K&R калькулятор обратной польской записи - C (СИ)

#include <stdio.h>
#include <stdlib.h>
#define MAXOP 100
#define NUMBER '0'
 
int getop(char []);
void push(double);
double pop (void);
 
main()
{
    int type;
    double op2;
    char s[MAXOP];
    while ((type = getop(s)) != EOF)
    {
        switch(type)
        {
        case NUMBER:
                push(atof(s));
            break;
            case '+':
                push(pop()+pop());
                break;
            case '-':
                op2 = pop();
                push(pop()-op2);
                break;
            case '*':
                push (pop()*pop());
                break;
            case '/':
                op2 = pop();
                if (op2 != 0.0)
                    push(pop()/op2);
                else
                    printf("Division by 0\n");
                break;
            case '\n':
                printf("\t%.8g\n", pop());
                break;
            default:
                printf("Error: unknown operation %s\n", s);
                break;
        }
    }
    return 0;
}
 
#include <ctype.h>
#include <stdio.h>
 
int getch(void);
void ungetch(int);
int NUMBER;
 
int getop(char s[])
{
    int i, c;
 
    while ((s[0] = c = getch()) == ' ' || c == '\t')
        ;
    s[1] = '\0';
    if (!isdigit(c) && c != '.')
        return c;
    i = 0;
    if (isdigit(c))
        while (isdigit(s[++i] = c = getch()))
        ;
    if (c == '.')
        while (isdigit(s[++i] = c = getch()))
        ;
    s[i] = '\0';
    if (c != EOF)
        ungetch(c);
    return NUMBER;
}
 
int sp;
double val[];
 
double pop(void)
{
    if (sp > 0)
        return val[--sp];
    else
    {
        printf("Error: stack is empty\n");
        return 0.0;
    }
}
 
#define MAXVAL 100
 
int sp = 0;
double val[MAXVAL];
 
void push(double f)
{
    if (sp < MAXVAL)
        val[sp++] = f;
    else
        printf("Error: stack is full, %g can not be placed\n", f);
}