problems with strcmp

I cant seem to get this to do what I want, which is to check for duplicate ISBN's and if a duplicate is there return the error message, can anyone help??

Code:

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


typedef struct
{
    char ISBN[20];
    char Author[30];
    char Title[50];

}

Library_Book;

FILE * bookfile;
Library_Book Books;
Library_Book NewBooks;
int choice=0;

void Menu (void);
void Add_Book (void);
void List_Books (void);
void Search_ISBN (void);
void Search_Title (void);

int main (int argc, char *argv[])
{
    {
    Menu();
    }
        while (choice != 5);
        system("PAUSE");
        return 0;
}


/******************************************************************************/
/*   Menu Function                  */

void Menu(void)

{
    int choice=0;
    system ("cls");


/* Displays Menu */

      printf("******************************************************\n");
      printf("*                  LIBRARY MENU                      *\n");
      printf("*                                                    *\n");
      printf("*   1   Add a book to file                           *\n");
      printf("*   2   List of all books on file                    *\n");
      printf("*   3   Search for a book using an ISBN number       *\n");
      printf("*   4   Search for a book using the title            *\n");
      printf("*   5   Quit                                         *\n");
      printf("*                                                    *\n");
      printf("******************************************************\n");
      printf("*   Please select your choice 1 to 5                 *\n");
      printf("******************************************************\n");
      scanf ("%i", &choice);

/* Switch to take user to correct option from menu */

      switch (choice){
             case 1: Add_Book();
             break;
             case 2: List_Books();
             break;
             case 3: Search_ISBN();
             break;
             case 4: Search_Title();
             break;
             case 5: exit(EXIT_SUCCESS);
             break;
             default: printf("Incorrect selection, please select your choice 1 to 5.\n");
             getchar();
             system ("pause");
    }
}

/*Function to add a book */

  void Add_Book (void)
  {
       int found = 0;
       system ("cls");

       bookfile = fopen("BOOK.DAT","ab");



     if (bookfile == 0)
                {
                    printf("An error occurred whilst opening the file.\n");
                }
           {
              printf ("Please enter ISBN or q to quit.\n");
              scanf ("%s", &Books.ISBN);
              strcmp (Books.ISBN,NewBooks.ISBN);

    }

             /* Do  the following until user wishes to quit */
       while ((tolower(Books.ISBN[0]) != 113)&&(strcmp (NewBooks.ISBN,Books.ISBN)==1))
              found =0;

       {
              printf ("Please enter Author.\n");
              fflush(stdin);
              gets (Books.Author);

            }

        {
              printf ("Please enter the title of the book.\n");
              fflush(stdin);
              gets (Books.Title);

            }
              fwrite (&Books,sizeof(Books),1,bookfile);

/*Error message for ISBN if already on file */
              strcmp (NewBooks.ISBN,Books.ISBN);
              while (strcmp (NewBooks.ISBN,Books.ISBN)==1)

                {
                    printf ("Book is already held on file, Please Enter another ISBN\n");
                    scanf ("%s", &Books.ISBN);
                    found =1;
                }

              printf ("Please enter ISBN or q to quit.\n");
              scanf ("%s", &Books.ISBN);

fclose (bookfile);
return Menu();
}

/* Lists all of the books which have been input onto the system */

   void List_Books (void)
   {
        system ("cls");

        bookfile = fopen("BOOK.DAT","rb");

       if (bookfile == 0)
                {  printf ("An error occurred while opening the file.\a\n");

                }
        else
        printf ("              LISTS ALL BOOKS ON FILE\n\n");
        printf ("ISBN                \t Author            \t Title\n\n");
        while (!feof(bookfile))
        {
        fread(&Books, sizeof(Books),1,bookfile);
        if(!feof(bookfile))
        printf ("%-20s \t %-20s \t %s\n", Books.ISBN, Books.Author,
        Books.Title);
        }
fclose (bookfile);
        system("PAUSE");
        return Menu();
}


/* Search the database using an ISBN */

   void Search_ISBN (void)
      {
        char Search_ISBN [20];
        int found =0;

        system ("cls");

        bookfile = fopen("BOOK.DAT","rb");

        printf("Please enter ISBN that you wish to search for.\n");
        scanf ("%s", &Search_ISBN);

        {
        while (!feof(bookfile))
        fread(&Books, sizeof(Books),1,bookfile);


           { if (Search_ISBN == Books.ISBN)
             {found = 1;

             }

             }
             if (found ==1)
        { if(!feof(bookfile))
            printf ("Title is %s\n Author is %s\n", Books.Author, Books.Title);
            }
           else

           {printf ("ISBN is not found.\n");}


        }

        fclose (bookfile);
        system("PAUSE");
        }

/* Search the database using the Title of a Book */

void Search_Title (void)
{

        char Search_Title[50];
        int found =0;

        system ("cls");

        bookfile = fopen("BOOK.DAT","rb");

        printf("Please enter book Title that you wish to search for.\n");
        scanf ("%s", &Search_Title);

        {
        while (!feof(bookfile))
        fread(&Books, sizeof(Books),1,bookfile);


           { if (Search_Title == Books.Title)
             {
                    found = 1;
             }
           }
             if (found ==1)
{           if(!feof(bookfile))
            printf ("Title is %s\n Author is %s\n", Books.Author, Books.Title);
            }
           else

    {
                printf ("Title is not found.\n");}

    }

        fclose (bookfile);
        system("PAUSE");
}

How does it not work?

Originally Posted by laserlight

How does it not work?

it doesnt recognise a duplicate isbn and doesnt loop properly either lol. Im a beginner and this is starting to annoy me

Can't compare the return value of strcmp() to 1, instead test whether it is equal to, greater than, or less than zero.

And, it is returning ZERO for equal!

--
Mats

Code:

fflush(stdin);
gets (Books.Title);

both should not be used - FAQ explains why.

when you are do

Code:

    strcmp (Books.ISBN,NewBooks.ISBN);

you are do nothing (it is helpful by its return value only, hence if you aren't check this value, call is useless)
Maybe you wanted use strcpy (to copy string from one place to another)

Code:

    fflush(stdin);

it is incorrect, try fflush(stdout) and only when you have no newline ('\n') in the end of output

Code:

	
	gets (Books.Author);

it will not control the length of input line (if it will be longer than Books.Author string (30 chars), it will erase Title[50]; in the structure, because it goes after the Author string)
you can use fgets (it differs from gets by saving newline character in the line only and can control line length fgets(line, MAXLINE, stdin) )

Code:

return Menu();

what is this mean ?

Code:

    char Search_ISBN [20];
    ...
    if (Search_ISBN == Books.ISBN)

you can't compare strings directly
only char by char

Code:

    if (strcmp(Search_ISBN, Books.ISBN) == 0)
        ;

here too

Code:

    if (Search_Title == Books.Title)

Originally Posted by loopymoo26

I cant seem to get this to do what I want, which is to check for duplicate ISBN's and if a duplicate is there return the error message, can anyone help??

I would use an array for strings

• make it
• load it from a file (only first field of every structure) by fread and strcpy functions
• then sort them
• count duplicates of every string and print error with number of them or skip duplicates of every string and print an error for every duplicate occurrence

You can load not only first string (ISBN), if you will load full structures, you will get avail to delete duplicates and save cleared structure array back to the file, but it will be longer than the processing of only first strings and it will need linked list rather than simply array of structure (you are beginner, so it can be hard).