How to sort an array of structs using qsort

Question

It's supposed to sort students in alphabetical order by their last names. I tried to do it with qsort, but it doesn't work.

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);
}

For one thing, you can't assign a string value with group[0].lastName = "Malaska"; please use strcpy(group[0].lastName, "Malaska"); Another, the compare() function is casting to a different type that you have not shown. — Weather Vane
– Weather Vane, Commented Dec 30, 2020 at 19:41
You should pass sizeof(struct student) instead of sizeof(group) — anotherOne
– anotherOne, Commented Dec 30, 2020 at 19:41
The sizeof(group) should be sizeof(group[0]) or sizeof(struct student) — Weather Vane
– Weather Vane, Commented Dec 30, 2020 at 19:43

ryyker · Accepted Answer · 2020-12-31 21:38:57Z

The following is an adaptation of your code with corrections and comments:

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);
    //                             ^^^^^^^^^^^^^^^^^
}

The following is also an adaptation of your code, but with some additional changes to construct that if in scope of what you are learning, should provide a few more examples of readability, use of scope, use of typedef, and dynamic memory allocation and freeing...

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);
    }
}

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