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Message |
< Erlang ~ Set comparision |
| darkolee |
 Posted: Thu Apr 01, 2010 4:22 pm |
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Joined: 02 Dec 2009
Posts: 12
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Hi
how should two sets be compared? Should the == operator be used?
If so, try out the following code
Set1=sets:from_list([a,q]).
Set2=sets:from_list([q,a]).
Set1==Set2.
The last statement returns false! Does anyone know of the proper way to compare sets which returns true if the elements of both sets are the same (without taking care of the order of these elements)?
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| zajda |
| Posted: Thu Apr 01, 2010 10:20 pm |
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Joined: 22 Aug 2009
Posts: 83
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hi,
== fails because [q,a] == [a,q] fails also.
if you dont care about performance use just something like:
Code: 3> [] =:= sets:to_list(sets:subtract(S1,S2)).
true
Writing custom code can be tricky, because in set structure there can be some pre-allocated empty segments (it depends what happened before with this set). Btw. thats why in above example we cannot pattern match on new() set.
I would start writing my code from combining chain of functions from library, in above example, into one custom (to cut down number of iterations). Maybe use at the beginning contract_seg/1 (which is not exported from sets) - to cut down length of iteration.
Michał Zajda
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Erlang Solutions Ltd |
Last edited by zajda on Fri Apr 02, 2010 1:04 pm; edited 1 time in total |
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| darkolee |
| Posted: Thu Apr 01, 2010 11:28 pm |
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Joined: 02 Dec 2009
Posts: 12
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Thanks for your reply.
I think the correct "heavy-weight" set comparision should be:
Code: [] =:= sets:to_list(
sets:union(
sets:subtract(S1,S2),
sets:subtract(S2,S1)
))
Well, it is inefficient as ugly as it looks, however that is the only way out. Shouldn't there be a simple equal/2 function in the sets module which returns a boolean indicating set equality? As far as I know, the set equality semantics are not affected by any sort of ordering of elements.
Quote: I would start writing my code from combining chain of functions from library, in above example, into one custom (to cut down number of iterations). Maybe use at the beginning contract_seg/1 (which is not exported from sets) - to cut down length of iteration.
I did not understand your final paragraph :s
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| zajda |
| Posted: Fri Apr 02, 2010 1:15 pm |
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Joined: 22 Aug 2009
Posts: 83
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Quote:
I did not understand your final paragraph :s
yep;) my English sucks sometimes;) let me rephrase it.
What I mean, is to go light weight not heavy weight.
To do it, we need to take a look of internals of the sets module,
to cut down to minimum iterations over set structure. By cut down I mean
make less of them and each one shorter. Maybe the cheapest solution would
be anyway rewriting it to some ordered structure? It need some profiling definitely. |
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| rvirding |
| Posted: Thu Apr 08, 2010 10:48 pm |
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Joined: 30 Aug 2006
Posts: 452
Location: Stockholm, Sweden
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Sets (and dict) uses a hash table internally. One effect of this is that two sets which contain the same elements may not have the same internal structure, how it looks internally depends on how you got there, as in your example. All set operations are, of course, guaranteed to give the same results.
You get the same property with gb_sets which uses trees internally, two sets with the same elements are not guaranteed to have the same internal structure. The only set module I know of which has this property is ordsets.
A simple way to test set equality is Code: sets:size(sets:subtract(Set1, Set2)) == 0
This may be a useful function to add to the API. |
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| darkolee |
| Posted: Sun Apr 18, 2010 7:00 pm |
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Joined: 02 Dec 2009
Posts: 12
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Thanks for your answers. Shouldn't the quick set comparison check be:
Code:
( sets:size(sets:subtract(Set1, Set2)) == 0)
and
( sets:size(sets:subtract(Set2, Set1)) == 0)
???
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| zajda |
| Posted: Sun Apr 18, 2010 8:28 pm |
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Joined: 22 Aug 2009
Posts: 83
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this is the most elegant and simplest solution from given above in this thread.
It should be the fastest, because, if I am not wrong, set holds in its structure, its own size. No conversion = better performance.
You can always prepare some primitive test case and measure the execution time with timer:tc.
Michał Zajda
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Erlang Solutions Ltd |
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| timrila |
| Posted: Tue Jun 12, 2012 9:26 am |
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| goods |
| Posted: Fri Jun 29, 2012 2:03 am |
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| wuji |
| Posted: Fri Aug 17, 2012 8:00 am |
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| sowei |
| Posted: Mon Aug 27, 2012 3:04 am |
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| dongdongwu |
| Posted: Thu Sep 20, 2012 2:11 am |
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