K and R C, Solution to Exercise 4-4:
K and R C Programs Exercises provides the solution to all the exercises in the C Programming Language (2nd Edition). You can learn and solve K&R C Programs Exercise.
C program to implement the following:
Show the top item of the stack without permanently popping it.
Swap the top two items on the stack.
Duplicate the top item on the stack.
Clear the stack, with the previous cases i. e K and R C Programs Exercise 4-3. Read more about C Programming Language .

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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] = '