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

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< 99 questions | Solutions

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(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.) |
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This is <hask>length</hask> in <hask>Prelude</hask>. |
This is <hask>length</hask> in <hask>Prelude</hask>. |
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+ | |||

+ | -- length returns the length of a finite list as an Int. |
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+ | length :: [a] -> Int |
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+ | length [] = 0 |
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+ | length (_:l) = 1 + length l |
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+ | |||

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

A fancier one! :-)

```
myLength = foldl (const . (+1)) 0
```