Understanding Ruby Symbols

Ruby is an interpreted language. It is dynamically typed and uses a new memory for a variable. A variable has a name and a value. Symbols are an optimized variable that holds single instance of memory. It is good for variables that assume the same values across the program such as hash table keys.

h = {'my_key' => 123}

The storage for my_key is allocated each time my_key is used. That’s waste of memory and many related bookkeeping tasks by the Ruby interpreter.

So declaring the key as a symbol makes sense as only one copy of my_key is kept in memory.

h = {:my_key => 123}

You have to use the : operator with each usage of a symbol.

irb(main):003:0> new_hash={:my_key => 123}
=> {:my_key=>123}

irb(main):004:0> new_hash[:my_key]
=> 123

# You must use the :

irb(main):005:0> new_hash[my_key]
NameError: undefined local variable or method `my_key' for main:Object
    from (irb):5
    
irb(main):006:0> new_hash['my_key']
=> nil

Written with StackEdit.