文章标题

#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 s[]);
void push(double f);
double pop(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 '-':
op2 = pop();
push(pop()-op2);
break;
case '*':
push(pop()*pop());
break;
case '/':
op2 = pop();
if(op2 != 0.0)
push(pop()/op2);
else
printf("error: zero divisor\n");
break;
case '\n':
printf("\t%.8g\n", pop());
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;
}
}

#include <ctype.h>

int getch(void);
void ungetch(int c);

/* 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;
i = 0;
while(isdigit(c)){
s[i++] = c;
c = getch();
}
if(c == '.'){
s[i] = c;
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)
{
return (bufp > 0)? buf[--bufp] : getchar();
}

void ungetch(int c)
{
if(bufp >= BUFSIZE)
printf("ungetch: too many characters\n");
else
buf[bufp++] = c;
}