Evaluate Postfix Expression using Stack

PostFixEvaluation.cpp

#include <iostream>
#include <string>
#include <stack>

bool isOperator(char x) {
    if (x == '+' || x == '-' || x == '*' || x == '/' )
        return true;
    return false;
}

bool isNumber(char x) {
    if (x >= '0' && x <= '9') return true;
    return false;
}

int performOperation(char operation, int op1, int op2) {
    if (operation == '+') return op1 + op2;
    if (operation == '-') return op1 - op2;
    if (operation == '*') return op1 * op2;
    if (operation == '/') return op1 / op2;
    return -1;
}

double evaluatePostfix(std::string exp) {
    std::stack<int> S;
    for (int i = 0; i < exp.length(); i++) {
        if (exp[i] == ' ' || exp[i] == ',')
            continue;
        if (isNumber(exp[i])) {
            int digit = 0;
            while (i < exp.length() && isNumber(exp[i])) {
                // *10 is necessary for multiple numbers base 10
                digit = digit * 10 + exp[i] - '0'; // - '0' is a hack to convert ASCII chart to INT
                i++;
            }
            i--;
            S.push(digit);
        }
        if (isOperator(exp[i])) {
            int op1 = S.top(); S.pop();
            int op2 = S.top(); S.pop();
            int result = performOperation(exp[i], op1, op2);
            S.push(result);
        }
        
    }
    return S.top();
}

int main() {
    std::string exp = "1 12 2*+";
    double result = evaluatePostfix(exp);
    std::cout << result << std::endl;
    return 0;
}