How can I marshal a hash with arrays?

Question

What should I do to marshal an hash of arrays? The following code only prints {}.

s = Hash.new
s.default = Array.new
s[0] <<          


        

Answer 1

You might be misled about how Hash.default works.

Before you Marshal.dump, print the data structure. It is {}. That's because you are concatenating each string into nil, not into an empty array. The code below illustrates and solves your problem.

s = Hash.new
s.default = Array.new
s[0] = []
s[0] << "Tigger"
s[7] = []
s[7] << "Ruth"
s[7] << "Puuh"
p s
data = Marshal.dump(s)
ls = Marshal.restore( data )
p ls

Returns:

{0=>["Tigger"], 7=>["Ruth", "Puuh"]}
{0=>["Tigger"], 7=>["Ruth", "Puuh"]}

EDIT:

I insert a lot of data into the hash

So maybe some helper code would make the insertion process smoother:

def append_to_hash(hash, position, obj)
  hash[position] = [] unless hash[position]
  hash[position] << obj
end

s = Hash.new
append_to_hash(s, 0, "Tigger")
append_to_hash(s, 7, "Ruth")
append_to_hash(s, 7, "Puuh")
s.default = Array.new // this is only for reading
p s
data = Marshal.dump(s)
ls = Marshal.restore( data )
p ls