From: "Peña, Botp" <botp@...>
Date: 2007-07-04T17:24:56+09:00
Subject: Re: Isn't hash default value behavior just a little bizarre?

On Behalf Of Farhad Farzaneh:
# Still, I do find it pretty surprising.  I got stung when I 
# was actually 
# using the feature to initialize any entry to an array just to 
# avoid the 
# business of
# 
# hash[key] ||= []

just be careful w hash#[]<< method (but not too careful, otherwise, you would not enjoy ruby :). What you really wanted then was hash#[]=

irb(main):041:0> y = Hash.new
=> {}
irb(main):043:0> y[1]
=> nil
irb(main):044:0> y[1] << "test"
NoMethodError: undefined method `<<' for nil:NilClass
        from (irb):44
        from :0

here, since y.default == nil  or  y[1] == nil, we are actually doing  nil << "test" wc is obviously wrong.

irb(main):045:0> y[1] = "test"
=> "test"

take a look also at hash#default