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comp.lang.ruby

Re: Hash order bug?

Berger, Daniel

7/25/2006 2:13:00 PM

> -----Original Message-----
> From: Javier Valencia [mailto:jvalencia@log01.org]
> Sent: Tuesday, July 25, 2006 7:48 AM
> To: ruby-talk ML
> Subject: Re: Hash order bug?
>
>
> dblack@wobblini.net wrote:
>
> > Hi --
> >
> > On Tue, 25 Jul 2006, Javier Valencia wrote:
> >
> >> I have this piece of simple code:
> >>
> >> ----------------------------------------------
> >> def foo
> >> return 5
> >> end
> >>
> >> a = {:gr => false, :und => false, :det => true, :sta => false, :inv
> >> => false}
> >> puts a.inspect
> >> ----------------------------------------------
> >>
> >> when I execute it i get a totally unordered hash:
> >>
> >> ----------------------------------------------
> >> ruby pro.rb
> >> {:inv=>false, :gr=>false, :und=>false, :det=>true, :sta=>false}
> >> ----------------------------------------------
> >>
> >> Now, i delete the foo function from the code (the foo
> function don't
> >> do nothing at all), and i get a well ordered hash:
> >>
> >> ----------------------------------------------
> >> ruby pro.rb
> >> {:gr=>false, :und=>false, :det=>true, :sta=>false, :inv=>false}
> >> ----------------------------------------------
> >>
> >>
> >> What's happening? all my code is behaving wrong because of that.
> >
> >
> > Hashes are unordered. If you need an ordered collection,
> you'll need
> > to use an array.
> >
> >
> > David
> >
>
> Oh my god, i didn't know it, sorry. Is that a missing feature?

No, it's just the way hashes behave. Note that there are ordered hash
implementations on the RAA:

http://raa.ruby-lang.org/project/or...

You can also use a Struct to get roughly the same effect.

Regards,

Dan


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1 Answer

Hal E. Fulton

7/26/2006 12:36:00 AM

0

Berger, Daniel wrote:
>
>
> No, it's just the way hashes behave. Note that there are ordered hash
> implementations on the RAA:
>
> http://raa.ruby-lang.org/project/or...
>
> You can also use a Struct to get roughly the same effect.

In case anyone tries to refer me to these, I'll point out that
there are *three* reasons I like a hash.

1. It indexes on an arbitrary value, not on an integer like
an array.
2. It consists of explicit pairings of key and value.
3. There is a convenient sybtax for literals:
{a=>b, c=>d, e=>f}

Note that there are *no* ordered hash solutions that preserve
the third criterion.

Most would require me to do something like:

oh = OrdHash.new(a,b, c,d, e,f)

or

oh = OrdHash.new([a,b], [c,d], [e,f])

or something just as bad.

Don't underestimate the importance of a convenient notation!
What if arrays in Ruby were like arrays in C? In C, you can
specify an array literal (barely) when you declare it; that's
about it.


Hal