CODEWITHSUNDEEP
listhaskelltupleslist-comprehension
matthieu
matthieu

Reputation: 109

haskell list of tuples, with unique tuples

I come from a Python and Java background so Haskell is quite different for me. I'm trying little activities to learn but I am stuck on this .

I have an ordered list of tuples, [(name, studentNumber)], and I want to filter this list so that each student and each studentNumber appears only once. Since the tuples are ordered, I want to keep the first instance of a name or studentNumber and remove any others that may show up.

I tried doing a list comphrenshion, but I'm not sure how to check if a name or number has already been added to the list.

Upvotes: 2

Views: 1583

Answers (3)

ben w
ben w

Reputation: 2535

It sounds as if you'd want (as a first, inefficient approximation) something like this:

import Data.List (nubBy)
import Data.Function (on)

filt = nubBy ((==) `on` snd) . nubBy ((==) `on` fst)

The first call to nubBy will result in a list in which each name appears only once, and that will then be passed to the second, resulting in a list in which each number appears only once.

Just using nub will result in a list in which each (name,number) pair occurs only once; there might still be repetitions of names with different numbers and numbers with different names.

(Of course something custom with an accumulator would be faster.)

Upvotes: 5

Matvey Aksenov
Matvey Aksenov

Reputation: 3900

You can spy on Data.List sources and write your extended nub function:

type Student = (Name, Number)
type Name = String
type Number = Int

unique :: [Student] -> [Student]
unique = go [] [] 
  where
    go unames unumbers (s@(name, number):ss)
      | name `elem` unames || number `elem` unumbers = go unames unumbers ss
      | otherwise = s : go (name:unames) (number:unumbers) ss
    go _ _ [] = []

Should do what you want.

Upvotes: 1

Harry Lachenmayer
Harry Lachenmayer

Reputation: 970

To unique-ify a list there's always the nub function from the prelude, I think that should do exactly what you need!

Upvotes: 0

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