#include <stdio.h>
#include <stdlib.h> /* for atof() */
#define MAXOP 100 /* max size of operand or operator */
#define NUMBER '0' /* signal that a number was found */
int getop(char []);
void push(double);
double pop(void);
/* reverse Polish calculator */
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 '*':
          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;
}
#define MAXVAL 100 /* maximum depth of val stack */
int sp = 0;         /* next free stack position */
double val[MAXVAL]; /* value stack */
/* 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 value from stack */
double pop(void)
{
   if (sp > 0)
       return val[--sp];
   else {
       printf("error: stack empty\n");
       return 0.0;
   }
}
#include <ctype.h>
int getch(void);
void ungetch(int);
/* getop: get next character or numeric operand */
int getop(char s[])
{
   int i, c;
   while ((s[0] = c = getch()) == ' ' || c == '\t')
       ;
   s[1] = '\0';
   if (!isdigit(c) && c != '.')
       return c;    /* not a number */
   i = 0;
  if (isdigit(c))   /* collect integer 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;
}
#define BUFSIZE 100
char buf[BUFSIZE];    /* buffer for ungetch */
int bufp = 0;        /* next free position in buf */
int getch(void) /* get a (possibly pushed-back) character */
{
   return (bufp > 0) ? buf[--bufp] : getchar();
}
void ungetch(int c) /* push character back on input */
{
   if (bufp >= BUFSIZE)
       printf("ungetch: too many characters\n");
   else
       buf[bufp++] = c;
}