Initializing user defined type array in C

Question

I need to write a simple library program using the structure Book:

typedef struct book {
    char name[NAME_LENGTH];
    char author[AUTHOR_NAME_LENGTH];
    char publisher[PUBLISHER_NAME_LENGTH];
    char genre[GENRE_LENGTH];
    int year;
    int num_pages;
    int copies;
} Book;

When the program runs the user can take out books from the library/add books/print books etc:

#define BOOK_NUM        50

int main(){

Book books[BOOK_NUM] = { 0 };
int opt = 0, books_counter = 0, *books_counter_p;
books_counter_p = &books_counter;


do {
    print_menu();
    scanf("%d", &opt);
    while (opt < 1 || opt > 5) {
        printf("Invalid option was chosen!!!\n");
        print_menu();
        scanf("%d", &opt);
    }

    switch (opt) {
        case 1:
            // Add book to library
            add_book(books, books_counter_p);


            break;
        case 2:
            // Take a book from library
            break;
        case 3:
            // Return book
            break;
        case 4:
            // Print all books
            break;
        case 5:
            // Release all memory allocated for library
            break;
        default:
            printf("We should not get here!\n");
    }
} while (opt != 5);

return 0;
}

I need some way to store the books so I can change the library as the program runs(I don't know in advance how many books the user will add/remove during the program run).

I don't want to use dynamic memory, can I define an empty array of books this way: Book books[BOOK_NUM] = { 0 }; ? It seems to work but 0 is an int type and not a Book type, will I run into troubles?

Is there any reason as to why you want to avoid dynamically allocating memory? — StereoBucket
– StereoBucket, Commented May 27, 2018 at 11:11
books[BOOK_NUM] = { 0 } just initializes elements to zero, I don't understand what dynamic memory has to do with that? — kingW3
– kingW3, Commented May 27, 2018 at 11:14
Book books[BOOK_NUM] = { 0 }; I think it's ok or use loop or make it global. — purec
– purec, Commented May 27, 2018 at 11:16

Achal · Accepted Answer · 2018-05-27 11:32:27Z

1

Book books[BOOK_NUM] = { 0 } /* creating & initializing array of structure variable */

when you initialize a struct variable with initializer { }, all of its other members will be initialized to default value & since in your case struct book declaration is global, all members of structure will automatically initialized to 0 for int & floating point,'\0' for char and NULL for pointer variable.

Or you can use memset() like

memset(&books, 0, sizeof(books));

answered May 27, 2018 at 11:32

Achal

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2 Comments

Cubic Over a year ago

@Venuce no, don't. If you have a new question, ask a new question. Don't try to change history, that's just confusing.

purec Over a year ago

struct book is type. books[BOOK_NUM] declared in main it is not global. Zero is just default state of memory.

purec · Accepted Answer · 2018-05-27 12:32:22Z

0

books[BOOK_NUM] = { 0 }; // copiler gives warning

Proper way to do this is:

books[BOOK_NUM] = {{0}}; //set the first field of the first struct to 0 and all the rest to 0 implicitly

answered May 27, 2018 at 12:32

purec

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