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Date: Fri, 2 Dec 2011 16:00:10 +1100
Subject: Re: order independent hash?
From: Chris Angelico <rosuav@gmail.com>
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On Fri, Dec 2, 2011 at 3:29 PM, 88888 Dihedral
<dihedral88888@googlemail.com> wrote:
> I clear my point a hash is a collection of (key, value) pairs that have
> well defined methods and behavior to be used in programming.
>
> The basic operations of a hash normally includes the following:
>
> 1. insertion of a (key, value) pair =A0into the hash
> 2. deletion of a (key, value) from the hash
> 3. inquiring =A0a hash by a key to retrieve the value if the (key, value)
> pair available in the hash. If no key matched, the hash will return
> a not found result.
>
> The hash can grow with (k,v) pairs accumulated in the run time.
> An auto memory management mechanism is required for a hash of a non-fixed=
 size of (k,v) pairs.

That's a hash table - think of a Python dictionary:

On Fri, Dec 2, 2011 at 3:33 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> Python dicts are hash tables.

Although strictly speaking, isn't that "Python dicts are implemented
as hash tables in CPython"? Or is the hashtable implementation
mandated? Anyway, near enough.

Cryptography and data verification use hashing too (look at the
various historic hashing algorithms - CRC, MD5, SHA, etc). The concept
of a hash is a number (usually of a fixed size) that is calculated
from a string or other large data type, such that hashing the same
input will always give the same output, but hashing different input
will usually give different output. It's then possible to identify a
large object solely by its hash, as is done in git, for instance; or
to transmit both the data and the hash, as is done in message
protection schemes (many archiving programs/formats include a hash of
the uncompressed data). These have nothing to do with (key,value)
pairs, but are important uses of hashes.

ChrisA