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2017-04-08
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Symbols

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Syntax

  • :symbol
  • :'symbol'
  • :"symbol"
  • "symbol".to_sym
  • %s{symbol}

Remarks

Advantages of using symbols over strings:

1. A Ruby symbol is an object with O(1) comparison

To compare two strings, we potentially need to look at every character. For two strings of length N, this will require N+1 comparisons

def string_compare str1, str2
  if str1.length != str2.length
        return false
  end
  for i in 0...str1.length
    return false if str1[i] != str2[i]
  end
  return true
end
string_compare "foobar", "foobar"

But since every appearance of :foobar refers to the same object, we can compare symbols by looking at object IDs. We can do this with a single comparison.(O(1))

def symbol_compare sym1, sym2
  sym1.object_id == sym2.object_id
end
symbol_compare :foobar, :foobar

2. A Ruby symbol is a label in a free-form enumeration

In C++, we can use “enumerations” to represent families of related constants:

enum BugStatus { OPEN, CLOSED };
BugStatus original_status = OPEN;
BugStatus current_status  = CLOSED;

But because Ruby is a dynamic language, we don’t worry about declaring a BugStatus type, or keeping track of the legal values. Instead, we represent the enumeration values as symbols:

original_status = :open
current_status  = :closed

3. A Ruby symbol is a constant, unique name

In Ruby, we can change the contents of a string:

"foobar"[0] = ?b # "boo"

But we can’t change the contents of a symbol:

:foobar[0]  = ?b # Raises an error

4. A Ruby symbol is the keyword for a keyword argument

When passing keyword arguments to a Ruby function, we specify the keywords using symbols:

# Build a URL for 'bug' using Rails.
url_for :controller => 'bug',
        :action => 'show',
        :id => bug.id

5. A Ruby symbol is an excellent choice for a hash key

Typically, we’ll use symbols to represent the keys of a hash table:

options = {}
options[:auto_save]     = true
options[:show_comments] = false

Related Examples