how to allocate memory to array of pointers?

Question

i have been given a structure and a pointer to an array.

each index of the array is a letter of the alphabet. i need to receive a name, last name and phone number, and allocate memory to a struct (phonebook).

then, each struct needs to be accessed from the array using the last name's first letter.

if the function is called again i need to use linked list to add another contact.

i dont know how to allocate memory for a certain index of an array. when i try to do

    phonebook[letter] = (Contact**)malloc(sizeof(Contact));

i keep having de reference warnings, and i cant seem to figure out how to point the address of phonebook[letter] to a structure properly.

this is what i have tried:

typedef struct Contact {
    char* firstName;
    char* lastName;
    char* phoneNum;
    struct Contact* next;
} Contact;

int main(){
Contact* phonebook[26];
addNewContact(phonebook)
}

int addNewContact(Contact** phonebook) {
    char newFirstName[SIZE], newLastName[SIZE], newPhoneNum[SIZE];
    int letter;
    printf("Enter a contact details \
(<first name> <last name> <phone number>):\n");
    scanf("%s%s%s", newFirstName, newLastName, newPhoneNum);
    //get number of the letter in the alphabet
    letter = newLastName[0] - 'A';

    //allocate memory to pointer
    Contact *current;
    phonebook = (Contact**)malloc(sizeof(Contact));
    if (phonebook == NULL) {
        printf("The addition of the contact has failed!");
        //free
        exit(1);
    }

    current = phonebook[letter];

    //check if details are being used
    do {
        //if the name already exists
        if (phonebook[letter]->firstName == newFirstName \
 && phonebook[letter]->lastName == newLastName) {
            printf("The addition of the contact has failed, \
since the contact %s %s already exists!\n", newFirstName, newLastName);
            //free
            return 0;
        }

        //if the phone number already exists
        if (phonebook[letter]->phoneNum == newPhoneNum) {
            printf("The addition of the contact has failed, \
since the phone number %s already exists!", newPhoneNum);
                        //free
            return 0;
        }

        current = current->next;
    } while (current != NULL);

    //assigning
    phonebook[letter]->firstName = newFirstName;
    phonebook[letter]->lastName = newLastName;
    phonebook[letter]->phoneNum = newPhoneNum;
    return 0;
}

in addition, i havent figured out the linked list part at all, i managed before to enter the details to the structure (though im not sure if i even pointed the struct to the right place) but i dont know how to iterate after the first addition of the name. if i intialize current->next to be NULL for the first time, it will also happen the next time i call the function.

currently the code stops due do access violation but it seemed that after the first name i had error with reading the inputs that only occured after the second time.

The problem is that `(Concat **)` should be `(Contact *)`. But you shouldn't cast the result of `malloc()`, see https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — Barmar, Dec 16 '22 at 22:29
You can't assign to `phonebook`. It's an array of pointers, not a pointer itself. — Barmar, Dec 16 '22 at 22:30
The whole `addNewContact` function is a huge mess fraught with errors. Break it up. Write a function that allocates nad fills a new `Contact` and returns a pointer to it, another function that adds a `Contact` to a given linked list, and finally a function that adds a `Contact` to a phomebook by selecting the correct linked list from it. — n. m. could be an AI, Dec 16 '22 at 22:36
This will be an O(n) structure. Are the names unique? Do they have to be in order? Consider a [multimap](https://en.wikipedia.org/wiki/Multimap) or [hash table](https://en.wikipedia.org/wiki/Hash_table)? With your code, you have to assign extra space for the contents of the phonebook; you are now assigning local stack data that is destroyed. — Neil, Dec 17 '22 at 01:51

Neil · Accepted Answer · 2022-12-23T05:01:52.417

You are actually pretty close to making it work. Probably the main problem with this code is that it has only pointers to the data, doesn't have space to store the data. You are pointing to phonebook[letter]->lastName = newLastName, a local variable that is destroyed when the function returns. This will make your strings a dangling pointer; when you try to access this data, undefined things happen. Probably the easiest way to fix that is to make a char array of maximum length instead of the pointers. You don't need to typedef in most cases, and sometimes it's confusing. I would recommend that you take the typedef out and just have,

#define SIZE 128
struct Contact {
    char firstName[SIZE];
    char lastName[SIZE];
    char phoneNum[SIZE];
    struct Contact* next;
};

Double-pointer Contact** phonebook is valid, but a recipe for confusion. Instead of naked pointers, use struct to encapsulate a phone book, using a list of struct Contact.

struct Phonebook {
    struct Contact *letter[26];
};

printf is defined in stdio.h. strcmp is defined in string.h. malloc and exit are stdlib.h.

See What's the correct declaration of main()? C doesn't scan ahead; switch the order of the functions or have a prototype above.

Perhaps in the future you will read from a file? It's easier do if you read the input separately from the interface. Maybe split it up into a separate function. This should be static, since it doesn't need to be published to other translation units.

static struct Contact *inputContact(void)

To allocate memory, current = malloc(sizeof *current);. Did you have a typo using phonebook? If you allocate it first, you can read directly into the allocated memory and not do any copying. Make sure you limit the size. (scanf is not meant for this.)

#define STR(n) STR2(n)
#define STR2(n) #n
    if(scanf("%" STR(SIZE) "s%" STR(SIZE) "s%" STR(SIZE) "s",
        current->firstName, current->lastName, current->phoneNum) != 3)
        exit(EXIT_FAILURE);

Then,

static int addNewContact(struct Phonebook* phonebook, struct Contact *contact)

do-while checks that the current is valid after you use it; you want while.

    //get number of the letter in the alphabet
    int letter = contact->lastName[0] - 'A'; // might want to check that
    struct Contact *current = phonebook->letter[letter];
    //check if details are being used
    while (current != NULL) {

In C, equality means they are the same memory location, not in general the same text. You probably want a library function strcmp. (But are you sure your logic is correct in that it prevents the same number in appearing with the same first letter of the last name?)

    if (!strcmp(current->phoneNum, contact->phoneNum))

Assigning will just overwrite the one letter that is there. You probably meant for it to be appended to the list.

    contact->next = phonebook->letter[letter];
    phonebook->letter[letter] = contact;
    return 1;

And lastly, be sure to initialize any data that is of automatic duration.

    struct Phonebook phonebook = {0}; /* Initialize null lists. */
    addNewContact(&phonebook, inputContact());

--edit

This can be seen as creating a static-26-bucket separately-chained hash-table, with your hash function being hash(struct Contact *entry) { return entry->lastName[0] - 'A'; }.

I think you are doing this, which also makes sense. Make sure that you allocate Contact and 3 text fields, for a total of 4 mallocs per entry. If you read the data into a temporary buffer, you can use strcpy to transfer it to a longer-lived string that you've just allocated.

First of all, thanks for the detailed response! After seeing your comment I played around with it and I understand what went wrong in most places, and I still have one more question. I ended up allocating memory for the 3 fields and tried to do: **current->firstname = newFirstName** for example. Even after allocating enough memory for both the structure and the firstname, i am still not able to get the information to save, and it only works as intended using **strcpy**. what is the reason for that? And both the struct Phonebook and the define tricks are actually pretty cool, thanks :) — olmervii, Dec 22 '22 at 10:33
@olmervii That should work as well, and is the same as, `current->firstname = malloc(SIZE) . . . fgets(current->firstname, SIZE, stdin)`. In the case where they are pointers, make sure you allocate `current` _as well_ as the 3 buffers for your inputs. Copying data is also valid, you might want to have an auto maximum, read, and copy it to a malloced smaller buffer. Or you could have a big static buffer for inputs and store pointers to that buffer. — Neil, Dec 22 '22 at 22:59
It is a little hard to explain without a diagram, so I created one. — Neil, Dec 23 '22 at 04:39

how to allocate memory to array of pointers?

1 Answers1