Ruby Symbols VS Strings in Hashes

Why use symbols as hash keys in Ruby?

TL;DR:

Using symbols not only saves time when doing comparisons, but also saves memory, because they are only stored once.

Ruby Symbols are immutable (can't be changed), which makes looking something up much easier

Short(ish) answer:

Using symbols not only saves time when doing comparisons, but also saves memory, because they are only stored once.

Symbols in Ruby are basically "immutable strings" .. that means that they can not be changed, and it implies that the same symbol when referenced many times throughout your source code, is always stored as the same entity, e.g. has the same object id.

Strings on the other hand are mutable, they can be changed anytime. This implies that Ruby needs to store each string you mention throughout your source code in it's separate entity, e.g. if you have a string "name" multiple times mentioned in your source code, Ruby needs to store these all in separate String objects, because they might change later on (that's the nature of a Ruby string).

If you use a string as a Hash key, Ruby needs to evaluate the string and look at it's contents (and compute a hash function on that) and compare the result against the (hashed) values of the keys which are already stored in the Hash.

If you use a symbol as a Hash key, it's implicit that it's immutable, so Ruby can basically just do a comparison of the (hash function of the) object-id against the (hashed) object-ids of keys which are already stored in the Hash. (much faster)

Downside:
Each symbol consumes a slot in the Ruby interpreter's symbol-table, which is never released.
Symbols are never garbage-collected.
So a corner-case is when you have a large number of symbols (e.g. auto-generated ones). In that case you should evaluate how this affects the size of your Ruby interpreter.

Notes:

If you do string comparisons, Ruby can compare symbols just by comparing their object ids, without having to evaluate them. That's much faster than comparing strings, which need to be evaluated.

If you access a hash, Ruby always applies a hash-function to compute a "hash-key" from whatever key you use. You can imagine something like an MD5-hash. And then Ruby compares those "hashed keys" against each other.

Every time you use a string in your code, a new instance is created - string creation is slower than referencing a symbol.

Starting with Ruby 2.1, when you use frozen strings, Ruby will use the same string object. This avoids having to create new copies of the same string, and they are stored in a space that is garbage collected.

Long answers:

https://web.archive.org/web/20180709094450/http://www.reactive.io/tips/2009/01/11/the-difference-between-ruby-symbols-and-strings

http://www.randomhacks.net.s3-website-us-east-1.amazonaws.com/2007/01/20/13-ways-of-looking-at-a-ruby-symbol/

https://www.rubyguides.com/2016/01/ruby-mutability/

How are Symbols faster than Strings in Hash lookups?

There's no obligation for hash to be equivalent to object_id. Those two things serve entirely different purposes. The point of hash is to be as deterministic and yet random as possible so that the values you're inserting into your hash are evenly distributed. The point of object_id is to define a unique object identifier, though there's no requirement that these be random or evenly distributed. In fact, randomizing them is counter-productive, that'd just make things slower for no reason.

The reason symbols tend to be faster is because the memory for them is allocated once (garbage collection issues aside) and recycled for all instances of the same symbol. Strings are not like that. They can be constructed in a multitude of ways, and even two strings that are byte-for-byte identical are likely to be different objects. In fact, it's safer to presume they are than otherwise unless you know for certain they're the same object.

Now when it comes to computing hash, the value must be randomly different even if the string changes very little. Since the symbol can't change computing it can be optimized more. You could just compute a hash of the object_id since that won't change, for example, while the string needs to take into account the content of itself, which is presumably dynamic.

Try benchmarking things:

require 'benchmark'

count = 100000000

Benchmark.bm do |bm|
  bm.report('Symbol:') do
    count.times { :symbol.hash }
  end
  bm.report('String:') do
    count.times { "string".hash }
  end
end

This gives me results like this:

       user     system      total        real
Symbol:  6.340000   0.020000   6.360000 (  6.420563)
String: 11.380000   0.040000  11.420000 ( 11.454172)

Which in this most trivial case is easily 2x faster. Based on some basic testing the performance of the string code degrades O(N) as the strings get longer but the symbol times remain constant.

In Ruby, how to choose whether a symbol or string to be used in a given scenario?

a = :foo
b = :foo

a and b refer to the same object in memory (same identity)

a.object_id # => 898908
b.object_id # => 898908

Strings behave differently

a = 'foo'
b = 'foo'

a.object_id # => 70127643805220
b.object_id # => 70127643805200

So, you use strings to store data and perform manipulations on data (replace characters or whatnot) and you use symbols to name things (keys in a hash or something). Also see this answer for more use cases for symbol.

Ruby Symbols vs Strings in Hashes

Since a symbol is not the same as a string:

:url == 'url' #=> false

As hash keys they would be different. Perhaps you have seen this behavior in Rails? Ruby on Rails uses HashWithIndifferentAccess which maps everything to a String internally, so you can do this:

h = HashWithIndifferentAccess.new
h['url'] = 'http://www.google.com/'
h[:url] #=> 'http://www.google.com/'

Strings vs Symbols when using HashWithIndifferentAccess?

Ruby Strings are mutable, which can bring some unpredictability and reduced performance. For these reasons Ruby also offers the choice of Symbols. The big difference would be that Symbols are immutable. While mutable object can be changed the immutable ones can only be overwritten. Symbols play more nicely with the memory and thus gain on performance, but if not careful enough the memory footprint of your app will increase.

Using Strings or Symbols comes down to understanding both of the terms and how they will serve the purpose in benefit of the overall app health and performance. This is the reason why maybe there is no strict definition of where to use String and where Symbol.

What I would maybe give as a guidance of what is recommended where:

Symbol - internal identifiers, variables not meant to change
String - variables that are changing or printed out

HashWithIndifferentAccess ends up mapping internally all the symbols to strings.

h = ActiveSupport::HashWithIndifferentAccess.new(test: 'test')

If you try to retrieve the keys of the created hash (h) you will get the keys as strings

h.keys # => ["test"]

Compare two hashes no matter symbols or strings, rails

How about this?

require 'set'

def sorta_equal?(sym_hash, str_hash)
  return false unless sym_hash.size == str_hash.size
  sym_hash.to_a.to_set == str_hash.map { |pair|
    pair.map { |o| o.is_a?(String) ? o.to_sym : o } }.to_set
end

sym_hash= {:id=>58, :locale=>:"en-US"}

sorta_equal?(sym_hash, {"id"=>58, "locale"=>"en-US"})           #=> true 
sorta_equal?(sym_hash, {"locale"=>"en-US", "id"=>58 })          #=> true 
sorta_equal?(sym_hash, {"id"=>58, "local"=>"en-US", "a"=>"b" }) #=> false 
sorta_equal?(sym_hash, {"id"=>58, "lacole"=>"en-US"})           #=> false 
sorta_equal?(sym_hash, {"id"=>58, [1,2,3]=>"en-US"})            #=> false 
sorta_equal?({}, {})                                            #=> true 

class A; end
a = A.new
sorta_equal?({:id=>a, :local=>:b}, {"id"=>a, "local"=>"b"})     #=> true

Rails using Symbol vs String as key in params hash

The reason used to be security. It's no longer relevant in Ruby 2.2 or later.

Before Ruby 2.2 symbols were not garbage collected. This means that once a symbol was created via a literal (:my_symbol) or #to_sym it would be around forever.

If Rails used symbols instead of strings then it would create symbols corresponding to names of params in a request. An attacker could send requests with params named param1, param2, ... and exhaust server memory by making the app allocate hundreds of thousands of symbols.

It's no longer the case in Ruby 2.2 or later

string vs symbols rails

When using a Hash in Ruby, any object such a string or a symbol can be used as the key. However, :email and "email" are different objects so to look up an object you must use the same object that was used to store the value.

What might be confusing you is that Rails uses a HashWithIndifferentAccess in some places. This is a customised hash where keys :foo and "foo" are considered to be the same. This allows you to use e.g. params[:user] and params["user"] interchangeably, however that is not the general case.

As Stefan spotted, the reason why strings were being used for the keys in the parsed JSON object was just down to a small typo in the option: symbilize_names where it should be symbolize_names.

When to use symbols instead of strings in Ruby?

TL;DR

A simple rule of thumb is to use symbols every time you need internal identifiers. For Ruby < 2.2 only use symbols when they aren't generated dynamically, to avoid memory leaks.

Full answer

The only reason not to use them for identifiers that are generated dynamically is because of memory concerns.

This question is very common because many programming languages don't have symbols, only strings, and thus strings are also used as identifiers in your code. You should be worrying about what symbols are meant to be, not only when you should use symbols. Symbols are meant to be identifiers. If you follow this philosophy, chances are that you will do things right.

There are several differences between the implementation of symbols and strings. The most important thing about symbols is that they are immutable. This means that they will never have their value changed. Because of this, symbols are instantiated faster than strings and some operations like comparing two symbols is also faster.

The fact that a symbol is immutable allows Ruby to use the same object every time you reference the symbol, saving memory. So every time the interpreter reads :my_key it can take it from memory instead of instantiate it again. This is less expensive than initializing a new string every time.

You can get a list all symbols that are already instantiated with the command Symbol.all_symbols:

symbols_count = Symbol.all_symbols.count # all_symbols is an array with all 
                                         # instantiated symbols. 
a = :one
puts a.object_id
# prints 167778 

a = :two
puts a.object_id
# prints 167858

a = :one
puts a.object_id
# prints 167778 again - the same object_id from the first time!

puts Symbol.all_symbols.count - symbols_count
# prints 2, the two objects we created.

For Ruby versions before 2.2, once a symbol is instantiated, this memory will never be free again. The only way to free the memory is restarting the application. So symbols are also a major cause of memory leaks when used incorrectly. The simplest way to generate a memory leak is using the method to_sym on user input data, since this data will always change, a new portion of the memory will be used forever in the software instance. Ruby 2.2 introduced the symbol garbage collector, which frees symbols generated dynamically, so the memory leaks generated by creating symbols dynamically it is not a concern any longer.

Answering your question:

Is it true I have to use a symbol instead of a string if there is at least two the same strings in my application or script?

If what you are looking for is an identifier to be used internally at your code, you should be using symbols. If you are printing output, you should go with strings, even if it appears more than once, even allocating two different objects in memory.

Here's the reasoning:

Printing the symbols will be slower than printing strings because they are cast to strings.
Having lots of different symbols will increase the overall memory usage of your application since they are never deallocated. And you are never using all strings from your code at the same time.

Use case by @AlanDert

@AlanDert: if I use many times something like %input{type: :checkbox} in haml code, what should I use as checkbox?

Me: Yes.

@AlanDert: But to print out a symbol on html page, it should be converted to string, shouldn't it? what's the point of using it then?

What is the type of an input? An identifier of the type of input you want to use or something you want to show to the user?

It is true that it will become HTML code at some point, but at the moment you are writing that line of your code, it is mean to be an identifier - it identifies what kind of input field you need. Thus, it is used over and over again in your code, and have always the same "string" of characters as the identifier and won't generate a memory leak.

That said, why don't we evaluate the data to see if strings are faster?

This is a simple benchmark I created for this:

require 'benchmark'
require 'haml'

str = Benchmark.measure do
  10_000.times do
    Haml::Engine.new('%input{type: "checkbox"}').render
  end
end.total

sym = Benchmark.measure do
  10_000.times do
    Haml::Engine.new('%input{type: :checkbox}').render
  end
end.total

puts "String: " + str.to_s
puts "Symbol: " + sym.to_s

Three outputs:

# first time
String: 5.14
Symbol: 5.07
#second
String: 5.29
Symbol: 5.050000000000001
#third
String: 4.7700000000000005
Symbol: 4.68

So using smbols is actually a bit faster than using strings. Why is that? It depends on the way HAML is implemented. I would need to hack a bit on HAML code to see, but if you keep using symbols in the concept of an identifier, your application will be faster and reliable. When questions strike, benchmark it and get your answers.