From: Robert Klemme <shortcutter@...>
Date: 2007-05-25T06:27:04+09:00
Subject: Re: Enumerator for Hash

2007/5/24, Ryan Hinton <iobass@email.com>:
> On May 23, 12:18 pm, Robert Klemme <shortcut...@googlemail.com> wrote:
> > You could wrap your keys in a custom class.  For added convenience you
> > could also wrap Hash with another class which wraps keys on insertion
> > and unwraps on reading.
> > ...
> >
> > In that case I'd probably rather do this:
> >
> > my_array.inject({}) {|hs, obj| hs[obj.key] ||= obj; hs }.values
>
> Now I remember why I wanted my earlier approach.  My object has
> several Array member variables.  By default, my #hash and #eql?
> methods use all of them.  In one case I need to ignore 2 of the 4
> arrays, e.g. for a uniqueness hash.  I can use your second method
> thus.
>
>   my_array.inject({}) {|hs, obj| hs[obj.arr1 + obj.arr2] ||= obj;
> hs }.values

Appending is not safe:

[1,2] + [3,4,5] -> [1,2,3,4,5]
[1,2,3] + [4,5] -> [1,2,3,4,5]

You want to be creating new arrays with two elements:

my_array.inject({}) {|hs, obj| hs[[obj.arr1, obj.arr2]] ||= obj; hs }.values

> But won't this create a bevy of new Array objects with the "+" method
> (two for each item in the original array)?

Yes, but I'd rather first try the suggestion above out before starting
to think about optimizations.

>  I could instead pre-hash
> the values something like the following.
>
>   my_array.inject({}) {|hs, obj| hs[obj.arr1.hash ^ obj.arr2.hash] ||=
> obj; hs }.values

That's a bad idea because of the nature of hash values, as you
correctly identified:

> This avoids the new Array spew, but I will lose unique objects if my
> fabricated hash keys clash (i.e. same hash key for unique arrays).  If
> I can specify different methods to use in place of #hash and #eql?, I
> can get the same behavior without the performance penalty.  (My code
> is already slow.)
>
> I think I'll use your wrapping Hash idea.  Thanks!

I'd start with the other approach which is considerably easier.

robert


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