eval.cpp

// Eval Function for C++
#include <bits/stdc++.h>
using namespace std;
int precedence(char op){
    if(op == '+'||op == '-')
    return 1;
    if(op == '*'||op == '/')
    return 2;
    return 0;
}
int applyOp(int a, int b, char op){
    switch(op){
        case '+': return a + b;
        case '-': return a - b;
        case '*': return a * b;
        case '/': return a / b;
    }
}
int eval(string expression){
    int i;
     
    // stack to store integer values.
    stack <int> values;
     
    // stack to store operators.
    stack <char> ops;
     
    for(i = 0; i < expression.length(); i++){
        if(expression[i] == ' ')
            continue;
         
        else if(expression[i] == '('){
            ops.push(expression[i]);
        }
         
        else if(isdigit(expression[i])){
            int val = 0;
            
            while(i < expression.length() &&
                        isdigit(expression[i]))
            {
                val = (val*10) + (expression[i]-'0');
                i++;
            }
             
            values.push(val);
            
              i--;
        }
         
       
        else if(expression[i] == ')')
        {
            while(!ops.empty() && ops.top() != '(')
            {
                int val2 = values.top();
                values.pop();
                 
                int val1 = values.top();
                values.pop();
                 
                char op = ops.top();
                ops.pop();
                 
                values.push(applyOp(val1, val2, op));
            }
             
    
            if(!ops.empty())
               ops.pop();
        }
         
        else
        {
            while(!ops.empty() && precedence(ops.top())
                                >= precedence(expression[i])){
                int val2 = values.top();
                values.pop();
                 
                int val1 = values.top();
                values.pop();
                 
                char op = ops.top();
                ops.pop();
                 
                values.push(applyOp(val1, val2, op));
            }
         
            ops.push(expression[i]);
        }
    }
    
    while(!ops.empty()){
        int val2 = values.top();
        values.pop();
                 
        int val1 = values.top();
        values.pop();
                 
        char op = ops.top();
        ops.pop();
                 
        values.push(applyOp(val1, val2, op));
    }
   
    return values.top();
}
 
int main()
{
    string expression = "10 + 20 * 30";
    cout << eval(expression) << endl;
}


// Output
// 610