[C] 逆波兰计算器

来源：网络

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#define MAXOP  100
#define NUMBER  '0'

int getop(char []);
void push(double);
double pop(void);

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

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;
}



#define BUFSIZE 100
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;
}
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 '/n':
                    printf("\t%.8g\n",pop());
                    break;*/
            case 0x0a:
                    printf("\t%.8g\n",pop());
                    break;

            default:
                    printf("error:unknown command %s\n",s);
                    break;
        }
    }
    return 0;
}


#define MAXVAL 100

int sp=0;
double val[MAXVAL];

void push(double f)
{
    if (sp<MAXVAL)
        val[sp++]=f;
    else
        printf("error:strack 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;
    }
}