Learning About Symbols

Rails
1

Hi Everyone,

I kind of get the idea of symbols. So I need help to understand them
more fully.

— config.yml
config:
some_setting: ‘setting_one’
another_setting:
- ‘element1’
- ‘element2’

So how do I ingest this:
yaml = YAML.load(‘config.yml’)

Are these true or false?
yaml[‘config’] == yaml[:config]
yaml[‘config’][‘another_setting’] == yaml[:config][:another_setting]

How else do you use symbols?

Thank you.

P

2

One reason to use Symbols is that they are immutable. When you’re
passing one around as an argument or Hash key, it won’t change.
Another is that multiple instances of a Symbol are the same object,
making a smaller memory footprint than Strings.

A string is not a symbol. Some structures will use to_s or to_sym to
allow you to pass either as an argument, but that’s not their default
behaviour.

{‘a’=>1,:a=>2}
=> {“a”=>1, :a=>2}

{‘a’=>1,‘a’=>2}
=> {“a”=>2}

3

On Friday, July 12, 2013 12:49:11 AM UTC-7, Ruby-Forum.com User wrote:

{‘a’=>1,:a=>2}
=> {“a”=>1, :a=>2}

{‘a’=>1,‘a’=>2}
=> {“a”=>2}

I know about this behavior, but this doesn’t answer the question I
posted.

I tested my question above.
I can create a hash in ruby and explicitly give it symbols for keys and
it
will work.
But when I retrieve a hash from a yml file, I cannot reference the keys
as
symbols.

Are these true or false?

yaml[‘config’] == yaml[:config]
yaml[‘config’][‘another_setting’] == yaml[:config][:another_setting]

The answer to both is false.

So that said, where/how else can symbols be used outside of being hash
keys? Thanks.

4

Are these true or false?

yaml[‘config’] == yaml[:config]
yaml[‘config’][‘another_setting’] == yaml[:config][:another_setting]

The answer to both is false.

yaml[‘config’] shows me the values from the yml file. yaml[:config]
doesn’t.

How else do you use symbols? I have read all the definitions. I
understand
the explanations, but the rest of where I am confused can now only be
answered by example usages.

5

Symbols are used all the time in Ruby/Rails when sending methods.

For example, look at the following method call:

instance.do_stuff(:foo => :bar, :answer => 3)

What you are actually sending to the do_stuff method is a hash that
looks
like the following:

{ :foo => :bar, :answer => 3 }

For these purposes, they are basically human-readable identifiers. The
advantage they have over strings in this case is that they are re-used,
so
if you use :foo in your application 100 times, it’s only in memory once.
Whereas if you use “foo” 100 times, it’s in memory 100 times.

6

On 2013-Jul-12, at 14:53 , Peter wrote:

As it pertains to YAML, String keys and Symbol keys are stored
differently:

irb2.0.0> require ‘yaml’
#2.0.0 => true
irb2.0.0> puts({:symbol => “I am a Symbol”, ‘string’ => “I am a
String”}.to_yaml)

:symbol: I am a Symbol
string: I am a String
#2.0.0 => nil

Note that there is actually a : (colon) in front of the :symbol and not
in front of the string. In both cases there is a : after the key and
before the value. (You might also note that the default value is a
string and quoting is often implied.)

Here are more types so you can see how they get formatted as YAML:

irb2.0.0> puts({:symbol => “I am a Symbol”, ‘string’ => “I am a String”,
42 => “I am the Answer”, “41” => “I am not a number”, “NumberFive” => 5,
“String5” => “5”, ’ quote me ’ => " I need quotes to keep my
leading/trailing spaces"}.to_yaml)

:symbol: I am a Symbol
string: I am a String
42: I am the Answer
‘41’: I am not a number
NumberFive: 5
String5: ‘5’
’ quote me ‘: ’ I need quotes to keep my leading/trailing spaces’
#2.0.0 => nil

-Rob

7

Thanks guys, that has been very informative. Although, each example has
been symbols used in a hash array. Any other uses? Perhaps outside of a
“hash”? Thanks.