Question

以下是Stephen Kochan撰写的“Programming in C”一书的练习10.4。它说我应该创建一个函数，它从输入字符串派生一个部分并将该部分返回到main()（作为字符串，而不是指针）并显示它。我的代码如下。

#include <stdio.h>

char subString (const char source[], int start, int count, char result[count + 1] ){ //result will be number of characters (count) + 1 (because of null)
int i, j, end = start + count;

// the part excluded must start from i = start and "count" number of characters must be derived and then put on result
for( i = start, j = 0; i < end; ++i, ++j)
    result[j] = source[i];

result[j] = '\0';

return result[count + 1];
}

int main (void){
char result[20] = {0};

const char text1[] = "character";

result[20] = subString( text1, 4, 3, result );
printf("From \"%s\" this part is being excluded-> \"%s\"\n", text1, result);

return 0;
}

输出

From "character" this part is being excluded-> "act"

Process returned 0 (0x0)   execution time : 0.332 s
Press any key to continue.

请注意，上面的代码运行得非常好 - 没有警告。

我无法理解的是当我更换下面的两行时

result[20] = subString( text1, 4, 3, result );
printf("From \"%s\" this part is being excluded-> \"%s\"\n", text1, result);

这一行

printf("From \"%s\" this part is being excluded-> \"%s\"\n", text1, subString( text1, 4, 3, result ) );

我得到了输出：

From "character" this part is being excluded-> "(null)"

Process returned 0 (0x0)   execution time : 0.332 s
Press any key to continue.

为什么？我怎样才能使用那一行代替它呢？另外，我对返回字符串/数组的函数有点困惑。他们倾向于引导我犯错误，所以如果有人能给我提出一些建议，那么在与他们合作时我应该始终牢记这一点，这对我非常有帮助。提前谢谢。

Answer 1

第1点：

在第一种情况下，您使用的result值通过作为参数传递给subString()而被修改。你没有使用subString () fucntion的返回值。

OTOH，在第二种方法中，你试图使用subString ()函数的重新转换值，这也是使用错误的格式说明符。您可以在man page of printf()

中详细了解正确的格式说明符

第2点

C中的数组索引从0开始。因此，没有名为result[20]的有效元素。因此，

result[20] = subString( text1, 4, 3, result );

导致一个错误，然后调用undefined behaviour。

Answer 2

printf("From \"%s\" this part is being excluded-> \"%s\"\n", text1, subString( text1, 4, 3, result ) );

请注意subString()返回的字符不是char*而您正在使用%s来打印会导致未定义行为的字符。

您的数组正在函数subString()中进行修改，在第一种情况下，您通过返回result[count + 1]来打印数组结果，并且还有一个未定义的行为

result[20] = subString( text1, 4, 3, result );

数组越界访问

您需要修改代码，如

char *subString (const char source[], int start, int count, char result[]){ //result will be number of characters (count) + 1 (because of null)
int i, j, end = start + count;

// the part excluded must start from i = start and "count" number of characters must be derived and then put on result
for( i = start, j = 0; i < end; ++i, ++j)
    result[j] = source[i];

result[j] = '\0';

return result;
}

int main()
{
  // Keep your array here
  char *p = subString( text1, 4, 3, result );
  printf("%s\n",p);
}

或

void subString (const char source[], int start, int count, char result[]){ //result will be number of characters (count) + 1 (because of null)
    int i, j, end = start + count;

    // the part excluded must start from i = start and "count" number of characters must be derived and then put on result
    for( i = start, j = 0; i < end; ++i, ++j)
        result[j] = source[i];

    result[j] = '\0';

    }

在main()

printf("%s\n",result);

简单的C程序。从函数返回的字符串会导致Error

2 个答案: