Efficient way to compare more 50 strings

I have method which takes two parameter one as string and other as int.

The string has to compare with more than 50 string and Once the match is found int value need to be mapped with hard coded string as Example below

EX:

  string Compare_Method(std::string str, int val) {

     if(str == "FIRST")
{
  std::array<std::string, 3> real_value = {"Hello1","hai1","bye1"}
  return real_value[val];
}

     else if(str == "SECOND")
{
  std::array<std::string, 3> real_value = {"Hello2","hai2","bye2"}
  return real_value[val];
}

     else if(str == "THIRD")
{
  std::array<std::string, 3> real_value = {"Hello3","hai3","bye3"}
  return real_value[val];
}

//----- 50+ else if

}

My approach is as above. What will be the efficient way

1.To compare more than 50 string.

2. create std::array for each if case