Rails - Oct 2006 - Ruby on Rails Symbol Question

There is good explanation about symbol in the book. One thing that is still not
clear to me is
that symbol sounds like a reference in languages like Java or pointers in C++.
Can someone please
clarify if my understanding is correct? TIA

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This is more of a Ruby-Talk question, but symbols are nothing like  
pointers. They are more like constants, except their type is Symbol.  
Some people like to think of them as immutable strings. Perhaps they  
are more like C++ macros, if an analogy is possible. Pointers/ 
references refer to other objects that actually exist. Symbols really  
*are* objects but their most commonly-used attribute is their name.

Hope this helps.


On Oct 7, 2006, at 3:53 PM, Bala Paranj wrote:
>
> There is good explanation about symbol in the book. One thing that  
> is still not clear to me is
> that symbol sounds like a reference in languages like Java or  
> pointers in C++. Can someone please
> clarify if my understanding is correct? TIA
>
> >

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On Oct 7, 2006, at 5:53 PM, Bala Paranj wrote:
> There is good explanation about symbol in the book. One thing that  
> is still not clear to me is
> that symbol sounds like a reference in languages like Java or  
> pointers in C++. Can someone please
> clarify if my understanding is correct?

Not really: a symbol is simply a name. When we say :fred, we mean the  
name fred. When we say :has_many, we mean the name has_many.

They''re a bit like read-only strings, but without any string methods.  
They''re also very similar to integers: just like there''s only
one
object that represents 42, there''s only one symbol :forty_two.  
Whereever you type 42 in your program, you''re referencing that one  
integer. Similarly, whereever you type :forty_two, you''re referencing  
that same symbol. That''s their power: you can use them as efficient  
ways of naming things, safe in the knowledge that a :duck is a :duck  
is a :duck.


Cheers


Dave

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Hi --

On Sat, 7 Oct 2006, Bala Paranj wrote:
>
> There is good explanation about symbol in the book. One thing that
> is still not clear to me is that symbol sounds like a reference in
> languages like Java or pointers in C++. Can someone please clarify
> if my understanding is correct? TIA
No, a symbol in Ruby isn''t like a reference or pointer; a symbol does
not have any special relation to any other object.

I think one source of confusion with symbols is that the word
"symbol" can be used in two ways: to mean an instance of Symbol and,
more informally, as a general term for an identifier or token.  So
here:

   x = :hello

:hello is a Symbol object, but x is also a symbol, in the informal
sense.  A Symbol object -- a symbol, in that sense -- does not point
to or represent anything other than itself.

See Dave Thomas''s reply, especially concerning the similarity between
symbols and integers.  I''d add that symbols are similar to integers
also in the sense that symbols, like integers, are used both by Ruby,
internally, and by us as programmers.  But there''s no connection
between Ruby''s use and our use.

For example, consider this:

irb(main):020:0> s = "Hello"
=> "Hello"

# Ruby has entered the symbol :s into the local variable
# symbol table:
irb(main):021:0> Symbol.all_symbols.find {|s| s.to_s == "s" }
=> :s

# I can make use of :s myself:
irb(main):022:0> sym = :s
=> :s

I''ve got a variable called s, which means that Ruby creates a symbol
:s, internally.  I can discover this via Symbol.all_symbols.

Now I assign the symbol :s to my variable sym.  But this has nothing
to do with Ruby''s use of the symbol :s.  My two variables, sym and s,
have no connection to each other.  Also, the fact that Ruby is using
:s to track something (local variables) has no connection to the fact
that I''m using the symbol :s in my program.

It''s just like an integer -- say, 75.  Ruby may have some reason to
store an internal count of 75 somethings; and I may have some reason
to use 75 in my program.  But there''s no connection between the two
uses.


David

-- 
                   David A. Black |
dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
Author of "Ruby for Rails"   [1] | Ruby/Rails training &
consultancy [3]
DABlog (DAB''s Weblog)        [2] | Co-director, Ruby Central, Inc.  
[4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

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--- dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
wrote:
> 
> No, a symbol in Ruby isn''t like a reference or pointer; a symbol
does
> not have any special relation to any other object.
Is it correct to say that Symbol represents a unique entity in the system?
(unique label?)

Maybe I am beating the dead horse here. I have to read the replies several times
to get this new
concept into my head.

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Hi --

On Sun, 8 Oct 2006, Bala Paranj wrote:
> --- dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org wrote:
>>
>> No, a symbol in Ruby isn''t like a reference or pointer; a
symbol does
>> not have any special relation to any other object.
>
> Is it correct to say that Symbol represents a unique entity in the system?
(unique label?)
Yes, a symbol is unique; that is, there''s only one :hello.  The symbol
:hello can be used in many different places, but there''s still only
one of it.  Again, it''s very similar to integers: there''s only
one
100, but it can be used repeatedly.


David

-- 
                   David A. Black |
dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
Author of "Ruby for Rails"   [1] | Ruby/Rails training &
consultancy [3]
DABlog (DAB''s Weblog)        [2] | Co-director, Ruby Central, Inc.  
[4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

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Same here, maybe someone has a good example of why :symbols are  
*really* useful?

-ryan






On Oct 8, 2006, at 2:11 AM, Bala Paranj wrote:
>
>
> --- dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org wrote:
>>
>> No, a symbol in Ruby isn''t like a reference or pointer; a
symbol does
>> not have any special relation to any other object.
>
> Is it correct to say that Symbol represents a unique entity in the  
> system? (unique label?)
>
> Maybe I am beating the dead horse here. I have to read the replies  
> several times to get this new
> concept into my head.
>
> >


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On 10/9/06, Ryan Marsh
<marsh.ryanw-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> Same here, maybe someone has a good example of why :symbols are *really*
> useful?
> -ryan
>
Firstly, there is the advantage of less memory consumption. Depending on
what you do with the symbols, this may or may not be significant.
Secondly - and don''t discount this - symbols look nicer in a
syntax-highlighting editor. This increases readability of code, which is
significant.

There may be other reasons that I''m not aware of.

Cheers,
Max


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> Not really: a symbol is simply a name. When we say :fred, we mean the  
> name fred. When we say :has_many, we mean the name has_many.
> 
> They''re a bit like read-only strings, but without any string
methods.
> They''re also very similar to integers: just like there''s
only one
> object that represents 42, there''s only one symbol :forty_two.  
> Whereever you type 42 in your program, you''re referencing that one
> integer. Similarly, whereever you type :forty_two, you''re
referencing
> that same symbol. That''s their power: you can use them as
efficient
> ways of naming things, safe in the knowledge that a :duck is a :duck  
> is a :duck.
When creating links in Rails, I see code that specify controller using symbols
but the application
has multiple controllers. So :controller will be pointing to different
controller for different
links. How does :controller remain constant in this case?

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On 09 Oct 2006, at 16:37, Bala Paranj wrote:
> When creating links in Rails, I see code that specify controller  
> using symbols but the application
> has multiple controllers. So :controller will be pointing to  
> different controller for different
> links. How does :controller remain constant in this case?
The symbol :controller is the key in a hash in this case, and the  
hash is a different instance for each link.


Best regards

Peter De Berdt



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Hi --

On Mon, 9 Oct 2006, Bala Paranj wrote:
>
>> Not really: a symbol is simply a name. When we say :fred, we mean the
>> name fred. When we say :has_many, we mean the name has_many.
>>
>> They''re a bit like read-only strings, but without any string
methods.
>> They''re also very similar to integers: just like
there''s only one
>> object that represents 42, there''s only one symbol :forty_two.
>> Whereever you type 42 in your program, you''re referencing that
one
>> integer. Similarly, whereever you type :forty_two, you''re
referencing
>> that same symbol. That''s their power: you can use them as
efficient
>> ways of naming things, safe in the knowledge that a :duck is a :duck
>> is a :duck.
>
> When creating links in Rails, I see code that specify controller
> using symbols but the application has multiple controllers. So
> :controller will be pointing to different controller for different
> links. How does :controller remain constant in this case?
The same way the number 100 remains constant.  Here''s a hash:

   { 100 => "hi" }

and another:

   { 100 => "bye" }

I''m using 100 twice as a hash key.  There''s no connection
between the
two hashes; the two uses of 100 don''t "know" about each
other.

Similarly, I can do this:

   { :x => "hi" }

and

   { :x => "bye" }

Here, I''m using the symbol :x twice as a hash key.

That''s what Rails does with symbols like :controller.  The symbol
:controller can serve any number of times as a hash key in different
hashes.  The use of a hash key in one has does not bind the key to the
value, except in the context of that hash.


David

-- 
                   David A. Black |
dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
Author of "Ruby for Rails"   [1] | Ruby/Rails training &
consultancy [3]
DABlog (DAB''s Weblog)        [2] | Co-director, Ruby Central, Inc.  
[4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

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"The use of a hash key in one has does not bind the key to the
value, except in the context of that has"

..so why is this an advantage? why is this good? why go the extra mile 
and learn about symbols? to gain...what?

?


-- 
Posted via http://www.ruby-forum.com/.

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wannaknow wrote the following on 09.10.2006 19:03 :
> "The use of a hash key in one has does not bind the key to the
> value, except in the context of that has"
>
> ..so why is this an advantage? why is this good? why go the extra mile 
> and learn about symbols? to gain...what?
>   
1/ A string is a String object which is far more costlier than a Symbol
object.
You gain on memory usage and CPU time used for instanciation.

2/ A symbol doesn''t look like a string -> it can convey a different
semantic content.
For example, I use symbols instead of strings everywhere there''s an API
defined (everywhere I need to pass a hash of parameters, I use symbols
as keys, symbols can be used to represent possible object state values
too). Everywhere you don''t need to treat a String object like a string
(printing it, calling concat, gsub, ...) you can probably assume that
using a symbol is more adequate (note that there''s a place for
constants
in there too, but it''s a different subject). Having the IDE colorize
symbols differently from strings when you follow these conventions
presents cleaner code to the reader.

Lionel.

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I did not notice that each hash has its own unique key and they are different
hashes. Thanks for
the explanation. Now it is clear. 
 
> That''s what Rails does with symbols like :controller.  The symbol
> :controller can serve any number of times as a hash key in different
> hashes.  The use of a hash key in one has does not bind the key to the
> value, except in the context of that hash.
> 
> 
> David

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Bala Paranj wrote:
> I did not notice that each hash has its own unique key and they are 
> different hashes. Thanks for
> the explanation. Now it is clear.


cool thread. i enjoyed it throughly. thanks all for the help. (not that 
i truly understand why i am using it, aside from an intuative point of 
view) [[ which i guess is good enough ]]

10x all.


-- 
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