使用 stdlib 的 qsort() 对字符串数组进行排序 [英] Using stdlib's qsort() to sort an array of strings

问题描述

一些前言:我是一名计算机工程专业的学生，​​在学习了 3 个学期的 Java(直至数据结构)后，我正在上 C 的第一堂课.这个问题与家庭作业有关，但离我解决它还有几个步骤.

我有一个读入内存的输入文件，它存储在 char[9][500] 中.我最多读入 500 个最大长度为 8 的字符串.我正在尝试使用 stdlib 内置的 qsort() 函数对该数组进行排序，但出现了一些内存错误.

重要的代码片段:

字符数据[4][500][60];字符调试[500][9];size_t 计数 = 0;/* 初始化文件，打开读取 */文件* p用户日志；pUserlog = fopen("userlog","r");而(！feof(pUserlog)){fscanf(pUserlog, "%9s %8s %16s",debug[count], data[1][count], data[2][count]);fgets(data[3][count], 60, pUserlog);计数++；}

本节将数据读入数组.这部分感兴趣的数组是调试".这是上面指定的数组.这是我对 qsort 的比较函数:

int compare(const void* a, const void* b){常量字符 **ia = (常量字符 **)a;const char **ib = (const char **)b;puts("我在比较！");返回 strncmp(*ia, *ib,8);}

这是我尝试调用 qsort:

size_t debug_len = sizeof(debug)/sizeof(char*);printf("debug len: %d, count: %d, sizeof(char*): %d
",debug_len,count,sizeof(char*));qsort(调试，计数，sizeof(char *)，比较)；

我尝试在 count 所在的调用中替换 debug_len，但仍然存在段错误.这是输出:

<上一页>$ ./测试调试长度:1125，计数:453，sizeof(char*):4我在比较！分段错误(核心转储)

谢谢！

解决方案

比较函数将接收指向正在比较的元素的指针.您正在有效地尝试使用 strncmp() 比较字符.由于您有指向每个字符串的指针，因此将其转换为 char * 并进行比较.

int compare(const void* a, const void* b){常量字符 *ia = (常量字符 *)a;常量字符 *ib = (常量字符 *)b;puts("我在比较！");返回 strncmp(ia, ib, 9);}

还要记住，它是一个数组数组，而不是一个指针数组.所以元素的大小应该是数组的大小，9，而不是指针的大小，4.此时，只使用 sizeof debug[0] 会更容易，因为它是一个二维数组.如果您不使用正确的大小执行此操作，qsort() 只会破坏您的数组.

size_t elemsize = sizeof debug[0];/* 9 - 每个元素的大小 */size_t 计数 = sizeof(debug)/elemsize;/* 500 - 数组元素个数 */qsort(调试，计数，elemsize，比较)；

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.

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.

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++;
}

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);
}

This is my attempt to call qsort:

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

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)

Thank you!

解决方案

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);
}

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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