From: "Peña, Botp" <botp@...>
Date: 2007-09-03T17:36:23+09:00
Subject: Re: assigning to hash keys when there is a default value?

From: 7stud -- [mailto:dolgun@excite.com] 
# I don't think it's a bug; 

dBlack was referring to this:

irb(main):001:0> h = Hash.new 5
=> {}
irb(main):002:0> h[2]
=> 5
irb(main):003:0> h
=> {}
irb(main):004:0> h[2] = h[2]
=> 5
irb(main):005:0> h
=> {2=>5}
irb(main):006:0> 5 || 10
=> 5
irb(main):007:0> h[3] = 5 || 10
=> 5
irb(main):008:0> h
=> {2=>5, 3=>5}
irb(main):009:0> h[4] = h[4] || 10
=> 5
irb(main):010:0> h
=> {2=>5, 3=>5, 4=>5}
irb(main):011:0> h[5] ||= 10
=> 5

after that step above, you must have h[5] or 5=>5

irb(main):012:0> h
=> {2=>5, 3=>5, 4=>5}

sadly none :(

however, the ff works (wc is supposed to be the longcut version for h[5] ||= 10)

irb(main):013:0> h[5] = h[5] || 10
=> 5
irb(main):014:0> h
=> {5=>5, 2=>5, 3=>5, 4=>5}




# the two examples are set forth on p. 276 of 
# "The Ruby Way (2nd ed)".  However, no explanation is given 
# for why the  two examples work differently.

they should be different (note, that has nothing to do w the bug dBlack is pointing at) and the output should be,

5
{2=>5}    #<---dblack is complaining here because the ruby output is {}

nil
{2=>10}


pls refer to the hash#new document.

irb(main):015:0> system "qri hash.new"
-------------------------------------------------------------- Hash::new
     Hash.new                          => hash
     Hash.new(obj)                     => aHash
     Hash.new {|hash, key| block }     => aHash
------------------------------------------------------------------------
     Returns a new, empty hash. If this hash is subsequently accessed
     by a key that doesn't correspond to a hash entry, the value
     returned depends on the style of new used to create the hash. In
     the first form, the access returns nil. If obj is specified, this
     single object will be used for all default values. If a block is
     specified, it will be called with the hash object and the key, and
     should return the default value. It is the block's responsibility
     to store the value in the hash if required.

        h = Hash.new("Go Fish")
        h["a"] = 100
        h["b"] = 200
        h["a"]           #=> 100
        h["c"]           #=> "Go Fish"
        # The following alters the single default object
        h["c"].upcase!   #=> "GO FISH"
        h["d"]           #=> "GO FISH"
        h.keys           #=> ["a", "b"]

        # While this creates a new default object each time
        h = Hash.new { |hash, key| hash[key] = "Go Fish: #{key}" }
        h["c"]           #=> "Go Fish: c"
        h["c"].upcase!   #=> "GO FISH: C"
        h["d"]           #=> "Go Fish: d"
        h.keys           #=> ["c", "d"]


kind regards -botp