[Haskell-beginners] removing duplicate tuples (including
ozgurakgun at gmail.com
Tue Sep 28 06:24:29 EDT 2010
I'll first try to give you a more understandable syntax. (I hope.)
nubSym :: Ord a => [(a,a)] -> [(a,a)]
nubSym xs = nub (map fix xs)
where fix (a,b) | a > b = (b,a)
| otherwise = (a,b)
The two changes above are getting rid of the point-free style and trading
the pattern matching syntax for guards.
In the first version the p was for matching with any parameter that didn't
match the first definition. You may want to have a look at how pattern
matching in Haskell works. (Well, you can find lots of resources about this
one but I would have a look at )
happy hacking! :)
On 28 September 2010 11:14, Martin Tomko <martin.tomko at geo.uzh.ch> wrote:
> Hi Ozgur,
> well, I am getting a list of tuples from a previous function, and they
> relate to edges in graphs, so I am not too keen to change that, although
> that could be possible. But I never worked with sets in Haskell, so will
> have to study.
> Regarding your suggestion - I have to study it, it is a bit advanced.
> First, I see there is no paramter to nubSym - I have never used that
> syntax, shouldn't there be something like nymSym (x:xs) or so?
> Second, obviously there is a local function, fix. I understand this: fix
> (a,b) | a > b = (b,a)
> but I am not sure how to interpret this:
> fix p = p. Where does p come from? How does haskell know that it relates to
> (a,b), or the x as parameter?
> Just asking for clarification ,as I am new to all this.
> On 9/28/2010 12:05 PM, Ozgur Akgun wrote:
> On 28 September 2010 10:33, Martin Tomko <martin.tomko at geo.uzh.ch> wrote:
>> I have a list of (a,a) tuples, and am trying something like nub, but also
>> matching for symmetrical tuples.
> You can of course do this. One approach would be to simply 'fix' the
> tuples according to some ordering, and then use standard nub - or a better
> But to me, the real question is this: If the order of your tuples to
> don't matter, do you actually need tuples? There are other types in which
> the order of the elements in a container does not change the meaning; such
> as a set. You may want to use a Set from Data.Set, or you can define a pair
> type in which ordering doesn't matter. It will end up being a cardinality
> restricted set type though.
> If you just want to get it working, here is some code for the first
> nubSym :: Ord a => [(a,a)] -> [(a,a)]
> nubSym = nub . map fix
> where fix (a,b) | a > b = (b,a)
> fix p = p
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