Exercise 4-4. Add the commands to print the top element of the stack without popping, to duplicate it, and to swap the top two elements. Add a command to clear the stack.

/* Exercise 4-4. Add the commands to print the top element of the stack without popping, to duplicate it, and to swap the top two elements. 
   Add a command to clear the stack. */

#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);
double top(void);
void duplicate_top(void);
void swap_top2(void);
void clear(void);
void view_stack(void);

/* reverse Polish calculator */
int 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 '%':
                op2 = pop();
                if (op2 != 0.0)
                    push((int)pop() % (int)op2);
                else
                    printf("error: zero divisor\n");
                break;              
            case '\n':
                printf("\t%.8g\n", pop());
                break;
            case 't':
                printf("Top element in a stack = %g\n", top());
                break;  
            case 'v':
                view_stack();
                break;  
            case 's':
                swap_top2();
                break;      
            case 'd':
                duplicate_top();
                break;  
            case 'c':
                clear();
                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;
    }
}

/* print the top element of the stack without popping */
double top(void)
{
    if (sp > 0)
        return val[sp - 1];
    else 
    {
        printf("error: stack empty\n");
        return 0.0;
    }
}

/* duplicate the top element of the stack */
void duplicate_top(void)
{
    if (sp > 0)
    {
        val[sp] = val[sp - 1];
        sp++;
    }
    else 
        printf("error: stack empty\n");
}

/* swap the top two elements of the stack*/
void swap_top2(void)
{
    double tmp;
    if (sp > 1)
    {
        tmp = val[sp - 2];
        val[sp - 2] = val[sp - 1];
        val[sp - 1] = tmp;
    }
    else 
        printf("error: stack contains less than 2 elements\n");
}

/* clear the stack*/
void clear(void)
{
    sp = (sp > 0) ? 0 : sp;
}

/* print stack elements */
void view_stack(void)
{
    int vp = sp;
    while(--vp >= 0)
        printf("%g\n", val[vp]);
}

#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;
}