CODEWITHSUNDEEP
haskellfunctional-programming
swaffelay
swaffelay

Reputation: 323

groupBy on snd element of tuple

So I would like to use groupBy to group my list of tuples based on their snd element. Every tuple with the same snd element should be in the same list

group ::  [(Int,Int)] -> [[(Int,Int)]]
group = groupBy (\a b -> snd a == snd b) lijst

A groupBy of the list [(1,2),(8,9),(5,2),(9,2),(3,9),(1,1)] should return [[(1,2),(5,2),(9,2)],[(8,9),(3,9)],[1,1]].

However it returns [[(1,2)],[(8,9)],[(5,2),(9,2)],[(3,9)],[(1,1)]].

How can I make this function work ?

Upvotes: 1

Views: 576

Answers (1)

typetetris
typetetris

Reputation: 4867

Reading in the documentation of groupBy we are referred to the documentation of group, which states

The group function takes a list and returns a list of lists such that the concatenation of the result is equal to the argument.

That's a not very obvious way of saying: "We only group equal, adjacent elements."

So sortBy your list first.

Upvotes: 6

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