c-如何使用qsort对结构数组进行排序

Question

它应该根据学生的姓氏按字母顺序对他们进行排序。我试着用qsort，但它不起作用。

struct student {
char name[30];
char lastName[30];
char grades[30][5];
int numberOfGrades;
};
int compare(const void *a, const void *b) {
struct group *s1 = (struct group *)a;
struct group *s2 = (struct group *)b;

return strcmp(s1->lastName, s2->lastName);
}
int main()
{
struct student group[30];
group[0].lastName = "Malaska";
group[1].lastName = "Kowalski";
group[2].lastName = "Adamczyk";
group[3].lastName = "Bakiewicz";

int numberOfStudents = 4;
qsort(group, numberOfStudents, sizeof(group), compare);
}

Answer 1

以下是对您的代码的修改和注释：

struct student {
char name[30];
char lastName[30];
char grades[30][5];
int numberOfGrades;
};
//Note, error incomplete type occurs 
//when using `struct group *s1 = (struct group *)a;`
//will replace with `struct student` 
int compare(const void *a, const void *b) {
const struct student *s1 = (struct student *)a;
const struct student *s2 = (struct student *)b;

return strcmp(s1->lastName, s2->lastName);
}
int main()
{
struct student group[30];//local scope, will not be recognized in 'compare()'                      
strcpy(group[0].lastName, "Malaska");//string assignments need to use strcpy or similar, not =
strcpy(group[1].lastName, "Kowalski");
strcpy(group[2].lastName, "Adamczyk");
strcpy(group[3].lastName, "Bakiewicz");

int numberOfStudents = 4;
//sending correct size of each element 
qsort(group, numberOfStudents, sizeof(group[0]), compare);
//                             ^^^^^^^^^^^^^^^^^
}

以下也是对您的代码的改编，但有一些额外的更改要构建，如果在您正在学习的范围内，则应提供更多可读性、范围的使用、typedef的使用以及动态内存分配和释放的示例。。。

typedef struct {  //use of `typedef struct {` rather than 'struct name {` 
char name[30];
char lastName[30];
char grades[30][5];
int numberOfGrades;
}student_s; //new type, can be used in place of `struct student` anywhere.
int compare(const void *a, const void *b) {
const student_s *s1 = (student_s *)a;
const student_s *s2 = (student_s *)b;

return strcmp(s1->lastName, s2->lastName);
}
int main()
{
int numberOfStudents = 4;//moved to top so can use group
//to properly size instances of group
//dynamic memory allocation, and clean up at bottom
student_s *group = calloc(numberOfStudents, sizeof *group);
if(group)//test here to ensure success
{
strcpy(group[0].lastName, "Malaska");
strcpy(group[1].lastName, "Kowalski");
strcpy(group[2].lastName, "Adamczyk");
strcpy(group[3].lastName, "Bakiewicz");
qsort(group, numberOfStudents, sizeof(group[0]), compare);
//... more code as necessary
//when finished with 'group', free it.
free(group);
}
}