1 reply to this topic

[Guenhwyvar]

Hiya, I am new to haskell and I need to use pattern matching to extract the list a from a tuple list. I have tried

where [(a,_,_)] = a

But is says the type if then a, not [a]? How to I extract the full list of a's?

Thanks

[Sysop_fb]

Sorry didn't see your question.

What you said seems a bit vague to me.
If you have a list with a single triple element tuple in it then all you need to do is pull out the tuple and then do your pattern matching

tst = [(1,2,3)]
fst' (a, _, _) = a

ghci> fst' (head tst)
1

If you have a list of multiple tuples then you'd have to iterate through the list

tst = [(1,2,3),(4,5,6),(7,8,9)]
fst' (a, _, _) = a

ghci> [fst' x | x <- tst]
[1,4,7]

Edit:
Oh you could also do pattern matching in list comprehension

let tst = [(1,2,3), (1,2,3), (4,5,1), (5,6,6)]

ghci> [a | (a, b, c) <- tst]
[1,1,4,5]

Edited by Sysop_fb, 21 December 2011 - 05:39 PM.