# 99 questions/Solutions/4

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< 99 questions | Solutions(Difference between revisions)

(correction (foldl) and shortened version of foldr) |
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myLength [] = 0 | myLength [] = 0 | ||

myLength (_:xs) = 1 + myLength xs | 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) | ||

</haskell> | </haskell> | ||

<haskell> | <haskell> | ||

− | myLength' | + | myLength' = foldl (\n _ -> n + 1) 0 |

− | myLength'' | + | myLength'' = foldr (\_ n -> n + 1) 0 |

− | myLength''' = foldr (\_ -> (+1)) 0 | + | myLength''' = foldr (\_ -> (+1)) 0 |

+ | myLength'''' = foldr ((+) . (const 1)) 0 | ||

+ | myLength''''' = foldr (const (+1)) 0 | ||

+ | myLength'''''' = foldl (const . (+1)) 0 | ||

+ | </haskell> | ||

+ | |||

+ | <haskell> | ||

+ | myLength' 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 easier | ||

+ | </haskell> | ||

+ | |||

+ | <haskell> | ||

+ | myLength = sum . map (\_->1) | ||

</haskell> | </haskell> | ||

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.] | ||

+ | |||

+ | |||

+ | [[Category:Programming exercise spoilers]] |

## Revision as of 19:47, 18 January 2014

(*) 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)) 0 myLength'''''' = foldl (const . (+1)) 0

myLength' 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 easier

myLength = sum . map (\_->1)

length

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.