# Some newbie questions - bare with me ;) **URL:** \ **Category:** Ruby\ **Created:** [October 11, 2010, 11:48am UTC](https://www.ruby-forum.com/t/some-newbie-questions-bare-with-me/196305 "2010-10-11T11:48:44Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![egervari](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/9dc877/32.png) [@egervari](https://www.ruby-forum.com/u/egervari)\ **Post date:** [October 11, 2010, 11:48am UTC](https://www.ruby-forum.com/t/some-newbie-questions-bare-with-me/196305/1 "2010-10-11T11:48:44Z")
Question 1 I was curious what “:” before a identifier was exactly. It seems that it’s often used as a map key… but what is it exactly? Printing :bob to the screen results in “bob”… but testing for equality between :bob and “bob” results in a falsehood. I figure this has to be some literal notation for something, but what? Question 2 In RSpec, it uses == for equality. I overrode the eql?() method in object, but == doesn’t use it. I am used to languages providing an equals() method that you can override. So if you want 1:1 equals, you must use eql? and == is just comparing the object id’s? So does that mean that the literal 1 uses the same object no matter where that literal is used? It just seems a little weird to me that if everything is an object… then my assumption would be that == would compare the actual state and not the object references like it does in Java. Can you clear this up? Thanks! Question 3 Ruby seems to @ in front of an identifier to denote that it’s an instance variable. I’m curious, what is the reason for this? And if these variables are in fact private to the object, is there some kind of short-hand to create getter/setters?, or do you have to define these 2 def’s yourself? I guess if that’s the case, it’s no worse than in Java, but Scala provides several ways to get around this. I’m curious if such shorthands exist. Thanks! ---
**Author:** ![egervari](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/9dc877/32.png) [@egervari](https://www.ruby-forum.com/u/egervari)\ **Post date:** [October 11, 2010, 11:55am UTC](https://www.ruby-forum.com/t/some-newbie-questions-bare-with-me/196305/2 "2010-10-11T11:55:46Z")
You should have written “bear with me”. ---
**Author:** ![egervari](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/9dc877/32.png) [@egervari](https://www.ruby-forum.com/u/egervari)\ **Post date:** [October 11, 2010, 11:57am UTC](https://www.ruby-forum.com/t/some-newbie-questions-bare-with-me/196305/3 "2010-10-11T11:57:31Z")
On Mon, Oct 11, 2010 at 11:35 AM, egervari [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Question 1 > I was curious what “:” before a identifier was exactly. It seems that > it’s often used as a map key… but what is it exactly? > > Printing :bob to the screen results in “bob”… but testing for > equality between :bob and “bob” results in a falsehood. > > I figure this has to be some literal notation for something, but what? It’s a Symbol. [http://ruby-doc.org/core/classes/Symbol.html](http://ruby-doc.org/core/classes/Symbol.html) A Symbol is an object of which there only exists one. Every time you use the literal :bob, it’s the same object. They are usually used as Hash keys, as flags, and so on. > not the object references like it does in Java. Can you clear this up? > Thanks! Check this: [http://ruby-doc.org/core/classes/Object.html](http://ruby-doc.org/core/classes/Object.html) methods ==, eql? and equals? Also this: [http://blog.rubybestpractices.com/posts/rklemme/018-Complete\_Class.html](http://blog.rubybestpractices.com/posts/rklemme/018-Complete_Class.html) There’s a very good explanation about the equivalence concepts and best practices in Ruby. > Question 3 > Ruby seems to @ in front of an identifier to denote that it’s an > instance variable. I’m curious, what is the reason for this? And if > these variables are in fact private to the object, is there some kind > of short-hand to create getter/setters?, or do you have to define > these 2 def’s yourself? I guess if that’s the case, it’s no worse than > in Java, but Scala provides several ways to get around this. I’m > curious if such shorthands exist. attr\_accessor :a # creates methods a= and a attr\_reader # creates method a attr\_writer # creates method a= Jesus. ---
**Author:** ![egervari](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/9dc877/32.png) [@egervari](https://www.ruby-forum.com/u/egervari)\ **Post date:** [October 11, 2010, 12:17pm UTC](https://www.ruby-forum.com/t/some-newbie-questions-bare-with-me/196305/4 "2010-10-11T12:17:26Z")
On Monday 11 October 2010, egervari wrote: > |Question 1 > |I was curious what “:” before a identifier was exactly. It seems that > |it’s often used as a map key… but what is it exactly? > | > |Printing :bob to the screen results in “bob”… but testing for > |equality between :bob and “bob” results in a falsehood. > | > |I figure this has to be some literal notation for something, but what? It is a Symbol. Symbols are sequences of characters with the property that each sequence has a unique identity. This means that the symbol :abc will be represented by the same object everywhere I use it. For example, in the following code: x = :abc y = :abc x and y contain the same object. This is unlike what happens with strings, where each string literal produces a new object: x = ‘abc’ y = ‘abc’ x and y contain two strings with the same contents but different identity (as you can see by using the object\_id method). Because of their properties, symbols are often used instead of strings when you’re not interested in the content of the string, but are only using it as a unique identifier. This is what happens in hashes. As for the equality test, a string is not a symbol, so ‘bob’==:bob fails. You can learn more about the difference between strings and symbols searching this mailing list for things like “difference between string and symbol”. > |not the object references like it does in Java. Can you clear this up? > |Thanks! Ruby provides three methods to compare objects: - equal? this method compares the identity of objects. That is, obj1.equal? (obj2) is true if and only if their object\_id is the same. This method is defined in Object and shouldn’t (usually) be overridden - == this method is used for comparing tow objects basing on their contents. For class Object, this is the same as equal? (comparing object identity), but other classes may override it to give results basing on the objects’ contents. For example, Array overrides this method to return true if the two arrays have the same size and their contents are the same (according to the == method of each object) - eql? this is a method used by hashes to check for the equality of keys. It usually have the same meaning as ==, but some class change it. You usually don’t need to change this Your problem comes from the fact that you overrode the eql? method rather than the == method > |Question 3 > |Ruby seems to @ in front of an identifier to denote that it’s an > |instance variable. I’m curious, what is the reason for this? And if > |these variables are in fact private to the object, is there some kind > |of short-hand to create getter/setters?, or do you have to define > |these 2 def’s yourself? I guess if that’s the case, it’s no worse than > |in Java, but Scala provides several ways to get around this. I’m > |curious if such shorthands exist. Use the attr\_reader, attr\_writer and attr\_accessor class methods. Attr reader defines a getter method named as the instance variable, attr\_writer defines a setter method named as the instance variable followed by an = and attr\_accessor does both. For example: class C attr\_reader :x attr\_writer :y attr\_accessor :z end is the same as class C def x @x end def y= value @y = value end def z @z end def z= value @z = value end end I hope this helps Stefano ---
**Author:** ![egervari](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/9dc877/32.png) [@egervari](https://www.ruby-forum.com/u/egervari)\ **Post date:** [October 11, 2010, 1:11pm UTC](https://www.ruby-forum.com/t/some-newbie-questions-bare-with-me/196305/5 "2010-10-11T13:11:53Z")
On Mon, Oct 11, 2010 at 12:15 PM, egervari [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Ah, thanks for all of those answers. It helps very much. I didn’t know > I could override == instead of eql?, and this fixed the problem with > RSpec 😉 Makes much more sense, and I’d rather use == than equals, or > eql. I’m glad that it’s not just a bad design decision carried over > from Java that even Java should not have had in it’s original design. But please note that #== in Ruby is not the same as == in Java. Ruby’s #equal? compares for object identity (in Java ==). Methods #== and #eql? are (most of the time) just two ways to access the same underlying logic. If you implement #== you should also implement #eql? because the latter method is used by Hash and Set (see my blog article that Jesus referred you to). Kind regards robert ---
**Author:** ![egervari](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/9dc877/32.png) [@egervari](https://www.ruby-forum.com/u/egervari)\ **Post date:** [October 11, 2010, 12:16pm UTC](https://www.ruby-forum.com/t/some-newbie-questions-bare-with-me/196305/6 "2010-10-11T12:16:15Z")
Ah, thanks for all of those answers. It helps very much. I didn’t know I could override == instead of eql?, and this fixed the problem with RSpec 😉 Makes much more sense, and I’d rather use == than equals, or eql. I’m glad that it’s not just a bad design decision carried over from Java that even Java should not have had in it’s original design. The symbol object makes sense. I guess one thing that is hard to understand is that Ruby seems to keep a flyweight for some classes, but not others. I’m guessing numbers, symbols and others have this automatic flyweight thing going on. I suppose I can just use == for them, and then override == in my personal classes to make them behave the same way. Thanks again for all the info.