear, then erase them.

#include <iostream>
#include <vector>
#include <algorithm>


int main()
{

    std::vector<int> V {1,2,2,4,5,5,6};

    auto it = std::remove_if(V.begin(), V.end(), [&](const auto& val) 
    { 
        return std::count(V.begin(), V.end(), val) > 1;
    });

    V.erase(it, V.end());

    for (const auto& val : V)
        std::cout << val << std::endl;
    
    return 0;
}

Output:

1
4
6

For demo: https://godbolt.org/z/j6fxe1