c - 实现了名为 atof_beta 的 atof，但结果与 atof 不同

Question

我写了一个atof函数，名为atof_beta。我运行它，但结果与atof的结果不一样。

例如:输入:1111111111111111111111111111111111111111111111111111111111111111111111111111111111

输出:(fabs(val-atof(s)) <= EPSILON) 等于 0。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <limits.h>
#include <float.h>
#include <math.h>

double check_overflow(double val)
{
    if (val > DBL_MAX){
        return DBL_MAX;
    }else if (val <  DBL_MIN){
        return DBL_MIN;
    }
    return val;
}


double get_exp(char *s)
{
    double exp = 10;
    int num = 0x00;
    char c = *s;

    if (c == '-'){
        exp = 0.1;
        s++;
    }else if (c == '+'){
        s++;
    }

    while ((c = *s++) != '\0'){
        if ((c >= '0') && (c <= '9')){
            num = num*10 + c - '0';
        }
        else{
            return 0x01;
        }
    }

    double tmp = 0x01;
    while (num--){
        tmp *= exp;
    }

    return tmp;
}

double atof_beta(char *s)
{
    double integer = 0x00;
    double fraction = 0x00;
    int sign = 0x01;
    int fraction_flag = 0x00;
    double divisor = 0x01;
    double exp = 0x01;

    char c = *s;

    if (c == '-'){
        sign = -1;
        s++;
    }else if (c == '+'){
        sign = 1;
        s++;
    }

    while ((c = (*s++)) != '\0'){
        if ((c >= '0') && (c <= '9')){
            if (!fraction_flag){
                integer = integer*10 + (c -'0');
            }else{
                fraction = fraction*10 + c -'0';
                divisor *= 10;
            }
        }else if (c == '.'){
            fraction_flag = 0x01;
        }else if ((c == 'e') || (c == 'E')){
            exp = get_exp(s);
            break;
        }else{
            break;
        }
    }

    integer = check_overflow(integer);
    fraction = check_overflow(fraction);
    exp = check_overflow(exp);
    double val = sign*((integer + (fraction/divisor))*exp);
    return check_overflow(val);
}

#define EPSILON 1e-6
void main()
{
    char s[1024], *p;
    double val;

    while(1)
    {
        printf("enter string : ");
        fgets(s, sizeof(s), stdin);
        if ((p = strchr(s, '\n')) != NULL) { 
            *p = '\0'; /* Remove newline */
        }
        puts(s); /* No need to cast */
        val = atof_beta(s);
        printf("\nfinal value : %lf, %lf, %d\n", val, atof(s), (fabs(val-atof(s)) <= EPSILON));
    }

    return;
}

Answer 1

这是错误的:

char *s;
... 
printf("enter string : ");
s = (char *)malloc(sizeof(char));
gets((char *)s);
puts((const char *)s);

你为单个char保留空间，但你想要一个字符串，还要注意gets在C99中已被弃用，C11删除了它，我建议fgets:

char s[32], *p;
...
printf("enter string : ");
/* No need to malloc */
fgets(s, sizeof s, stdin);
if ((p = strchr(s, '\n')) != NULL) { 
    *p = '\0'; /* Remove newline */
}
puts(s); /* No need to cast */