[Haskell-beginners] first number is a list

[Mats Rauhala]

On 12:24 Mon 27 Jun     , Roelof Wobben wrote:
>  Oke,  Now figure out how I can find the 3 in the list [1,2,3,4] I was thinking about using tail [1,2,3,4] and after that use last [1,2,3] So something like last [ tail [1,2,3,4] Or : let outcome = tail [1,2,3,4]let outcome2 = last [outcome]

tail returns the list except for the head. So for example the list
[1,2,3,4] becomes [2,3,4]. Using last on that would return 4, not 3.
However the function init returns the list except for the last item, so
using init on [1,2,3,4] would return [1,2,3] and then using last on that
would return the 3 you wanted. So for example:

 ghci> last $ init [1,2,3,4]
 3

But Data.List exports a function `find :: (a -> Bool) -> [a] -> Maybe
a`, which you can use to find 3.

 ghci> find (== 3) [1,2,3,4]
 Just 3

Another way could be using head and dropWhile:

 ghci> head $ dropWhile (/= 3) [1,2,3,4]
 3



-- 
Mats Rauhala
MasseR
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