Setting hash equal to another hash in Ruby

Question

I want to keep track of the counts of some arbitarily-named strings and then reset the counts to zero. My thought was to do the following:

reset_hash={"string1"=>0,"string2"=>0,"string3"=>0}
=> {"string1"=>0, "string2"=>0, "string3"=>0} 

new_hash = reset_hash
=> {"string1"=>0, "string2"=>0, "string3"=>0} 

new_hash["string1"]=1
new_hash["string3"]=1
new_hash
=> {"string1"=>1, "string2"=>0, "string3"=>1}

...

Now I want to reset new_hash back to reset_hash:

new_hash = reset_hash
=> {"string1"=>1, "string2"=>0, "string3"=>1}
reset_hash
=> {"string1"=>1, "string2"=>0, "string3"=>1} 

What is going on here? It seems that reset_hash has actually been set to new_hash, which is the opposite of what I wanted. How do I implement the desired behavior?

Answer 1

When you have a variable pointing to an object, you really just have a reference to the object. If both a and b point to the hash {1=>3, "foo" => 54}, changing a or b will change the other.

However, you can use a combination of two methods to make what you want.

A default value for the hash:

new_hash = Hash.new(0)

This gives unused values a default value of 0:

new_hash["eggs"]  # -> 0

You can then add counts:

new_hash["string1"] += 1 # => 1

When you're done just call

new_hash.clear # => {}

and your hash will be reset, but new accesses will still default to 0.

Note that if you set the default value to a type of object other than a number you may be able to change things due to the whole reference issue noted above.

irb(main):031:0> b = Hash.new("Foo") #=> {}
irb(main):032:0> b[3] #=> "Foo"
irb(main):033:0> b[33] #=> "Foo"
irb(main):034:0> b[33].upcase! #=> "FOO"
irb(main):035:0> b[3] # => "FOO"

To get around this you can pass a block to you hash:

h = Hash.new {|hash, key| hash[key] = "new default value"}

This creates a new object at key each time, so changes to one won't ripple:

d = Hash.new{ |hash,key| hash[key] = "string"} #=> {}
d[3] # => "string"
d[3].upcase! #=> "STRING"
d[5] #=> "string"

Answer 2

As others mentioned you have to use clone. Your task should look like this:

reset_hash={"string1"=>0,"string2"=>0,"string3"=>0}
new_hash = reset_hash.clone

new_hash["string1"]=1
new_hash["string3"]=1
new_hash

new_hash = reset_hash.clone
reset_hash