Symbols

Creating a Symbol

The most common way to create a Symbol object is by prefixing the string identifier with a colon:

:a_symbol       # => :a_symbol
:a_symbol.class # => Symbol

Here are some alternative ways to define a Symbol, in combination with a String literal:

:"a_symbol"
"a_symbol".to_sym

Symbols also have a %s sequence that supports arbitrary delimiters similar to how %q and %Q work for strings:

%s(a_symbol)
%s{a_symbol}

The %s is particularly useful to create a symbol from an input that contains white space:

%s{a symbol} # => :"a symbol"

While some interesting symbols (:/, :[], :^, etc.) can be created with certain string identifiers, note that symbols cannot be created using a numeric identifier:

:1 # => syntax error, unexpected tINTEGER, ...
:0.3 # => syntax error, unexpected tFLOAT, ...

Symbols may end with a single ? or ! without needing to use a string literal as the symbol’s identifier:

:hello?  # :"hello?" is not necessary.
:world!  # :"world!" is not necessary.

Note that all of these different methods of creating symbols will return the same object:

:symbol.object_id == "symbol".to_sym.object_id
:symbol.object_id == %s{symbol}.object_id

Since Ruby 2.0 there is a shortcut for creating an array of symbols from words:

%i(numerator denominator) == [:numerator, :denominator]

Converting a String to Symbol

Given a String:

s = "something"

there are several ways to convert it to a Symbol:

s.to_sym
# => :something
:"#{s}"
# => :something

Converting a Symbol to String

Given a Symbol:

s = :something

The simplest way to convert it to a String is by using the Symbol#to_s method:

s.to_s
# => "something"

Another way to do it is by using the Symbol#id2name method which is an alias for the Symbol#to_s method. But it’s a method that is unique to the Symbol class:

s.id2name
# => "something"

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