Re: Comparing DS_HASH_TABLE objects

Paul Cohen wrote:
> On Sun, May 31, 2009 at 8:43 PM, Eric Bezault <ericb-D6Qt/9opevxWk0Htik3J/[email protected]> wrote:
>>> How do I compare two DS_HASH_TABLE objects for object equality?
>> I don't know what you mean. Do you want to consider two
>> hash tables equal if the items they contain are equal?
> 
> I am aware that there are different ways of defining equality between
> two tables t1 and t2. My assumption was something like:
> 
> t1.keys.is_equal (t2.keys) and then
>    for each key k1 in t1:
>        (t1 @ k1).is_equal (t2 @ k2)
> 
> I don't think the 'equality_tester':s should be used since they have
> more with how DS_HASH_TABLE:s work, than what they contain (though how
> they work will of course affect what they contain!).

I don't see how `equality_tester' has more to do with how hash
table works than what you try to do. I guess you're confusing
`equality_tester' with `key_equality_tester'. Which brings to
my attention the fact that in order to be correct, hash table
equality should use `key_equality_tester' to compare keys in
your code above, not `is_equal'. Indeed keys might be equal
in terms of `is_equal', but they might nevertheless represent
two different keys in the hash table (which uses `key_equality_
tester' for that).


>> If so, what does it mean for two items in the hash tables
>> to be equal? Should they have the same key?
> 
> Yes and the items should be 'is_equal'.
> 
>> If so, I don't
>> think there is a feature for that in DS_HASH_TABLE.
>> The implementation of `is_equal' uses '=' to compare
>> items.
> 
> Yes. I saw that. Why '=' and not 'is_equal'?

But why `is_equal' and not any other kind of comparison?
It uses '=' to compare items because `is_equal' is used
in the postcondition of `copy/twin', and their implementation
in hash table does not clone the items.


> 
> I was just asking to make sure I hadn't overlooked anything and to see
> if anyone had a standard idiom for this problem.
> 
> I'm writing a test case for a situation where a DS_HASH_TABLE [ANY,
> UC_STRING] is converted to another representation (JSON object) and
> then back again, and I want to check that the two tables are "equal".
> Obviously they won't contain the same objects so comparing by
> reference isn't of much help. Note that I'm using ANY as the type for
> my items.
> 
> /Paul
> 


-- 
Eric Bezault
mailto:ericb-D6Qt/9opevxWk0Htik3J/[email protected]
http://www.gobosoft.com


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