Hash assignment [:x] vs ['x']


#1

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?


#2

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.


#3

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 :slight_smile:


#4

On Mon, May 25, 2009 at 6:19 PM, Mk 27 removed_email_address@domain.invalid
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


#5

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


#6

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
irb(main):002:0>

:wink:

Kind regards

robert


#7

On Mon, May 25, 2009 at 1:56 PM, Bertram S.
removed_email_address@domain.invalid 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

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