Compute intersection of two lists - Method 1

list_intersection_1.cpp

#include <iostream>
#include <string>
#include <unordered_set>
#include <list>
#include <algorithm>
using namespace std;

list<string> compare_list_warning(list<string> const& list_one, list<string> const& list_two)
{
  list<string> const& small_list = list_one.size() < list_two.size() ? list_one : list_two;
  list<string> const& big_list = list_one.size() < list_two.size() ? list_two : list_one;

  list<string> duplicates;
  unordered_set<string> duplicate_finder;
  duplicate_finder.insert(begin(small_list), end(small_list));
  copy_if(begin(big_list), end(big_list), back_inserter(duplicates), [&](string const& v) { return (duplicate_finder.find(v) != end(duplicate_finder)); });
  return  duplicates;
}

int main() {
  list<string> common_names = compare_list_warning({"Roger", "Marcel", "Camille", "Hubert"}, {"Huguette", "Cunegond", "Marcelle", "Camille"});
  for(string const& common : common_names) {
    std::cout << "Common element: " << common << "\n";
  }
  std::cout << std::flush;
  return 0;
}

Hi this is AK_, I really loved the way you optimized my code and used set instead of unordered_map since we dont really care about the stored values. Could you please explain line 16: copy_if ?

Also can you edit my code on SO so we can accept it as an answer..

Hi AK_

I'm sorry, I only saw you comment right now, it does not appear in the regular github notification hub :p

The copy_if is just another way of writing the same thing than you : it iterates over the first collection and pushes the elements that satisfy the given condition back the second one (hence the back_inserter). The condition is provided as a function, here written as a lambda.

Did the code modification on SO, its waiting for peer review