The C Programming Language 练习题4-3

题目
在有了基本框架后，对计算器程序进行扩充就比较简单了。在该程序中加入取模（%）运算符，并注意考虑负数的情况。

题目分析
取模运算比较简单，直接在case部分加入即可。
考虑负数需要看‘-’后面是否数字。
之前程序很有问题，重新修改了下。顺便把整体代码都放上来，现在这个能够正确的处理负数了。为了方便调试，程序不是很简洁。
发现还是有很多问题，参考了别人的方法，更新一下。
代码实现

#include <stdio.h>#include <stdlib.h>  /* for atof() */#define MAXOP 100#define NUMBER '0'#define MAXVAL 100 /* maximum depth of val stack */int getop(char[]);void push(double);double pop(void);double val[MAXVAL]; /* value stack */int main(){    int type, i;    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 '%':            push((int)pop() % (int)pop());            break;        case '\n':            printf("\t%.8g\n", pop());            break;        case 'e':            printf("error: after '-' is not a number%s\n", s);            break;        default:            printf("error: unknown command %s\n", s);            break;        }    }    return 0;}int sp = 0; /* next free stack position *//*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);/* getop: get next character or numeric operand */int getop(char s[]){    int i, c, d;    while ((s[0] = c = getch()) == ' ' || c == '\t')        ;    s[1] = '\0';    if (!isdigit(c) && c != '.' && c != '-')        return c;   /*not a number */    if ((c == '-' || c == '+') && !isdigit(d = getch()))        {            ungetch(d);            return c;        }    i = 0;    if (isdigit(c))/* a number without '+' and '-' */        {            while(isdigit(s[++i] = c = getch()))        ;        }        else /* a number with '+' and '-' */        {            s[++i] = d;            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 1000char 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;}