Linked List Insertion, deletion, sorting

Question

Sorry that this is a rather large question. I cannot get the following C++ to work, I always get an error about not having a struct/class/union in the code from visiual studio. I am supposed to have the books getting put into a linked list in alphabetical order, but so far my insert method is broken.

    //********************************************************************
    //  BookList.cpp
    //
    //  Represents a collection of books.
    //*******************************************************************
    #include "BookList.h"

    //----------------------------------------------------------------
    //  Creates a new Book object and adds it to the end of
    //  the linked list.
    //----------------------------------------------------------------
    void BookList::add(Book *newBook) {

    BookNode *node = new BookNode(newBook);
    BookNode *current;

    if (head == NULL)
    head = node;
    else {
       current = head;
       while (current->next != NULL) {
           current = current->next;
       }
       current->next = node;
    }
    }

    char *BookList::getBookList(char *list) {

       list[0] = '\0';
       BookNode *current = head;

       while (current != NULL) {
          strcat( list, current->book->getBook() );
          strcat( list, "\n" );
          current = current->next;
          }

       return list;
       }

    void BookList::insert(Book *newBook) {
        BookNode *node = new BookNode(newBook);
        BookNode *current;

        if (head == NULL) {
    head = node;
        }
        else {
            current = head;
            int result = *newBook.compareTo(current->book->getBook());
            if (result == -1) {
                current->next = node;
                }
            else {
                while (result == 1) {
                    current = current->next;
                    result = *newBook.compareTo(current->book->getBook());
        }
    current->next = node;
    }
    }
    }

    //********************************************************************
    //  BookList.h
    //
    //  Represents a collection of books.
    //*******************************************************************
    #include "Book.h"

    class BookNode {
       public:
          BookNode() { };
          BookNode(Book *theBook) {
             book = theBook;
             next = NULL;
          };
          friend class BookList;

       private:
          Book *book;
          BookNode *next;
    };

    class BookList {
       public:
          void add(Book *);
          char* getBookList(char *);
      void delet(Book *);
      void insert(Book *);
          BookList() {
          head = NULL;
          };

       private:
          BookNode *head;

     };

    #include <cstring>

    //********************************************************************
    //  Book.h
    //
    //  Represents a single book.
    //*******************************************************************

    class Book {

       public:
          Book (char *newTitle) {
             strcpy( title, newTitle );
          }

          int compareTo(Book *newBook) {
              int compvar;
                      compvar = strcmp(newBook->getBook(), title);
                      return compvar;
          }

          char *getBook() {
             return title;
          }

       private:
          char title[81];

       };

There are certainly multiple problems with this code, so any help anybody can provide would be awesome. Thanks in advance!

Answer 1

This line of code is wrong (two lines that look just like this):

int result = *newBook.compareTo(current->book->getBook());

If you dereference, use parenthesis around it:

int result = (*newBook).compareTo(current->book->getBook());

But to make it easier to read, I would suggest:

int result = newBook->compareTo(current->book->getBook());

This should show you your real problem: compareTo() is expecting a Book object and getBook() returns a char *. Use the following and it compiles fine for me. Without a main function that uses the classes, I couldn't tell you if your code logically works right but now it compiles.

You should really overload the equality operator in Book.h for comparisons. Hope this helps.

Answer 2

You need to add a forward declaration of BookList class before you start declaring the BookNode class as you have the line friend class BookList; inside the definition of your BookNodeclass.

Change the following:

//********************************************************************
//  BookList.h
//
//  Represents a collection of books.
//*******************************************************************
#include "Book.h"

class BookNode {

to this:

//********************************************************************
//  BookList.h
//
//  Represents a collection of books.
//*******************************************************************
#include "Book.h"

class BookList;

class BookNode {

Without this, the compiler will complain that BookList is not a struct/class/union.

Obviously you'll have to get rid of the other compiler errors in your code after doing this.

Answer 3

Well, the logical error that jumps out at me is that you never say

node->next = current->next

before

current->next = node

Of course, there are STL libraries for linked lists, there's no bounds-checking in your printing function (which should probably be using a std::string anyway), and this never cleans up the memory it allocates... but I assume this is for an intro programming assignment. You won't be able to instantiate your class without a delet() method defined either.