#include <ctype.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXOP 100
#define HEIGHT 1000
#define BUFSIZE 100

#define NUM '0'
#define FUN '1'
#define VAR '2'

#define INDEX(x) (x <= 'Z' ? x - 'A' : x - 71) /* index of the variable */

double var[52];
double lout = 0.0;

void clear();
double pop();
double peek();
int getop(char[]);
void push(double);

/* Stack based reverse Polish notation calculator without ungetch()
      - single uppercase and lowercase characters are variables (syntax 2 A =);
      - special variable LOUT for the last printed value.
      - mathematical functions: sin, cos, tan, exp, pow.
      - custom functions: swp (swap top 2 values), dup (duplicate last value),
        top (peek at the stack top), cls (clear stack).
      - operators: +, -, *, /, % and =.
      - press enter to print the value at the top of the stack.
 */
int main(int argc, char *argv[]) {
  int type;
  double op1, op2;
  char s[MAXOP];

  while ((type = getop(s)) != 0) {
    switch (type) {
    case NUM:
      push(atof(s));
      break;
    case '+':
      op1 = pop();
      op2 = pop();
      push(op1 + op2);
      break;
    case '-':
      op2 = pop();
      op1 = pop();
      push(op1 - op2);
      break;
    case '*':
      op2 = pop();
      op1 = pop();
      push(op1 * op2);
      break;
    case '/':
      op2 = pop();
      op1 = pop();
      if (op1 == 0.0)
        printf("error: division by zero\n");
      else
        push((double)op1 / op2);
      break;
    case '%':
      op2 = pop();
      op1 = pop();
      if (op2 == 0.0)
        printf("error: division by zero\n");
      else
        push((int)op1 % (int)op2);
      break;
    case '=':
      var[INDEX(s[0])] = pop();
      break;
    case VAR:
      if (strcmp("LOUT", s) == 0)
        push(lout);
      else
        push(var[INDEX(s[0])]);
      break;
    case FUN:
      if (strcmp("sin", s) == 0)
        push(sin(pop()));
      else if (strcmp("cos", s) == 0)
        push(cos(pop()));
      else if (strcmp("tan", s) == 0)
        push(tan(pop()));
      else if (strcmp("exp", s) == 0)
        push(exp(pop()));
      else if (strcmp("pow", s) == 0)
        push(pow(pop(), pop()));
      else if (strcmp("cls", s) == 0)
        clear();
      else if (strcmp("top", s) == 0) {
        lout = peek();
        printf("\t%.8g\n", lout);
      } else if (strcmp("swp", s) == 0) {
        op2 = pop();
        op1 = pop();
        push(op2);
        push(op1);
      } else if (strcmp("dup", s) == 0)
        push(peek());
      else
        printf("error: unknown function %s\n", s);
      break;
    case '\n':
      lout = pop();
      printf("\t%.8g\n", lout);
      break;
    default:
      printf("error: unknown command %s\n", s);
    }
  }
  return 0;
}

int bufp = 0;
char buf[BUFSIZE];

int getch() { return (bufp > 0) ? buf[--bufp] : getchar(); }

int getop(char s[]) {
  int c, i, rc;
  static int pc = ' ';

  if (pc != ' ' && pc != '\t') {
    s[0] = c = pc;
    pc = ' ';
  } else {
    while ((s[0] = c = getch()) == ' ' || c == '\t')
      ;
    s[1] = 0;
  }

  // special characters and operators
  if (c == '+' || c == '*' || c == '/' || c == '%' || c == '\n' || c == EOF)
    return c;

  i = 0;

  if (isdigit(c) || c == '.' || c == '-') {
    while (isdigit(s[++i] = c = getch()) || c == '.')
      ;
    pc = c;
    rc = i == 1 && s[0] == '-' ? '-' : NUM;
  } else {
    while (isalnum(s[++i] = c = getch()))
      ;
    s[i] = 0;
    if (i == 1) {
      if (c == '=' || (c = getch()) == '=')
        rc = '=';
      else {
        pc = c;
        rc = VAR;
      }
    } else {
      pc = c;
      rc = strcmp("LOUT", s) == 0 ? VAR : FUN;
    }
  }

  s[i] = 0;
  return rc;
}

int sp = 0;
double stack[HEIGHT];

void push(double f) {
  if (sp < HEIGHT)
    stack[sp++] = f;
  else
    printf("error: stack empty\n");
}

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

double peek() {
  if (sp > 0)
    return stack[sp - 1];
  else {
    printf("error: stack empty\n");
    return 0.0;
  }
}

void clear() {
  for (; sp > 0; sp--)
    ;
}