将字母等级转换为数字等级

因此,我的一项任务遇到了一些麻烦。我需要让用户输入字母等级(B,B+/-),并将其转换为数字等级(3,2.7)。我可以让程序只与A、B、C一起工作,但我不知道当用户将分数加上+/-时如何让程序工作。这就是我目前所拥有的。

# include <iostream>

using namespace std;

int main()
{
   string input;
   string A;
   string A-;
   //string B+;
   string B;
   //string B-;
   //string C+;
   string C;
   //string C-;
   //string D+;
   string D;
   //string D-;
   string F;

   cout << "Enter a letter grade: ";

   cin >> input;
   if (input == "A")
       cout << "The numeric value is 4.0" <<endl;

   if (input == "A-")
       cout << "The numeric value is 3.7" << endl;

   if (input == "B+")
       cout << "The numeric value is 3.3" << endl;

   if (input == "B")
       cout << "The numeric value is 3" <<endl;

   if (input == "C")
       cout << "The numeric value is 2" <<endl;

   if (input == "D")
       cout << "The numeric value is 1" <<endl;

   if (input == "F")
       cout << "The numeric value is 0" <<endl;

   return 0;
   }