2Stacks expression evaluation

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <stack>
#include <cctype>
using namespace std;

bool error;

bool hasLowerPriority(char op1, char op2)   {
    switch (op1) {
        case '(': return false;
        case '-': return op2 == '-';
        case '+': return op2 == '-' || op2 == '+';
        case '*': return op2 != '/';
        case '/': return true;
        default : error = true; return false;
    }
}

stack < char > operators;
stack < int > operands;

void operation(char op) {
    if(operands.size() < 2) {
        error = true; return;
    }
    int op2 = operands.top(); operands.pop();
    int op1 = operands.top(); operands.pop();
    switch(op)  {
        case '+': operands.push(op1 + op2); break;
        case '-': operands.push(op1 - op2); break;
        case '*': operands.push(op1 * op2); break;
        case '/': operands.push(op1 / op2); break;
        default : error = true; return;
    }
}

int main()  {
    char exp[1000], *p;
    int len;

    while(true) {
        printf("\nEnter an expression (x to exit):\n");
        scanf("%[^\n]%*c", exp);
        if(exp[0] == 'x' || exp[0] == 'X') break;

        len = strlen(exp);
        if(len == 0) {
            getchar();
            continue;
        }
        exp[len] = ' ';
        exp[len + 1] = ')';
        exp[len + 2] = '\0';
        error = false;
        operators.push('(');

        p = strtok(exp, " ");
        while(p && !error) {
            if(isdigit(p[0]))
                operands.push(atoi(p));
            else switch(p[0]) {
                case '(':
                    operators.push('(');
                    break;
                case ')':
                    while(!operators.empty() && !error && operators.top() != '(') {
                        operation(operators.top());
                        operators.pop();
                    }
                    if (!operators.empty())
                        operators.pop();
                    else
                        error = true;
                    break;
                default:
                    while(!operators.empty() && !error && hasLowerPriority(operators.top(), p[0])) {
                        operation(operators.top()); operators.pop();
                    }
                    operators.push(p[0]);
            }
            p = strtok(NULL, " ");
        }
        if(error || !operators.empty() || operands.size() != 1) {
            printf("ERROR\n");
            while(!operands.empty())
                operands.pop();
            while(!operators.empty())
                operators.pop();
        }
        else printf("%d\n", operands.top()); operands.pop();
    }
    return 0;
}