C++ vector of strings into associative vector of ints

Im having trouble to convert a string vector with size of ~ 1.0000.0000 elements to an associative vector with integers.

Input:

std::vector<std::string> s {"a","b","a","a","c","d","a"};

Desired output:

std::vector<int> i {0,1,0,0,2,3,0};

I was thinking of an std::unordered_multiset as mentioned in Associative Array with Vector in C++ but i can't get it running.

The goal is to reduce the time it takes to convert c++ strings to R strings, which is so much faster if I just use numbers.

Thank you for your help!

Edit:

Thats how I tried to populate the set:

for (size_t i = 0; i < s.size(); i++)
{
        set.insert(s[i]);
}

This code will output your desired output for your given input. And it will process 1.000.000 strings of length 3 in 0.4s. So I think unordered_map is a viable choice.

#include <string>
#include <iostream>
#include <unordered_map>
#include <chrono>
#include <random>

// generator function for creating a large number of strings.
std::vector<std::string> generate_strings(const std::size_t size, const std::size_t string_length)
{
    static std::random_device rd{};
    static std::default_random_engine generator{ rd() };
    static std::uniform_int_distribution<int> distribution{ 'a', 'z' };

    std::vector<std::string> strings;
    std::string s(string_length, ' ');

    for (std::size_t n = 0; n < size; n++)
    {
        for (std::size_t m = 0; m < string_length; ++m)
        {
            s[m] = static_cast<char>(distribution(generator));
        }

        strings.emplace_back(s);
    }

    return strings;
}

int main() 
{
    std::vector<std::string> strings = generate_strings(1000000, 3);
    //std::vector<std::string> strings{ "a","b","a","a","c","d","a" };

    std::unordered_map<std::string, int> map;
    std::vector<int> output;

    // speed optimization, allocate enough room for answer
    // so output doesn't have to reallocate when growing.
    output.reserve(strings.size());

    auto start = std::chrono::high_resolution_clock::now();

    int id = 0;
    for (const auto& string : strings)
    {
        if (map.find(string) == map.end())
        {
            output.push_back(id);
            map.insert({ string, id });
            id++;
        }
        else
        {
            output.push_back(map.at(string));
        }
    }

    auto duration = std::chrono::high_resolution_clock::now() - start;
    auto nanoseconds = std::chrono::duration_cast<std::chrono::nanoseconds>(duration).count();

    auto seconds = static_cast<double>(nanoseconds) / 1.0e9;

    /*
    for (const auto& value : output)
    {
        std::cout << value << " ";
    }
    */

}