Cleaner if statement sequence in C++

I am a bit of a newbie so please go easy on me. I wanted to know if there was a way to shorten these if statement chains like the one shown below in C++.

int offset;
    
string S;
cin >> S;

if(S == "mon")
{ offset = 0; }
else if(S == "tues")
{ offset = 1; }
else if(S == "wed")
{ offset = 2; }
else if(S == "thurs")
{ offset = 3; }
else if(S == "fri")
{ offset = 4; }
else if(S == "sat")
{ offset = 5; }
else if(S == "sun")
{ offset = 6; }

I would suggest using std::map.

std::map<std::string, int> m { 
  {"mon", 0}, {"tues", 1}, {"wed", 2}, {"thurs", 3},
  {"fri", 4}, {"sat", 5}, {"sun", 6}
};

std::string s;
std::cin >> s;

int offset = m.at(s);

If s is not a valid key, the std::out_of_range exception will be thrown. You might handle that by putting a try block in a loop and gathering input until you get a valid key, as shown below.

#include <iostream>
#include <stdexcept>
#include <map>

int main() {
  std::map<std::string, int> m { 
    {"mon", 0}, {"tues", 1}, {"wed", 2}, {"thurs", 3},
    {"fri", 4}, {"sat", 5}, {"sun", 6}
  };

  std::string s;

  while (true) {
    try {
      std::cin >> s;
      int offset = m.at(s);
      std::cout << offset << std::endl;
      break;
    }
    catch (std::out_of_range e) {
      std::cerr << "Invalid input: " << s << std::endl;      
    }
  }

  return 0;
}

You could also refactor your original code to use ? : (the actual name is somewhat debatable). Of course, this requires us to provide an else case, so let's go with -1.

int offset =
  s == "mon"   ? 0 :
  s == "tues"  ? 1 :
  s == "wed"   ? 2 :
  s == "thurs" ? 3 :
  s == "fri"   ? 4 :
  s == "sat"   ? 5 :
  s == "sun"   ? 6 : -1;