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? A hash function is different from a hash or so called a hash table in my post. 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. 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. -- http://mail.python.org/mailman/listinfo/python-list