#include <stdio.h>
#include <stdlib.h>     /* for atof() */
#include <ctype.h>

#define MAXOP  100      /* max size of operand or operator */
#define NUMBER '0'      /* signal that a number was found */
#define MAXVAL 100      /* max depth of val stack */

#define BUFSIZE 100     /* buffer for ungetch */

int bufp = 0;           /* next free position in buf */

int sp = 0;             /* next free stack position */
extern double val[MAXVAL]       /* value stack */

int getops(char []);        /* function protos */
void push(double);
double pop(void);

int getch(void);
void ungetch(int);

/* reverse Polish notation calculator */
main()
{
    int type;
    double op2;
    char s[MAXOP];

    while ((type = getops(s)) != EOF)
    {
        switch (type)
        {
            case NUMBER:
                push(atof(s));
                break;

            case '+':
                push(pop() + pop());
                break;

            case '*':
                push(pop() * pop());
                break;

            case '-':
                op2 = pop();
                push(pop() - op2);
                break;

            case '/':
                op2 = pop();
                if (op2 != 0.0)
                    push(pop() / op2);
                else
                    printf("error: zero divisor\n");
                break;

            case '\n':
                printf("\t%.8g\n", pop());
                break;

            default:
                printf("error: unknown command %s\n", s);
                break;
        }
    }
    return 0;
}

/* push(): push f onto value stack */

void push(double f)
{
    if (sp < MAXVAL)
    {
        val[sp++] = f;
    }

    else
        printf("error: stack full, can't push %g\n", f);
}

/* pop(): pop and return top val from stack */

double pop(void)
{
    if (sp > 0)
    {
        return val[--sp];
    }
    else
    {
        printf("error: stack empty\n");
        return 0.0;
    }
}

/* getop(): get next operator or numeric operand */
int getops(char s[])
{
    int i, c;

    while ((s[0] = c = getch()) == ' ' || c == '\t')
        ;

    s[1] = '\0';

    if (!isdigit(c) && c != '.')
    {
        return c;       /* NaN */
    }

    i = 0;

    if (isdigit(c))     /* collect int part */
    {
        while (isdigit(s[++i] = c = getch()))
            ;
    }
    if (c == '.')       /* collect fraction part */
    {
        while (isdigit(s[++i] = c = getch()))
            ;
    }

    s[i] = '\0';

    if (c != EOF)
    {
        ungetch(c);
    }

    return NUMBER;
}

int getch(void)             /* get a (possibly pushed back) char */
{
    return (bufp > 0) ? buf[--bufp] : getchar();
}

void ungetch(int c)         /* push char back on input */
{
    if (bufp >= BUFSIZE)
        printf("ungetch: too many chars\n");
    else
        buf[bufp++] = c;
}