How to compare two arrays and return non matching values in C++

Question

I would like to parse through two vectors of strings and find the strings that match each other and the ones that do not.

Example of what I want get:
input vector 1 would look like: [string1, string2, string3]
input vector 2 would look like: [string2, string3, string4]

Ideal output:
string1: No Match
string2: Match
string3: Match
string4: No Match

At the moment I use this code:

vector<string> function(vector<string> sequences, vector<string> second_sequences){

 for(vector<string>::size_type i = 0; i != sequences.size(); i++) {
  for(vector<string>::size_type j = 0; j != second_sequences.size(); j++){
   if (sequences[i] == second_sequences[j]){
     cout << "Match: " << sequences[i];
    }else{
     cout << "No Match: " << sequences[i];
     cout << "No Match: " << second_sequences[j];
   }
  }
 }  
}

It works great for the ones that match, but iterates over everything so many times,
and the ones that do not match get printed a large number of times.

How can I improve this?

Answer 1

The best code is the code that you did not have to write.

If you take a (STL) map container it will take care for you of sorting and memorizing the different strings you encounter.

So let the container works for us.

I propose a small code quickly written. You need for this syntax to enable at least the C++ 2011 option of your compiler ( -std=c++11 on gcc for example ). The syntax that should be used before C++11 is much more verbose (but should be known from a scholar point of view ).

You have only a single loop. This is only a hint for you ( my code does not take into account that in the second vector string4 could be present more than once, but I let you arrange it to your exact needs)

#include <iostream>
#include <vector>
#include <string>
#include <map>


using namespace std;


vector<string> v1 { "string1","string2","string3"};
vector<string> v2 { "string2","string3","string4"};

//ordered map will take care of "alphabetical" ordering
//The key are the strings
//the value is a counter ( or could be any object of your own
//containing more information )
map<string,int> my_map;

int main()
{
    cout << "Hello world!" << endl;

    //The first vector feeds the map before comparison with
    //The second vector
    for ( const auto & cstr_ref:v1)
        my_map[cstr_ref] = 0;

    //We will look into the second vector ( it could also be the third,
    //the fourth... )
    for ( const auto & cstr_ref:v2)
    {
        auto iterpair = my_map.equal_range(cstr_ref);

        if ( my_map.end() != iterpair.first )
        {
            //if the element already exist we increment the counter
            iterpair.first->second += 1;
        }
        else
        {
            //otherwise we put the string inside the map
            my_map[cstr_ref] = 0;
        }

    }

    for ( const auto & map_iter: my_map)
    {
        if ( 0 < map_iter.second )
        {
            cout  << "Match    :";
        }
        else
        {
            cout  << "No Match :" ;
        }

        cout << map_iter.first << endl;
    }


    return 0;
}

Output:

No Match :string1
Match    :string2
Match    :string3
No Match :string4

Answer 2

std::sort(std::begin(v1), std::end(v1));
std::sort(std::begin(v2), std::end(v2));

std::vector<std::string> common_elements;
std::set_intersection(std::begin(v1), std::end(v1)
                    , std::begin(v2), std::end(v2)
                    , std::back_inserter(common_elements));

for(auto const& s : common_elements)
{
    std::cout<<s<<std::endl;
}