99 questions/Solutions/4: Difference between revisions
< 99 questions | Solutions
mNo edit summary |
(Their was a section for the length function in the prelude but it was missing so I took the function form the 2010 report and link to it.) |
||
| Line 32: | Line 32: | ||
This is <hask>length</hask> in <hask>Prelude</hask>. | This is <hask>length</hask> in <hask>Prelude</hask>. | ||
-- length returns the length of a finite list as an Int. | |||
length :: [a] -> Int | |||
length [] = 0 | |||
length (_:l) = 1 + length l | |||
The prelude for haskell 2010 can be found [http://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009 here.] | |||
A fancier one! :-) | A fancier one! :-) | ||
Revision as of 19:21, 30 November 2012
(*) Find the number of elements of a list.
myLength :: [a] -> Int
myLength [] = 0
myLength (_:xs) = 1 + myLength xs
myLength' :: [a] -> Int
myLength' list = myLength_acc list 0 -- same, with accumulator
where
myLength_acc [] n = n
myLength_acc (_:xs) n = myLength_acc xs (n + 1)myLength' = foldl (\n _ -> n + 1) 0
myLength'' = foldr (\_ n -> n + 1) 0
myLength''' = foldr (\_ -> (+1)) 0
myLength'''' = foldr ((+) . (const 1)) 0
myLength''''' = foldr (const (+1)) 0myLength' xs = snd $ last $ zip xs [1..] -- Just for fun
myLength'' = snd . last . (flip zip [1..]) -- Because point-free is also fun
myLength''' = fst . last . zip [1..] -- same, but easiermyLength = sum . map (\_->1)This is length in Prelude.
-- length returns the length of a finite list as an Int. length :: [a] -> Int length [] = 0 length (_:l) = 1 + length l
The prelude for haskell 2010 can be found here.
A fancier one! :-)
myLength = foldl (const . (+1)) 0