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.
--
http://mail.python.org/mailman/listinfo/python-list
