From: Konrad Meyer <konrad@...>
Date: 2007-09-10T04:35:04+09:00
Subject: Re: Hash

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On Sunday 09 September 2007 12:23:43 pm Ron Green wrote:
> Peter Cooper wrote:
> > On 9/9/07, Ron Green <rongreen1@mac.com> wrote:
> >>
> >> Marcel Molina Jr. wrote:
> >> > It should be noted though that String#hash isn't garaunteed to be
> >> > unique.
> >>
> >> Then,again I ask, what is it good for?
> >=20
> >=20
> > It's still useful as a hash. Marcel wasn't wrong, but *no* fixed size=20
> > hash
> > is "guaranteed" to be unique as that's absolutely impossible, per the
> > pigeonhole principle=20
> > (http://en.wikipedia.org/wiki/Pigeonhole_principle).
> > String#hash's hash is of a far lower "quality" than that offered by,=20
> > say,
> > SHA-1 or SHA-2.
> >=20
> > Regards,
> > Peter Cooper
> > http://www.rubyinside.com/
>=20
> Peter,
> If Its not guaranteed to be unique, then it can't be used for identity.
> Can you give me an example of how i would use string.hash?

Let's put it this way. MD5 and SHA-* hashes aren't *guaranteed* to be
unique either. There's just many more cases where strings will share a hash
with String#hash as opposed to something like MD5/SHA-*.

Hashes are useful for identify strings in hashtables. You use this every
time you say something like:

  foo =3D {"bar" =3D> "baz"}
  foo["bar"]                # =3D> "baz"

HTH,
=2D-=20
Konrad Meyer <konrad@tylerc.org> http://konrad.sobertillnoon.com/

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