Comparing values input to values in a vector

Question

I'm trying to input a form of data validation in which when a user enter's a book's ISBN number, if it has already been stored then it will output an error message. However, I'm having trouble doing this. I'm not sure if I'm overloading the == operator correctly, and I'm not sure how to compare the vector values in the store_ISBN() function.

Here is the code:

#include "std_lib_facilities.h"

// Classes ---------------------------------------------------------------------

class Book{
public:
       vector<Book> books; // stores book information
       Book() {}; // constructor
       friend ostream& operator<<(ostream& out, const Book& b);
       bool operator==(const Book& d);
       string what_title();
       string what_author();
       int what_copyright();
       void store_ISBN();
       void is_checkout();
private:
        char check;
        int ISBNfirst, ISBNsecond, ISBNthird;
        char ISBNlast;
        string title;
        string author;
        int copyright;
};

// Class Functions -------------------------------------------------------------

string Book::what_title()
{
       cout << "Title: ";
       getline(cin,title);
       cout << endl;
       return title;
}

string Book::what_author()
{
       cout << "Author: ";
       getline(cin,author);
       cout << endl;
       return author;
}

int Book::what_copyright()
{
    cout << "Copyright Year: ";
    cin >> copyright;
    cout << endl;
    return copyright;
}

void Book::store_ISBN()
{
     bool test = false;
     cout << "Enter ISBN number separated by spaces: ";
     while(!test){
     cin >> ISBNfirst >> ISBNsecond >> ISBNthird >> ISBNlast;
     for(int i = 0; i < books.size(); ++i)
             if(ISBNfirst == books[i]) cout << "test"; // no idea how to implement this line            
     if((ISBNfirst<0 || ISBNfirst>9) || (ISBNsecond<0 || ISBNsecond>9) || (ISBNthird<0 || ISBNthird>9))
                   error("Invalid entry.");
     else if(!isdigit(ISBNlast) && !isalpha(ISBNlast))
          error("Invalid entry.");
     else test = true;}     
     cout << endl;
}

void Book::is_checkout()
{
     bool test = false;
     cout << "Checked out?(Y or N): ";
     while(!test){
     cin >> check;
     if(check == 'Y') test = true;
     else if(check == 'N') test = true;                                
     else error("Invalid value.");}
     cout << endl;
}

// Operator Overloading --------------------------------------------------------

bool Book::operator==(const Book& d){ // is this right???
     if((ISBNfirst == d.ISBNfirst) && (ISBNsecond == d.ISBNsecond) 
             && (ISBNthird == d.ISBNthird) && (ISBNlast == d.ISBNlast)) return true;
     else return false;
}

ostream& operator<<(ostream& out, const Book& b){
         out << "Title: " << b.title << endl;
         out << "Author: " << b.author << endl;
         out << "ISBN: " << b.ISBNfirst << "-" << b.ISBNsecond << "-" << b.ISBNthird << "-" << b.ISBNlast << endl;
         out << endl;
         return out;
}

// Main ------------------------------------------------------------------------     

int main()
{
    Book store;
    string question;
    while(true){
        store.what_title();
        store.what_author();
        store.what_copyright();
        store.store_ISBN();
        store.is_checkout();
        store.books.push_back(store);
        cout << "Are you finished?(Y or N): ";
        cin >> question;
        if(question == "Y") break;
        else if(question == "N"){
              cout << endl;
              cin.ignore();}
        else error("Invalid value.");
        }
    cout << endl;
    cout << "Books stored -\n" << endl;
    for(int i = 0; i < store.books.size(); ++i)
            cout << store.books[i];
    keep_window_open();
}

Note that in the store_ISBN function I've only included testing for one variable since I don't want to type out the whole thing before I figure out how to do it.

As you can see each time a book passes through the loop in main, the data for that book is stored. I'm then able to output all the data input after the loop by overloading the << operator to print Title, Author, and ISBN. So I think I should be able to access that individual data in the vector to compare to the user input ISBN, but I don't know how. The parts that I am confused about have been commented as such.

Answer 1

books refers to a vector of Book class. You are comparing Book to an integer, which is undefined behavior. You need to dereference the Book object before you can access its data members.

First, don't access vectors using subscript [] notation. It is inefficient and makes life difficult. Use an iterator (something like, not sure on how you would want to implement):

for (std::vector::iterator it = books.begin(); it != books.end(); ++it)
{
}

That isn't your problem, however. You use the -> operator to dereference objects to get to their members. You made your members private, however, so you either need a get function like

ISBNf() { return ISBNfirst; }

Or make your members public, but that is a bad idea (people can fool with your data). However, for simplicity, assume they are public, this is what you want:

for (std::vector::iterator it = books.begin(); it != books.end(); ++it)
{
    if (*this == *it) cout << "test"; 
}

There is no good solution, here, because I have no idea what you are trying to achieve. I think you are trying to compare the number of digits on the integer, but this is not how to achieve that. If you are just trying to make sure you are assigning ISBNfirst properly, let me put your mind to rest: you are. However, you aren't accessing them correctly, which is where the -> operator comes in.

Next, this code is overkill:

else if(!isdigit(ISBNlast) && !isalpha(ISBNlast)

instead, use the isalphnum() function:

else if (!isalphnum(ISBNlast));

Posted; I will edit my post to point out all the flaws in your code.

Answer 2

I'm not sure quite what the user is expected to type for an ISBN.

Reading from a stream into an int will read digits up to a space, and convert the result to int (if all goes well, anyway). Reading into a char will store the char value. So at the moment you're validating than an ISBN looks like three single digits (0-9), and then the next char. That's not what I think an ISBN looks like.

Your operator== looks OK, although note that for a bool return value,

if (X) return true;
else return false;

can be replaced with

return X;

because conditionals are already of type bool.

After setting your ISBN values (and any other fields you plan to use in operator==, if it's not finished yet), the way to look for a matching book in the store is:

for(int i = 0; i < books.size(); ++i)
    if(*this == books[i]) cout << "test";

In other words, look for a book equal to this book. Or you could use std::find from <algorithms>, although in this case it wouldn't really be any more concise.

By the way, it is unusual to use the same class (Book) to represent both a single book, and the whole store. Unpicking that is a fairly complex set of changes and decisions, though, so I'll just say that a class should represent a single kind of thing, and an object of the class represent an example of that kind. So normally Book and Bookstore are different kinds of thing. The vector in Book is an instance variable, meaning that every Book has its own vector of Books. That doesn't really make sense.