【晨哥的C语言之旅】逆波兰表达式实现计算器功能

/*逆波兰表达式实现简单的计算器功能*/
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

#define MAXOP  100
#define NUMBER  '0'
#define MAXVAL 100
#define BUFSIZE 100

int getop(char []);
void push(double);
double pop(void);
int getch(void);
void ungetch(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 '-':
push(pop() + pop());
break;
case '/':
op2 = pop();
if (op2 != 0.0)
push(pop() /op2);
else
printf("error: zero divisor\n");
break;
default:
printf("error: unknown command %s\n", s);

}
}
return 0;
}

int sp = 0;
double val[MAXVAL];

void push(double f){
if(sp < MAXVAL)
val[sp++] = f;
else
printf("error: stack full, can't push %g\n", f);
}

double pop(void){
if (sp > 0)
return val[--sp];
else{
printf("error: stack empty\n");
return 0.0;
}
}

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;
if(isdigit(c))
while (isdigit(s[++i] = c = getch()))
;
if(c == '.')
while (isdigit(s[++i] = c = getch()))
;
s[i] = '\0';
if(c != EOF)
ungetch(c);
return NUMBER;
}

char buf[BUFSIZE];
int bufp = 0;

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;
}