如何在 C 中实现结构的动态多维数组

Question

My problem is that I need have the potential to store a number of courses each with a name, and in a course is a number of sections, and a number of students in each section each with a name (obviously), and each student has a number of assignments of type float .

So far I created a nested structure:

struct Student {
    float *Assignments;
};

struct Section {
    char Student_Name[30];
    struct Student *Students;
};

struct Course {
    char Course_Name[10];
    struct Section *Sections;
};

struct Test_Cases {
    struct Course *Courses;
};

Have a pointer to the root structure:

struct Test_Cases *ptr;

And seemingly allocated memory to the root structure and courses with:

ptr = (struct Test_Cases *)malloc(*Num_Cases * sizeof(struct Test_Cases));

ptr->Courses = (struct Course *)malloc(*Num_Courses * sizeof(struct Course));

Is the way I'm going about this correct? Thanks.

Answer 1

yes it seems correct starting.

Answer 2

Fortunately, your requirement does not involve any dynamicaly allocated multidimensional array, because it is far from easy in C language.

Here you only need to allocate various arrays of structures. There are 2 common ways to allocate arrays in C:

1. uninitialized memory:

    struct Test_Cases* ptr = malloc(*Num_Cases * sizeof(struct Test_Cases));

   or (IMHO better):

    struct Test_Cases* ptr = malloc(*Num_Cases * sizeof(*ptr));

   Remember: you should not cast malloc in C

2. zero initialized memory:

    struct Test_Cases* ptr = calloc(*Num_Cases, sizeof(struct Test_Cases));

   or:

    struct Test_Cases* ptr = calloc(*Num_Cases, sizeof(*ptr));

malloc is slightly faster, because its skips the 0 initialization part, but having that initialization may help to write shorter code in which case it should be prefered.

Answer 3

For all these structures, it would be better to add an integer member containing the number of elements allocated for the array:

struct Student {
    char Student_Name[30];
    size_t Num_Assignments;
    float *Assignments;
};

struct Section {
    char Section_Name[30];
    size_t Num_Students;
    struct Student *Students;
};

struct Course {
    char Course_Name[10];
    size_t Num_Sections;
    struct Section *Sections;
};

struct Test_Cases {
    size_t Num_Courses;
    struct Course *Courses;
};

When allocating these arrays, it is safer to use calloc() to get zero inintialized memory and to always test for memory allocation failure:

#include <stdlib.h>

struct Test_Cases *allocate_test_cases(int Num_cases, int Num_courses) {
    struct Test_Cases *ptr = calloc(Num_Cases, sizeof(*ptr));
    if (ptr == NULL)
        return NULL;
    ptr->Num_Courses = Num_Courses;
    ptr->Courses = calloc(Num_Courses, sizeof(*ptr->Courses));
    if (ptr->Courses == NULL) {
        free(ptr);
        return NULL;
    }
    ...
}