Forum: Ruby hash assignment [:x] vs ['x']

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68814ed988eee4f21b2cd07353e5e445?d=identicon&s=25 Mk 27 (mk27)
on 2009-05-25 19:19
Being totally new to ruby, I just noticed that using

bob['phone']

which coming from perl is a style I'm used to, results in a separate key
than

bob[:phone]

That seems slightly weird, I had assumed the :form is just a shorthand
like %w(  ).  So -- is there anything more that I need to understand
about this distinction?
3afd3e5e05dc9310c89aa5762cc8dd1d?d=identicon&s=25 Tim Hunter (Guest)
on 2009-05-25 19:26
(Received via mailing list)
Mk 27 wrote:
> like %w(  ).  So -- is there anything more that I need to understand
> about this distinction?

:x is a Symbol object. 'x' is a String. You can make a symbol from a
string with the intern method, and a string from a symbol with the to_s
method.

A symbol is a named number that Ruby guarantees will have the same value
throughout your program. You pick the name, Ruby picks the number.
753dcb78b3a3651127665da4bed3c782?d=identicon&s=25 Brian Candler (candlerb)
on 2009-05-25 19:30
Mk 27 wrote:
> Being totally new to ruby, I just noticed that using
>
> bob['phone']
>
> which coming from perl is a style I'm used to, results in a separate key
> than
>
> bob[:phone]
>
> That seems slightly weird, I had assumed the :form is just a shorthand
> like %w(  ).  So -- is there anything more that I need to understand
> about this distinction?

Yes. :phone is a Symbol literal, whereas 'phone' is a String literal.

Symbols and Strings are two different types of object entirely. In
particular:

* Symbols are immutable

* a literal like :foo will always give exactly the same object instance
throughout the lifetime of the program, whereas a string literal like
'foo' will give a fresh object every time it is used

For this reason, symbols are somewhat more efficient for hash lookups.
Some libraries use them for passing hashes of options, although I
suspect this is more because they are one character shorter to type,
rather than any particular performance reason :-)
146e52d49d361f85c0945487452fc6a0?d=identicon&s=25 Ben Lovell (benl)
on 2009-05-25 19:32
(Received via mailing list)
On Mon, May 25, 2009 at 6:19 PM, Mk 27 <halfcountplus@intergate.com>
wrote:

> like %w(  ).  So -- is there anything more that I need to understand
> about this distinction?


The first example is indexing via a string, the second is via a ruby
symbol.
You can think of a symbol as essentially a (very) lightweight string
usually
used for keying into hashes and general naming of things.

 Rgds,
Ben
C06869c119472a139eb163b72040b0db?d=identicon&s=25 Bertram Scharpf (Guest)
on 2009-05-25 19:56
(Received via mailing list)
Hi,

Am Dienstag, 26. Mai 2009, 02:19:29 +0900 schrieb Mk 27:
> bob['phone']
> bob[:phone]
>
> That seems slightly weird, I had assumed the :form is just a shorthand
> like %w(  ).  So -- is there anything more that I need to understand
> about this distinction?

  $ irb
  irb(main):001:0> :phone.class
  => Symbol
  irb(main):002:0> 'phone'.class
  => String

and

  irb(main):003:0> :phone.methods.sort - 'phone'.methods
  => ["id2name", "to_int"]
  irb(main):004:0> String.instance_methods.sort -
Symbol.instance_methods
  => [ ... (loads)]

:phone.methods  ==  Symbol.instance_methods

Bertram
8f6f95c4bd64d5f10dfddfdcd03c19d6?d=identicon&s=25 Rick Denatale (rdenatale)
on 2009-05-25 20:13
(Received via mailing list)
On Mon, May 25, 2009 at 1:56 PM, Bertram Scharpf
<lists@bertram-scharpf.de> wrote:
>  $ irb
>  => [ ... (loads)]
>
> :phone.methods  ==  Symbol.instance_methods

Strings are mutable, Symbols are immutable.

There is only one instance of a Symbol with a given string 'value' so

(hash1 == hash2) iff (hash1.object_id == hash2.object_id)

This is the main property of a Symbol, it's used for keys in Hashes
where lookup performance is important (like the method hashes used
internally by Ruby), since it means that Hash#== is O(1) and doesn't
depend on the 'length'

The downside is that Symbols can't be garbage collected (at least for
the MRI implementation) in order to preserve the id/value invariant a
list of all string values which have been 'interned' when a Hash has
been created needs to be kept.

In Rails, it's common to see Symbol and String keys used
'interchangeably', but this is only possible because Rails provides a
HashWithIndifferentAccess class which actually uses string keys
internally (because of the GC issue) but allows either to be used as
key arguments to methods.

--
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale
E0d864d9677f3c1482a20152b7cac0e2?d=identicon&s=25 Robert Klemme (Guest)
on 2009-05-25 21:50
(Received via mailing list)
On 25.05.2009 20:13, Rick DeNatale wrote:

> Strings are mutable, ...

Well, most of the time - but sometimes they aren't:

irb(main):001:0> "I am not".freeze.upcase!
RuntimeError: can't modify frozen string
         from (irb):2:in `upcase!'
         from (irb):2
         from /usr/local/bin/irb19:12:in `<main>'
irb(main):002:0>

;-)

Kind regards

  robert
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