On Fri, Jan 29, 2010 at 8:03 AM, spir <denis.s...@free.fr> wrote: > I recently discovered that Lua uses the data's address (read: id) as input to > the hash func. This allows Lua tables (a kind of more versatile associative > array) to use _anything_ as key, since the id is guaranteed not to change, > per definition. > [Aside this consideration, hashing addresses ensures a constant type as input > (so allows a specific efficient hash method) and the simplest possible one.]
This sounds like a bad idea to me. You generally want key lookup to be based on value, not identity. For example, if I say d = dict() d['key'] = value and later print d['key'] I want this to print 'value' regardless of whether I use the same instance of the string 'key' in both cases. Maybe Lua has some mechanism for ensuring that this can't happen? In general, for a hash table to work as expected, two keys that are equal in value should have the same hash value. Here is an explanation of why (for Java, but it pretty much applies to any hash table implementation): http://www.javaworld.com/javaqa/2002-06/01-qa-0621-hashtable.html Kent _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
