reverse polish notation (invalid)

#include <stdio.h>
#include <stdlib.h>
#include "stack.h"


void getInput()
{
    char ch;

    while(1)
    {
        makeEmpty();

        printf("Enter an RPN expression: ");

        scanf("%c", &ch);

        if(ch == 'q')
                break;

        while(1)
        {
            if(ch == '\n')
                break;

            if(ch >= 48 && ch <= 57)
            {
                if(!isFull())
                {
                    push(ch - 48);
                }
                else
                {
                    printf("Expression is too complex\n");
                    exit(EXIT_FAILURE);
                }

            }

            switch(ch)
            {
                case '+':
                    push(add(pop(), pop()));
                    break;
                case '-':
                    push(sub(pop(), pop()));
                    break;
                case '*':
                    push(mul(pop(), pop()));
                    break;
                case '/':
                    push(divide(pop(), pop()));
                    break;
                case '=':
                    printf("%d\n", pop());
                    break;
            }

            scanf("%c", &ch);
        }
    }
}

int add(int a, int b)
{
    return a + b;
}

int sub(int a, int b)
{
    return a - b;
}

int mul(int a, int b)
{
    return a * b;
}

int divide(int a, int b)
{
    return a / b;
}

int main()
{

    getInput();
    printf("%d\n", pop());

    return 0;
}