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calculating an expression

kali17 (5)
Hello,
I need help to make it less messy but also make it work. It was suppose to evaluate an arithmetic expression without conversion to postfix.
Ps: I'm a beginner and I'll appreciate any comment or advice to get better.
Thanks in advance.

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#include <iostream>
#include <stack>

using namespace std;


bool isOperand(char character);
bool isOperator(char character);
void Calculate(stack<char> operands, stack<char> operators, string expression);
void compute(stack<char> operands, stack<char> operators);

int main()
{
    stack<char> numbers ;
    stack<char> operators;
    string exp =  "1+2*3";
    Calculate(numbers,operators,exp);


  return 0;
}

bool isOperator(char character) {
    if (character == '+' || character == '-' || character == '*' || character == '/') {
       return true;
    }
    return false;
}
bool isOperand(char character) {
    if (!isOperator(character) && character != '(' && character != ')') {
        return true;
    }
    return false;
}

void Calculate(stack<char> operands, stack<char> operators, string expression)
{
  char c;
  char token;
  int result;
  for(int i = 0; i < expression.length(); i++)
{
  if (isOperand(c))
  token += c;
  if (c == ' ' || i == (expression.length() - 1)){
        if (isOperand(token)){
            operands.push(token);
            token = ' ';
        }
   }
   else if (isOperator(c))
   operators.push(c);
   if (operands.size() == 2){
       operands.top();
       operands.pop();
       operands.top();
       operands.pop();
       compute(operands,operators);
    }
 }
}

void compute (stack<char> operands,stack<char> operators){
    int num1;
    int num2;
    char op;
    switch(op)
    {
    case '+' :
        operands.push(num1 + num2);
        break;
    case '-' :
        operands.push(num1 - num2);
        break;
    case '*' :
        operands.push(num1 * num2);
        break;
    case '/' :
        operands.push(num1 / num2);
        break;
    }
}
mutexe (2372)
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 int num1;
    int num2;
    char op;

none of these in compute() have been initialised, so when you come to use them further down in that function they'll have nonsense values assigned to them.
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