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#include "stdlib.h"
#include"stdio.h"
#include "ctype.h"
#include "math.h"

#define MAXOP 100
#define NUMBER  0
#define TRUE 1
#define FALSE 0

int Getop(char s[]);
void push(double val);
double pop(void);
void showTop(void);
void duplicate(void);
void swapItems(void);
void clearStack();

int main(void)
{
 int type;
 double op2;
 char s[MAXOP];
 int flag = TRUE;

 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)
    push(pop() / op2);
   else
    printf("nError: division by zero!");
   break;
  case '%':
   op2 = pop();
   if(op2)
    push(fmod(pop(), op2));
   else
    printf("nError: division by zero!");
   break;
  case '?':
   showTop();
   break;
  case '#':
   duplicate();
   break;
  case '~':
   swapItems();
   break;
  case '!':
   clearStack();
  case 'n':
   printf("nt%.8gn", pop());
   break;
  default:
   printf("nError: unknown command %s.n", s);
   break;
  }
 }
 return EXIT_SUCCESS;
}

#define MAXVAL 100

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

/* push: push f onto stack. */
void push(double f)
{
 if(sp < MAXVAL)
  val[sp++] = f;
 else
  printf("nError: stack full can't push %gn", f);
}

/*pop: pop and return top value from stack.*/
double pop(void)
{
 if(sp > 0)
  return val[--sp];
 else
 {
  printf("nError: stack emptyn");
  return 0.0;
 }
}

void showTop(void)
{
 if(sp > 0)
  printf("Top of stack contains: %8gn", val[sp-1]);
 else
  printf("The stack is empty!n");
}


void duplicate(void)
{
 double temp = pop();

 push(temp);
 push(temp);
}

void swapItems(void)
{
 double item1 = pop();
 double item2 = pop();

 push(item1);
 push(item2);
}

/* pop only returns a value if sp is greater than zero. So setting the
stack pointer to zero will cause pop to return its error */

void clearStack(void)
{
 sp = 0;
}

int getch(void);
void unGetch(int);

/* Getop: get next operator or numeric operand. */
int Getop(char s[])
{
 int i = 0;
 int c;
 int next;

 /* Skip whitespace */
 while((s[0] = c = getch()) == ' ' || c == 't')
  ;
 s[1] = '';

 /* Not a number but may contain a unary minus. */
 if(!isdigit(c) && c != '.' && c != '-')
  return c;               

 if(c == '-')
 {
  next = getch();
  if(!isdigit(next) && next != '.')
  {
   return c;
  }
  c = next;
 }
 else
  c = getch();

 while(isdigit(s[++i] = c))
  c = getch();
 if(c == '.')                        /* Collect fraction part. */
  while(isdigit(s[++i] = c = getch()))
   ;
 s[i] = '';
 if(c != EOF)
  unGetch(c);
 return NUMBER;
}

#define BUFSIZE 100

char buf[BUFSIZE];
int bufp = 0;

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

/* unGetch: push character back on input. */
void unGetch(int c)
{
 if(bufp >= BUFSIZE)
  printf("nUnGetch: too many charactersn");
 else
  buf[bufp++] = c;
}

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