Radek Radek - 1 year ago 46
Ruby Question

How to assign hash["a"]["b"]= "c" if hash["a"] doesn't exist?

is there any other simpler way than

if (hash.has_key?("a") )
hash["a"]["b"] = "c"
hash["a"] = Hash.new
hash["a"]["b"] = "c"

Answer Source

The easiest way is to construct your Hash with a block argument:

hash = Hash.new { |h, k| h[k] = { } }
hash['a']['b'] = 1
hash['a']['c'] = 1
hash['b']['c'] = 1
puts hash.inspect
# "{"a"=>{"b"=>1, "c"=>1}, "b"=>{"c"=>1}}"

This form for new creates a new empty Hash as the default value. You don't want this:

hash = Hash.new({ })

as that will use the exact same hash for all default entries.

Also, as Phrogz notes, you can make the auto-vivified hashes auto-vivify using default_proc:

hash = Hash.new{ |h,k| h[k] = Hash.new(&h.default_proc) }

UPDATE: I think I should clarify my warning against Hash.new({ }). When you say this:

h = Hash.new({ })

That's pretty much like saying this:

h = Hash.new
h.default = { }

And then, when you access h to assign something as h[:k][:m] = y, it behaves as though you did this:

    h[:k][:m] = y
    h.default[:m] = y

And then, if you h[:k2][:n] = z, you'll end up assigning h.default[:n] = z. Note that h still says that h.has_key?(:k) is false.

However, when you say this:

h = Hash.new(0)

Everything will work out okay because you will never modified h[k] in place here, you'll only read a value from h (which will use the default if necessary) or assign a new value to h.