Ruby Tuesday – String#to_sym

Today’s Ruby Tuesday covers String#to_sym.

String#to_sym operates on a String object, and turns the string into a Symbol.

Why would someone want to have a Symbol instead of a String?

The main reason is that Symbols are created once for the lifetime of a program, and use the same reference. Every reference to the symbol :foo refers to the same instance of :foo, and it doesn’t matter when, or where, in the program lifetime it is.

Strings on the other hand are unique objects, so two strings of the same value are not necessarily the same object, and depends on when, where, and how they were created.

"foo".to_sym
# => :foo
"Foo".to_sym
# => :Foo
"foo".__id__ == "foo".__id__
# => false
:foo.__id__ == :foo.__id__
# => true

One case where this might be useful is where you have to tokenize strings, and are going to want to use them as keys for a map. Think header columns in a CSV file, but please don’t write your own CSV parser, as Ruby has a nice one built in.

"firstName,lastName,preferred name,age".split(",").map(&:to_sym)
# => [:firstName, :lastName, :"preferred name", :age]

The other place where you might want to use String#to_sym is when you are trying to create Symbols that cannot easily be created by just using the normal symbol notation.

":foo".to_sym
# => :":foo"
"peanut butter".to_sym
# => :"peanut butter"

–Proctor

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