Re: order independent hash?

From	Lie Ryan <lie.1296@gmail.com>
Subject	Re: order independent hash?
Date	2011-12-05 00:40 +1100
References	(3 earlier) <4ED7A2CE.6070306@davea.name> <mailman.3203.1322756312.27778.python-list@python.org> <6366868.86.1322800180523.JavaMail.geo-discussion-forums@pret21> <mailman.3216.1322802013.27778.python-list@python.org> <1576053.251.1322804908048.JavaMail.geo-discussion-forums@pruu5>
Newsgroups	comp.lang.python
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On 12/02/2011 04:48 PM, 88888 Dihedral wrote:
> On Friday, December 2, 2011 1:00:10 PM UTC+8, Chris Angelico wrote:
>> On Fri, Dec 2, 2011 at 3:29 PM, 88888 Dihedral
>> <dihedr...@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  into the hash
>>> 2. deletion of a (key, value) from the hash
>>> 3. inquiring  a 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....@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
>
> If one tries to insert a (k,v1) and then a (k,v2) pair into a
> hash with v1 not equals V2, what could happen in your understanding of
> a hash?

Don't try to argue, in English, `hash != hash` is true; it's just a 
typical occurence of homonyms. Just because they have the same name 
doesn't mean hash (function) has to have somewhat similar properties to 
hash (table).

> A hash function is different from a hash or so called a hash table in
> my post.

Indeed.

> If the hash collision rate is not specified, then  it is  trivial to write a hash function with the conditions you specified. A hash function applied to a set of data items only  is of very limited use at all.

It's trivial indeed, but a hashtable couldn't exist without hash 
function. And without a good hash function, a hash table's performance 
may degrade into O(n) access/insertion/deletion.

> A hash stores (k,v) pairs specified in the run time with auto memory
> management build in is not a simple hash function to produce data signatures only clearly in my post.
>
> What I said a hash which is lifted as a basic type in python  is called a dictionary in python.
>
> It is called a map in c++'s generics library.

Putting aside all these, it's pretty obvious from the beginning that OP 
was referring to hash functions, not hash tables.

Thread

Re: order independent hash? Peter Otten <__peter__@web.de> - 2011-11-30 13:47 +0100
  Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-01 07:35 -0800
  Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-01 07:35 -0800
    Re: order independent hash? Dave Angel <d@davea.name> - 2011-12-01 10:52 -0500
      Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-01 08:17 -0800
      Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-01 08:17 -0800
    Re: order independent hash? Chris Angelico <rosuav@gmail.com> - 2011-12-02 03:18 +1100
      Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-01 20:29 -0800
        Re: order independent hash? Chris Angelico <rosuav@gmail.com> - 2011-12-02 16:00 +1100
          Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-01 21:48 -0800
            Re: order independent hash? Lie Ryan <lie.1296@gmail.com> - 2011-12-05 00:40 +1100
          Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-01 21:48 -0800
            Re: order independent hash? Roel Schroeven <rschroev_nospam_ml@fastmail.fm> - 2011-12-04 14:41 +0100
              Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-04 07:39 -0800
                Re: order independent hash? Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-04 10:41 -0700
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-04 10:06 -0800
                Re: order independent hash? Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-04 13:13 -0700
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-04 15:17 -0800
                Re: order independent hash? Matt Joiner <anacrolix@gmail.com> - 2011-12-05 10:21 +1100
                Re: order independent hash? Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-04 16:24 -0700
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-04 16:52 -0800
                Re: order independent hash? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-05 02:00 +0000
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-04 16:52 -0800
                Re: order independent hash? Lie Ryan <lie.1296@gmail.com> - 2011-12-05 12:59 +1100
                Re: order independent hash? Ethan Furman <ethan@stoneleaf.us> - 2011-12-04 18:08 -0800
                Re: order independent hash? Ben Finney <ben+python@benfinney.id.au> - 2011-12-05 15:52 +1100
                Re: order independent hash? Matt Joiner <anacrolix@gmail.com> - 2011-12-05 14:02 +1100
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-06 21:24 -0800
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-06 21:24 -0800
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-04 15:17 -0800
                Re: order independent hash? Terry Reedy <tjreedy@udel.edu> - 2011-12-04 19:28 -0500
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-04 10:06 -0800
          Re: order independent hash? Hrvoje Niksic <hniksic@xemacs.org> - 2011-12-02 10:53 +0100
            Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-02 03:48 -0800
            Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-02 09:00 -0800
            Re: order independent hash? Terry Reedy <tjreedy@udel.edu> - 2011-12-02 16:40 -0500
              Re: order independent hash? Hrvoje Niksic <hniksic@xemacs.org> - 2011-12-04 14:55 +0100
                Re: order independent hash? Chris Angelico <rosuav@gmail.com> - 2011-12-05 01:08 +1100
                Re: order independent hash? Hrvoje Niksic <hniksic@xemacs.org> - 2011-12-07 14:28 +0100
                Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-07 09:48 -0800
                Re: order independent hash? Tim Chase <python.list@tim.thechases.com> - 2011-12-04 12:57 -0600
                Re: order independent hash? Hrvoje Niksic <hniksic@xemacs.org> - 2011-12-08 10:30 +0100
                Re: order independent hash? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-09 11:27 +0000
                Re: order independent hash? Hrvoje Niksic <hniksic@xemacs.org> - 2011-12-09 17:51 +0100
                Re: order independent hash? Chris Angelico <rosuav@gmail.com> - 2011-12-10 04:13 +1100
                Re: order independent hash? Lie Ryan <lie.1296@gmail.com> - 2011-12-11 10:58 +1100
                Re: order independent hash? Chris Angelico <rosuav@gmail.com> - 2011-12-11 11:17 +1100
                Re: order independent hash? Lie Ryan <lie.1296@gmail.com> - 2011-12-11 14:04 +1100
        Re: order independent hash? Lie Ryan <lie.1296@gmail.com> - 2011-12-05 00:29 +1100
      Re: order independent hash? 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-01 20:29 -0800
      Re: order independent hash? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-02 04:33 +0000

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