C++プログラムでの逆ポーランド記法の評価

理解を深めるために、次の実装を見てみましょう-

#include<iostream>
#include<cmath>
#include<stack>
#include<climits>
using namespace std;
float scanNum(char ch){
   int value;
   value = ch;
   return float(value-'0');//return float from character
}
int isOperator(char ch){
   if(ch == '+'|| ch == '-'|| ch == '*'|| ch == '/' || ch == '^')
      return 1;//character is an operator
   return -1;//not an operator
}
int isOperand(char ch){
   if(ch >= '0' && ch <= '9')
      return 1;//character is an operand
   return -1;//not an operand
}
float operation(int a, int b, char op){
   //Perform operation
   if(op == '+')
      return b+a;
   else if(op == '-')
      return b-a;
   else if(op == '*')
      return b*a;
   else if(op == '/')
      return b/a;
   else if(op == '^')
      return pow(b,a); //find b^a
   else
      return INT_MIN; //return negative infinity
}
float postfixEval(string postfix){
   int a, b;
   stack<float> stk;
   string::iterator it;
   for(it=postfix.begin(); it!=postfix.end(); it++){
      //read elements and perform postfix evaluation
      if(isOperator(*it) != -1){
         a = stk.top();
         stk.pop();
         b = stk.top();
         stk.pop();
         stk.push(operation(a, b, *it));
      }
      else if(isOperand(*it) > 0){
         stk.push(scanNum(*it));
      }
   }
   return stk.top();
}
main(){
   string post = "53+62/*35*+";
   cout << "The result is: "<<postfixEval(post);
}

入力

"53+62/*35*+"

出力

The result is: 39