gistfile1.rb

hash        = {:dog => 'woof', :cat => 'meow'}
without_dup = hash
with_dup    = hash.dup

# BEFORE CHANGE

puts "hash:        #{hash.inspect}"
puts "without_dup: #{without_dup.inspect}"
puts "with_dup:    #{with_dup.inspect}"

# => hash:        {:dog=>"woof", :cat=>"meow"}
# => without_dup: {:dog=>"woof", :cat=>"meow"}
# => with_dup:    {:dog=>"woof", :cat=>"meow"}

hash[:cat] = 'hiss'

# AFTER CHANGE

puts "hash:        #{hash.inspect}"
puts "without_dup: #{without_dup.inspect}"
puts "with_dup:    #{with_dup.inspect}"

# => hash:        {:dog=>"woof", :cat=>"hiss"}
# => without_dup: {:dog=>"woof", :cat=>"hiss"}
# => with_dup:    {:dog=>"woof", :cat=>"meow"}

hash[:dog].upcase!

# AFTER UPCASE!

puts "hash:        #{hash.inspect}"
puts "without_dup: #{without_dup.inspect}"
puts "with_dup:    #{with_dup.inspect}"