C程序设计语言（第二版） 4-4 栈操作添加复制、打印栈顶元素，交换栈顶元素等

#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("\n\t%.8g\n", 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 %g\n", f); 
} 
 
/*pop: pop and return top value from stack.*/ 
double pop(void) 
{ 
    if(sp > 0) 
        return val[--sp]; 
    else 
    { 
        printf("\nError: stack empty\n"); 
        return 0.0; 
    } 
} 
 
void showTop(void) //?
{ 
	double top=val[sp];
	
	printf("%u\n",top);
} 
 
 
void duplicate(void) //#
{ 
	int top=sp-1;
	double topVal;
	topVal=val[top];
	val[top+1]=topVal;
	sp++;
} 

void swapItems(void) //~
{ 
	double temp;
	int top= sp;
	int secTop = sp-1;
	temp = val[top];
	val[top]=val[secTop];
	val[secTop]=temp;


} 
 
/* 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] = '\0'; 
 
    /* 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] = '\0'; 
    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 characters\n"); 
    else 
        buf[bufp++] = c; 
}