How do I copy a hash in Ruby?

The clone method is Ruby's standard, built-in way to do a shallow-copy:

irb(main):003:0> h0 = {"John" => "Adams", "Thomas" => "Jefferson"}=> {"John"=>"Adams", "Thomas"=>"Jefferson"}irb(main):004:0> h1 = h0.clone=> {"John"=>"Adams", "Thomas"=>"Jefferson"}irb(main):005:0> h1["John"] = "Smith"=> "Smith"irb(main):006:0> h1=> {"John"=>"Smith", "Thomas"=>"Jefferson"}irb(main):007:0> h0=> {"John"=>"Adams", "Thomas"=>"Jefferson"}

Note that the behavior may be overridden:

This method may have class-specific behavior. If so, that behavior will be documented under the #initialize_copy method of the class.

As others have pointed out, clone will do it. Be aware that clone of a hash makes a shallow copy. That is to say:

h1 = {:a => 'foo'} h2 = h1.cloneh1[:a] << 'bar'p h2                # => {:a=>"foobar"}

What's happening is that the hash's references are being copied, but not the objects that the references refer to.

If you want a deep copy then:

def deep_copy(o)  Marshal.load(Marshal.dump(o))endh1 = {:a => 'foo'}h2 = deep_copy(h1)h1[:a] << 'bar'p h2                # => {:a=>"foo"}

deep_copy works for any object that can be marshalled. Most built-in data types (Array, Hash, String, &c.) can be marshalled.

Marshalling is Ruby's name for serialization. With marshalling, the object--with the objects it refers to--is converted to a series of bytes; those bytes are then used to create another object like the original.

If you're using Rails you can do:

h1 = h0.deep_dup

http://apidock.com/rails/Hash/deep_dup