From: Charles Mills <cmills@...>
Date: 2005-02-09T03:45:08+09:00
Subject: Re: Map keys and values behavior


Thomas E Enebo wrote:
> I am working on the JRuby project and I was running some of the tests
> in samples/test.rb from main ruby-1.8.1 source.  I ran across
something
> which seems like a bug to me (in the test).  It is something like:
>
> $y = {1, 2, 2, 4, 3, 6}
> test_ok($y.keys, [1,2,3])
> test_ok($y.values, [2,4,6])
>
>   Does ruby Maps really return keys and values in order of insertion?

It might or it might not.  But in general no.

> That seems strange to me.  Seems like the tests are missing a sort.
>
$ ruby -v
ruby 1.8.2 (2004-12-25) [i386-cygwin]
$ irb
irb(main):001:0> h = {1=>2, 2=>4, 3=>6}
=> {1=>2, 2=>4, 3=>6}
irb(main):002:0> h.keys
=> [1, 2, 3]
irb(main):003:0> h.values
=> [2, 4, 6]
irb(main):004:0> 3.hash
=> 7

In the above example the keys are in order, but that is only because
Ruby's hash's hashing algorithm.  Fixnums hash to themself (Hash does
not call Fixnum#hash to save time, so 3.hash really == 3).  Then (I
think, see st.c to verify) when the Hash is iterated over (to collect
the keys, values, etc) it is traversed in the order of the hash of the
keys.

Here is another example:
irb(main):003:0> "a".hash
=> 100
irb(main):004:0> "b".hash
=> 101
irb(main):005:0> h = {"a"=>1, "b"=>2}
=> {"a"=>1, "b"=>2}
irb(main):006:0> h.keys
=> ["a", "b"]
irb(main):007:0> h.values
=> [1, 2]
### Now try in different order ###
irb(main):008:0> h = {"b"=>2, "a"=>1}
=> {"a"=>1, "b"=>2}
irb(main):009:0> h.keys
=> ["a", "b"]
irb(main):010:0> h.values
=> [1, 2]

100 comes before 101.

-Charlie