使用STDLIB的的qsort（）排序字符串数组 [英] Using stdlib's qsort() to sort an array of strings

问题描述

有些preface：我是一名计算机工程专业的学生采取用C一流的Java之后的3个学期（最高数据结构）。这个问题是关于一个家庭作业，但几步从解决它为我删除。

Some preface: I'm a computer engineering student taking a first class in C after 3 semesters of Java (up to data structures). This question is in relation to a homework assignment, but a few steps removed from solving it for me.

我有读入存储器，使得它被存储在字符的输入文件[9] [500]。我最大长度为500最具串读8.我试图使用排序STDLIB的内置的qsort（）函数这个数组，和我有一些内存错误。

I have an input file that I read into memory such that it is stored in char[9][500]. I read in at most 500 strings of maximum length 8. I am attempting to sort this array using stdlib's built in qsort() function, and am having some memory errors.

code的重要片段：

Important snippets of code:

char data[4][500][60];
char debug[500][9];
size_t count = 0;

/* initialize file, open for reading */
FILE* pUserlog;
pUserlog = fopen("userlog","r");

while(!feof(pUserlog))
{
    fscanf(pUserlog, "%9s %8s %16s",debug[count], data[1][count], data[2][count]);
    fgets(data[3][count], 60, pUserlog);
    count++;
}

本部分读取数据到阵列。在这一部分感兴趣的数组是调试。这是上面指定的阵列。这是我比较函数的qsort为：

This section reads the data into the arrays. The array of interest in this part is "debug". This is the array specified above. Here is my comparison function for qsort:

int compare(const void* a, const void* b)
{
    const char **ia = (const char **)a;
    const char **ib = (const char **)b;
    puts("I'm in compare!");
    return strncmp(*ia, *ib,8);
}

这是我试图调用的qsort：

This is my attempt to call qsort:

size_t debug_len = sizeof(debug)/sizeof(char*);
printf("debug len: %d, count: %d, sizeof(char*): %d\n",debug_len,count,sizeof(char*));
qsort(debug,count, sizeof(char *), compare);

我试图在我的电话代debug_len其中count是的，但我仍然段错误。下面是输出：

I attempted substituting debug_len in my call where count is, but I am still segfaulting. Here is the output:

$ ./test
debug len: 1125, count: 453, sizeof(char*): 4
I'm in compare!
Segmentation fault (core dumped)

感谢您！

推荐答案

比较功能将接收指针所比较的元素。你实际上试图比较使用 STRNCMP字符（）。既然你有指向每个字符串，将它转换为一个的char * 和比较。

The compare function will receive pointers to the elements that are being compared. You are effectively trying to compare characters using strncmp(). Since you have pointers to each of the strings, cast it to a char * and compare.

int compare(const void* a, const void* b)
{
    const char *ia = (const char *)a;
    const char *ib = (const char *)b;
    puts("I'm in compare!");
    return strncmp(ia, ib, 9);
}

也请记住，这是一个数组的数组，而不是指针数组。因此，一个元素的大小应该是数组的大小， 9 ，而不是指针， 4 。在这一点上，它会更容易只使用 sizeof的调试[0] ，因为它是一个二维数组。如果你没有用正确的尺寸做到这一点，的qsort（）只会破坏你的数组。

Remember also, it's an array of arrays, not an array of pointers. So the size of an element should be the size of the array, 9 and not of the pointer, 4. At this point, it would be easier to just use sizeof debug[0] since it is a two-dimensional array. If you don't do this with the right sizes, qsort() will just destroy your array.

size_t elemsize = sizeof debug[0];      /*   9 - size of each element */
size_t count = sizeof(debug)/elemsize;  /* 500 - number of elements in array */
qsort(debug, count, elemsize, compare);

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