How can I access items in a dynamically allocated array of structs?

Question

I'm in a class right now that works with C and one of my assignments requires that I work with a struct that my professor wrote for us. It's actually two structs, with one struct basically containing an array of the first struct.

Here's what they look like:

typedef struct cityStruct
{
    unsigned int zip;
    char *town
} city;
typedef struct zipTownsStruct
{
    int *towns;
    city **zips;
    city *cities
} zipTowns;

And here's my function for allocating memory for the zipTowns structure:

void getArrs(zipTowns *arrs, int size)
{
    arrs->towns = malloc(sizeof(int) * size);
    arrs->zips = malloc(sizeof(city **) * size);
    arrs->cities = malloc(sizeof(city *) * size);
}

From what I understand, what I'm doing here is allocating space in memory for a certain number of ints, city pointers, and city structures, based on the size variable. I understand that this is basically what an array is.

I'm having trouble with understanding how I can access these arrays and manipulate items in it. Writing this gives me an error:

strcpy(arrs.cities[0]->town, "testTown\0");

You can see what I'm trying to do here. I want to access each "City" in the zipTowns struct by index and insert a value.

How can I access the items in these dynamically allocated array of structures?

Answer 1

Think of x->y as (*x).y.

arrs is not a structure, it's a pointer to a structure, and cities is not a pointer to a pointer to a structure, it's just a pointer to a structure.

Use arrs->cities[0].town instead of arrs.cities[0]->town.

However, you're still not allocating enough room for these structures. This should make it clearer what you're doing with the allocations, and should also give you enough room for your data:

arrs->towns = malloc(sizeof(*arrs->towns) * size);
arrs->zips = malloc(sizeof(*arrs->zips) * size);
arrs->cities = malloc(sizeof(*arrs->cities) * size);

With the second and third, you were only allocating enough room for a pointer to be stored instead of the actual data type.

With this approach, you will be able to access from arrs->cities[0] to arrs->cities[9] and you also will be able to access the members of each city by doing arrs->cities[<number>].<member>.

You also do not need to intentionally null-terminate your strings. This is already done for you. Therefore, you can replace "testTown\0" with "testTown".