没有在 C 中正确使用 strcmp

Question

我编写此方法是为了找到适合给定小数的 2 的最大幂。十进制为 char 数组格式，以避免数字存储时出现输入溢出错误。 2 的幂是用 float 的 pow(2, power) 格式计算的，即。 8.000000 这个数字然后被发送到一个方法来删除句点和 0 的那个轨迹。 IE。 8.000000 变成 8

1  #include <string.h>
2  #include <stdio.h>
3  #include <stdlib.h>
4  #include <memory.h>
5  #include <math.h>
6
7   int i;
16
17  void removeFloatZeros(char *floatvalue)
18  {
19      char *ptr = strchr(floatvalue, '.');
20      *ptr = '\0';
21  }
22
45
173 char *decimalToBinary(char *decimal)
174 {
176     int x;
177     double power = 0;
178     char *binary = malloc(sizeof(char *) * 1024);
179     char *twosPower = malloc(sizeof(char *) * 1024);
180
181     /* What is the greatest power of 2 that will fit into the decimal? */
182     for(x = 0; x <= 30; x++)
183     {
184         power = pow(2.0, x);
185         snprintf(twosPower, 1023, "%f", power);
186         removeFloatZeros(twosPower);
189         printf("strcmp(decimal, twosPower) = %d\n", strcmp(twosPower, decimal));  
190         memset(twosPower, '\0', 1023);
191     }
214 }
215
216 int main(int argc, char*argv[])
217 {
218     char *dec1 = argv[1];
219     decimalToBinary(dec1);
220     return 1;
221 }
222

例如，如果我在 argv[1] 中输入 20，它将输出：

strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = -1  
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1

我在哪里做错了？另外，忽略 for 循环的结束条件。它应该在第 6 次迭代之前输出所有 1，在第 6 次和第 6 次迭代之后输出所有 -1。

Answer 1

strcmp 返回值：

零值表示两个字符串相等。

大于零的值表示第一个不匹配的字符在str1中的值大于在str2中的值。

而小于零的值则相反。

您的输入：20 循环的第一次迭代： twosPower = "1" strcmp("20", "1")

第一个字符不匹配并且在 str2 ("1") 中的值小于在 str1 ("2") 中的值 -> 返回负值。

其余的迭代应该自己解释......

另外，编辑：

printf("strcmp(decimal, twosPower) = %d\n", strcmp(twosPower, decimal)); 

您的 printf 格式字符串与您在参数中所做的相反。

编辑：

str1    str2    
1       20  First char that differs is '1' vs. '2'. '1' (ASCII 49) is smaller than '2' (ASCII 50), 49 - 50 = -1 = return value
2       20  First char that differs is '\0' vs. '0'. '\0' (ASCII 0) is smaller than '0' (ASCII 48), 0 - 48 = -48 = return value
4       20  First char that differs is '4' vs. '2'. '4' (ASCII 52) is greather than '2' (ASCII 50), 52 - 50 = 2 = return value
8       20  First char that differs is '8' vs. '2'. '4' (ASCII 56) is greather than '2' (ASCII 50), 56 - 50 = 6 = return value
16      20  First char that differs is '1' vs. '2'. '1' (ASCII 49) is smaller than '2' (ASCII 50), 49 - 50 = -1 = return value

... and so on ...

也许这个输出有更多帮助

此外，您在数字中查找 2 的最大幂的方法存在缺陷，因为 strcmp 的返回值仅取决于不同的 FIRST char。 所以 strcmp("2", "16") 和 strcmp("200000000", "16") 总是返回相同的东西。

Answer 2

这个清理后的代码版本会产生如下所示的输出：

$ ./xx 20
strcmp(1, 20) = -1
strcmp(2, 20) = -48
strcmp(4, 20) = 2
strcmp(8, 20) = 6
strcmp(16, 20) = -1
strcmp(32, 20) = 1
strcmp(64, 20) = 4
strcmp(128, 20) = -1
strcmp(256, 20) = 5
strcmp(512, 20) = 3
strcmp(1024, 20) = -1
strcmp(2048, 20) = 52
strcmp(4096, 20) = 2
strcmp(8192, 20) = 6
strcmp(16384, 20) = -1
strcmp(32768, 20) = 1
strcmp(65536, 20) = 4
strcmp(131072, 20) = -1
strcmp(262144, 20) = 6
strcmp(524288, 20) = 3
strcmp(1048576, 20) = -1
strcmp(2097152, 20) = 57
strcmp(4194304, 20) = 2
strcmp(8388608, 20) = 6
strcmp(16777216, 20) = -1
strcmp(33554432, 20) = 1
strcmp(67108864, 20) = 4
strcmp(134217728, 20) = -1
strcmp(268435456, 20) = 6
strcmp(536870912, 20) = 3
strcmp(1073741824, 20) = -1
$

可以说有很多小变化。

#include <string.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

static void removeFloatZeros(char *floatvalue)
{
    char *ptr = strchr(floatvalue, '.');
    *ptr = '\0';
}

static void decimalToBinary(char *decimal)
{
    int x;
    double power = 0;
    char *twosPower = malloc(sizeof(char *) * 1024);

    /* What is the greatest power of 2 that will fit into the decimal? */
    for(x = 0; x <= 30; x++)
    {
        power = pow(2.0, x);
        snprintf(twosPower, 1023, "%f", power);
        removeFloatZeros(twosPower);
        printf("strcmp(%s, %s) = %d\n", twosPower, decimal, strcmp(twosPower, decimal));  
        //printf("strcmp(decimal, twosPower) = %d\n", strcmp(twosPower, decimal));  
        memset(twosPower, '\0', 1023);
    }
    free(twosPower);
}

int main(int argc, char*argv[])
{
    for (int i = 1; i < argc; i++)
        decimalToBinary(argv[i]);
    return 0;
}

显示比较值使事情更容易理解。您需要释放内存（或使用自动数组）。您需要使用标题。 static 声明不是 100% 必要的，但这意味着当我在我的 über-fussy 编译器设置下编译时不会收到任何警告。

但关键的变化是打印出正在比较的值 - 它可以理解来自 strcmp() 的数字。

（在进行比较之前，请考虑添加适当数量的前导零。）