使用cin.get（）从cin获取输入？

Question

I have a double question for you. I am pretty new to c++ and I trying to mutate this program so that it can accept variables and store them in a map. My problem is, I actually have no idea where the program gets its input from the user!

I understand how it goes through cin to evaluate the characters, but where it get the original string is a little mind boggling.

I assume it takes its input here?

   int result = 0;
   char c = cin.peek();

My basic problem is that I am trying to get the program to accept "x+3" as input. If x hasn't been used before, as the user for the input and then store the value in the map. If it has been used, retrieve it from the map. I don't want you guys to solve it for me, but a general direction would be really helpful.

So I guess my two questions are:

1.Where does the program get the user input?

2.What is the best way to get recognize if there is a character in the stream? (I see isalpha() can work, is that the right direction?) Should I copy the stream a string or something to work with it?

#include <iostream>
#include <cctype>

using namespace std;

int term_value();
int factor_value();

/**
   Evaluates the next expression found in cin.
   @return the value of the expression.
*/
int expression_value()
{
   int result = term_value();
   bool more = true;
   while (more)
   {
      char op = cin.peek();
      if (op == '+' || op == '-')
      {
         cin.get();
         int value = term_value();
         if (op == '+') result = result + value;
         else result = result - value;
      }
      else more = false;
   }
   return result;
}

/**
   Evaluates the next term found in cin.
   @return the value of the term.
*/
int term_value()
{
   int result = factor_value();
   bool more = true;
   while (more)
   {
      char op = cin.peek();
      if (op == '*' || op == '/')
      {
         cin.get();
         int value = factor_value();
         if (op == '*') result = result * value;
         else result = result / value;
      }
      else more = false;
   }
   return result;
}

/**
   Evaluates the next factor found in cin.
   @return the value of the factor.
*/
int factor_value()
{
   int result = 0;
   char c = cin.peek();
   if (c == '(')
   {
      cin.get();
      result = expression_value();
      cin.get(); // read ")"
   }
   else // Assemble number value from digits
   {
      while (isdigit(c))
      {
         result = 10 * result + c - '0';
         cin.get();
         c = cin.peek();
      } 
   }
   return result;
}

int main()
{

   cout << "Enter an expression: ";
   cout << expression_value() << "\n";
   return 0;
}

EDIT 1: My idea is this:

Take the input and copy it to a stringstream that I will pass by reference to the functions. So I can use peek and the such on the stringstream.

After that, I will take the user input from cin when I need more user input for variable values.

Answer 1

I'd recommend you to read user's input with std::getline and apply some expression parsing algorithm to lines being read. Parsing of user input is too diffucult to be done this way. Most people mase use of parser generators such as ANTLR or boost::spirit for such tasks.