# Difference between revisions of "99 questions/Solutions/4"

From HaskellWiki

< 99 questions | Solutions

(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.) |
(Grouped all fold solutions together) |
||

Line 14: | Line 14: | ||

<haskell> |
<haskell> |
||

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

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

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

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

Line 39: | Line 39: | ||

The prelude for haskell 2010 can be found [http://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009 here.] |
The prelude for haskell 2010 can be found [http://www.haskell.org/onlinereport/haskell2010/haskellch9.html#x16-1710009 here.] |
||

− | |||

− | A fancier one! :-) |
||

− | <haskell> |
||

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

− | </haskell> |

## 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)) 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)
```

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.