// This program evaluates simple expressions
//   number1 operator1 number2 operator2 number3
//   e.g., 3 + 4 * 5
// the program computes the answer using C++
// precedence, i.e., * comes before +.
// For this simple example, we will just use
// the operators *, +, /, -.
//
//
double evaluate(double, char, double );
int priority(char);

#include <iostream>
#include <math.h>
int main()
{
    double n1, n2, n3;
    double result;
    char op1, op2;
    int op1_priority, op2_priority;

    cout << "Enter an expression: ";
    cin >> n1 >> op1 >> n2 >> op2 >> n3;

    // decide whether op1 has higher precedence than op2

    op1_priority = priority(op1);
    op2_priority = priority(op2);

    // if op2 has higher priority than op1
    if (op2_priority < op1_priority)
    {
        result = evaluate(n2, op2, n3);
        result = evaluate(n1, op1, result);
    }
    else
    {
        // first evaluate op1, then evaluate op2
        result = evaluate(n1, op1, n2);
        result = evaluate(result, op2, n3);
    }
    
    cout << "The answer is " << result << endl;
}

double evaluate(double operand1, char op, double operand2)
{
    double value = 0;

    if (op == '+')
        value = operand1 + operand2;
    else if (op == '-')
        value = operand1 - operand2;
    else if (op == '*')
        value = operand1 * operand2;
    else if (op == '/')
        value = operand1 / operand2;
    else if (op == '%')
        value = int(operand1) % int(operand2);
    else if (op == '^')
        value = pow(operand1, operand2);
    else
        cout << "invalid operator " << op << endl;

    return value;
}

int priority(char op)
{
    int op_priority;

    op_priority = 1;
    if (op == '/' || op == '*' || op == '%')
        op_priority = 2;
    if (op == '+' || op == '-')
        op_priority = 3;

    return op_priority;
}