// from K&R section 4.3
#include <stdio.h>
#include <stdlib.h> // for atof()
#include <ctype.h>

#define MAXOP 100 // max size of operand
#define NUMBER '0' // signal that a number was found
#define MAXVAL 100 // max depth of value stack
#define BUFSIZE 100

int sp = 0; // next free stack position
double val[MAXVAL]; // value stack
char buf[BUFSIZE]; // buffer for ungetch
int bufp = 0; // next free position in buffer

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

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

// get next operator or 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;
}

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

// 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;
}