how can i change the struct in C array?

Question

i have a problem changing the struct that is an array.

I'm working on a library project in C language that need to add a book (Book struct) into a library, I have an array that has all my books, I need to add to this array, my new book.

i did this, could anyone help me , and give me a little bit information about it ?

#include <stdio.h>
#include <string.h>

#define BOOK_NUM 50
#define NAME_LENGTH 200
#define AUTHOR_NAME_LENGTH 100
#define PUBLISHER_NAME_LENGHT 50
#define GENRE_LENGHT 50

typedef struct _Book
    {
    char name[NAME_LENGTH];
    char author[AUTHOR_NAME_LENGTH];
    char publisher[PUBLISHER_NAME_LENGHT];
    char genre[GENRE_LENGHT];
    int year;
    int num_pages;
    int copies;
}Book;

Book books_arr[BOOK_NUM],*ptr=books_arr;

void add_book()
{
    char book_name[NAME_LENGTH],author_name[AUTHOR_NAME_LENGTH],publisher_name[PUBLISHER_NAME_LENGHT],book_genre[GENRE_LENGHT];
    int book_year,book_pages,book_copies,cnt=0,cnt2=0;
    printf("Please enter book name:\n");
    scanf("%s",&book_name);
    printf("Please enter author name:\n");
    scanf("%s",&author_name);
    printf("Please enter publisher name:\n");
    scanf("%s",&publisher_name);
    printf("Please enter book genre:\n");
    scanf("%s",&book_genre);
    printf("Please enter the year of publishment:\n");
    scanf("%d",&book_year);
    printf("Please enter the number of pages:\n");
    scanf("%d",&book_pages);
    printf("Please enter the number of copies:\n");
    scanf("%d",&book_copies);
    for (ptr=books_arr;ptr<&books_arr[BOOK_NUM];ptr++)
    {
        if (strcmp(book_name,(*ptr).name)==0)
            (*ptr).copies=(*ptr).copies+book_copies;
        if(strcmp(book_name,(*ptr).name)!=0)
            cnt++;
        if((*ptr).name!=NULL)
            cnt2++;
    }
    if(cnt==BOOK_NUM)
    {
        if(cnt2==BOOK_NUM)
            printf("There is no place in the library for this book\n");
        if(cnt2<BOOK_NUM)
        {
            (*ptr).name=book_name;
            (*ptr).author=author_name;
            (*ptr).publisher=publisher_name;
            (*ptr).genre=book_genre;
            (*ptr).year=book_year;
            (*ptr).num_pages=book_pages;
            (*ptr).copies=book_copies;
        }
    }
}

every time i compile the code, i have the problem, "expression must be a modifiable lvalue".

thank you

When you have an error, identify the line where the error is reported. We shouldn't have to guess and work it out for ourselves. — Jonathan Leffler
– Jonathan Leffler, Commented Dec 27, 2016 at 4:08
The arrow operator -> was invented to make it easier to write code like ptr->member rather than having to use the (*ptr).member. — Jonathan Leffler
– Jonathan Leffler, Commented Dec 27, 2016 at 4:13

dbush · Accepted Answer · 2016-12-27 04:09:33Z

You're attempting to assign an array to another array. That's not legal in C.

If you want to copy the contents of a character array containing a string to another character array, use strcpy.

strcpy(ptr->name, book_name);
strcpy(ptr->author, author_name);
strcpy(ptr->publisher, publisher_name);
strcpy(ptr->genre, book_genre);

Also note the use of the -> operator to access a pointer-to-member.

Besides this, you're also not reading these 4 string correctly. The %s format specifier to scanf expects a pointer to the first element in a char array. What you're doing is passing in the address of the array itself.

For this, just pass in the name of the array. The array name decays into a pointer to the first element when passed to a function.

scanf("%s",book_name);
printf("Please enter author name:\n");
scanf("%s",author_name);
printf("Please enter publisher name:\n");
scanf("%s",publisher_name);
printf("Please enter book genre:\n");
scanf("%s",book_genre);

Rohan · Accepted Answer · 2016-12-27 04:08:49Z

1

Instead of doing this

(*ptr).name=book_name;

You should copy a string using strcpy like

strcpy((*ptr).name,book_name);

Also, consider using other safe functions like strncpy().

Note use ptr->name instead of (*ptr).name.

The ptr->name is an array, you cannot change array but you can change contents of an array, which is what the error is suggesting.

answered Dec 27, 2016 at 4:08

Rohan

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1 Comment

Jonathan Leffler Over a year ago

Be cautious about strncpy() — it isn't a panacea by any means.

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