From: Ben Armstrong <BArmstrong@...>
Date: 2005-11-10T01:37:22+09:00
Subject: Nested hash constructor confusion

Can anyone explain the following?  Our "intuitive" solution (h1) to
creating a constructor for a hash nested within a hash did not work.  It
was only after attempting a workaround (h3) and finding that it
performed very poorly that we Googled (h2) a correct solution.  But it
is not obvious to us why this more complex constructor is required.

# Unexpected result, h1 hash is empty.
h1=Hash.new(Hash.new(0)) # => {}
h1[1][2]='h1'            # => "h1"
h1                       # => {}

# Expected result, h2 hash contains nested hash.
h2=Hash.new{|hash,key| hash[key]=Hash.new(0)}  # => {}
h2[1][2]='h2'            # => "h2"
h2                       # => {1=>{2=>"h2"}}

# Expected result but expensive.
h3=Hash.new(Hash.new(0)) # => {}
temp=h3[1].dup           # => {}
temp[2]='h3'             # => "h3"
h3[1]=temp               # => {2=>"h3"}
h3                       # => {1=>{2=>"h3"}}

Thanks,
Ben Armstrong