在C中沒有正確使用strcmp

Question

我編寫了此方法，以找到適合給定十進制數的2的最大冪。 小數點采用char數組格式，以避免在存儲數字時出現輸入溢出錯誤。 2的冪是使用float的pow（2，power）格式計算的。 8.000000然后將這個數字發送到一種方法來刪除句點和0的那個尾跡。 即。 8.000000變成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]中輸入20，它將輸出：

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

我在哪里錯呢？ 同樣，忽略for循環的結束條件。 它應該在第6次迭代之前輸出所有1，在第6次以及第6次迭代之后輸出所有-1。

Answer 1

strcmp返回值：

零值表示兩個字符串相等。

大於零的值表示不匹配的第一個字符在str1中的值大於在str2中的值。

小於零的值表示相反的意思。

您的輸入：20循環的第一次迭代：twosPower =“ 1” strcmp（“ 20”，“ 1”）

第一個字符不匹配，並且在str2（“ 1​​”）中的值小於在str1（“ 2”）中的值->返回負值。

其余的迭代應該自己解釋一下。

另外，編輯：

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

您的printf格式字符串表示您在參數中所做的相反的操作。

編輯：

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 ...

也許這個輸出有幫助

此外，由於strcmp的返回值僅取決於不同的FIRST字符，因此您找到數字最大2的冪的方法存在缺陷。 因此strcmp（“ 2”，“ 16”）和strcmp（“ 200000000”，“ 16”）總是會返回相同的內容。

Answer 2

此代碼的清理版本產生的輸出如下所示：

$ ./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
$

可以說，這里有很多小的變化。

#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;
}

顯示比較值使事情更容易理解。 您需要釋放內存（或使用自動數組）。 您需要使用標題。 static聲明不是100％必需的，但是意味着在über-fussy編譯器設置下進行編譯時，我不會收到任何警告。

但是關鍵的變化是打印出要比較的值-它使strcmp()中的數字有意義。

（在進行比較之前，請考慮添加適當數量的前導零。）