push object in a class array with ruby [closed] - arrays

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Hi i tried alot to push the variables which are in initialize to a class array but it always give error like this 'uninitialized class variable ##books in Object (NameError)'
code is here:
class Library
##books = Array.new
##books = []
attr_reader :bookName, :author
def initialize(bookName,author)
#bookName = bookName
#author = author
##books.push(self)
end
def countbooks
$count = 0
##books.each do |b|
count += 1
end
end
i used ##books.push(self) to take all variables of object to an array but it did not work.

Answer 1

DISCLAIMER: I haven't used Ruby for some time, so some of my knowledge might be outdated. Feel free to correct me.
There are multiple problems with your code. Let's get the code you provided into some sort of running state.
First of all, you are missing an end in your provided code within the countbooks method. It needs an end to close the loop. Second, you can use either Array.new or [] as they both create an empty array, but if you use both you just overwrite the first new, empty array.
If you now initialize an instance of your new class and call the method countbooks, you will receive an error because you don't access your global variable $count, but count instead, which has never been initialized. If you fix this, you can now just call the countbooks method and then a puts($count) and you get the current number.
However, global variables are generally bad practices, so you should avoid them. Remove the $ sign and add a return count after the loop ends, and we kind of replicated the Array.length method, just a lot slower :)
This should yield the following code:
class Library
##books = []
attr_reader :bookName, :author
def initialize(bookName,author)
#bookName = bookName
#author = author
##books.push(self)
end
def countbooks
count = 0
##books.each do |b|
count += 1
end
return count
end
end
The code should now be executable, but there is a big problem.
The class variable ##books:
You now have a single array for the whole class. That means, if we create a new instance of the class Library, the array will be shared across all of them.
x = Library.new("title","bintree")
y = Library.new("title2","bintree2")
puts(x.countbooks)
puts(y.bookName)
x.countsbooks will return 2 and you only push new entries to the array if you create a new instance of Library.
From your question, I understand that you now try to call ##books outside of the class. This will not work. It only exists in the Library class. You can not use the attr accessors to gain access, either.
You probably should read more on classes, instances, and especially the different variable types. These links helped me to answer your question:
More about classes, instances, and a bit about variables
More about arrays
Here is an example of how the code could look like if we ignore build-in functions such as Array.length to implement our own counter and avoid class (##) and global ($) variables. You can gain access to the #books if you now add attr_reader :books, but it would be better to write a method instead.
class LibraryBTD
def initialize
#books = Array.new
#counter = 0
end
def addBook(title,author)
#books.push([title,author])
#counter += 1
end
def listAllBooks
puts("There are currently " + #counter.to_s + " books in the library.")
#books.each do |book|
title = book[0]
author = book[1]
puts("Title: " + title + " | Author: " + author)
end
end
end
If we now create a new instance of LibraryBTD and add 2 books, we can list them with listAllBooks:
a = LibraryBTD.new
a.addBook("MyBook1","BTD")
a.addBook("MyBook2","BTD")
a.listAllBooks
This should output the following in the console:
There are currently 2 books in the library.
Title: MyBook1 | Author: BTD
Title: MyBook2 | Author: BTD
However, if we create a new instance, the list will be empty, because #books is only an instance variable.
b = LibraryBTD.new
b.listAllBooks
There are currently 0 books in the library.