How to implement a Dynamic multidimensional array of structures in C

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.

There is no multidimensional array in the code shown. Other than that it looks like an ok start. Not sure what else to say since your question is non-specific. One comment would be that Test_cases only contains a single field. If you are planning to add more fields then that's fine otherwise it may not be necessary to have that in a struct. — kaylum
– kaylum, Commented May 25, 2020 at 7:22
@kaylum I forgot to mention there's multiple tests scenarios as well. Which is what the structure represents. Is my method of allocating correct? — Cobra04
– Cobra04, Commented May 25, 2020 at 7:37
Design, Code, Test, Debug a single list of pointers, Make Sure It Works correctly, full, empty, whatever. Using That, DCTD a list manged vis a single pointer to the list, (or a metadata struct thereof), MSIW. UT, DCTD a resizeable list, MSIW. UT, make a list of lists of lists...to as many dimensions as you might wish. If you have correctly assembled all the building-blocks, your total assembly will not fall down:) — Martin James
– Martin James, Commented May 25, 2020 at 7:37
part from casting malloc it is an ok start. never cast malloc. if you "need" to cast malloc then you are compiling your code with the wrong compiler switch (C++) — AndersK
– AndersK, Commented May 25, 2020 at 10:02

user13453676 · Accepted Answer · 2020-05-25 07:32:25Z

1

yes it seems correct starting.

answered May 25, 2020 at 7:32

user13453676

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2 Comments

Cobra04 Over a year ago

Is my method of allocating correct? For example how would I allocate memory for the amount of sections in a class?

user13453676 Over a year ago

ptr = (struct Section*)malloc(*Num_Sections * sizeof(struct Section)); I'm not sure, I understand you'r question !! hope this help you

Serge Ballesta · Accepted Answer · 2020-05-25 08:30:52Z

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:

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

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.

chqrlie · Accepted Answer · 2020-05-25 09:56:59Z

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;
    }
    ...
}

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