# Difference between revisions of "99 questions/Solutions/20"

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< 99 questions | Solutions

(Adding a third solution.) |
(Added another solution that also indicates failure using Maybe (thanks to zygoloid from #haskell for the input regarding Maybe)) |
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removeAt 1 (x:xs) = (Just x, xs) |
removeAt 1 (x:xs) = (Just x, xs) |
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removeAt k (x:xs) = let (a, r) = removeAt (k - 1) xs in (a, x:r) |
removeAt k (x:xs) = let (a, r) = removeAt (k - 1) xs in (a, x:r) |
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+ | </haskell> |
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+ | |||

+ | Another solution that also uses Maybe to indicate failure: |
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+ | |||

+ | <haskell> |
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+ | removeAt :: Int -> [a] -> (Maybe a, [a]) |
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+ | removeAt _ [] = (Nothing, []) |
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+ | removeAt 0 xs = (Nothing, xs) |
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+ | removeAt nr xs | nr > length xs = (Nothing, xs) |
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+ | | otherwise = (Just (xs !! nr), fst splitted ++ (tail . snd) splitted) |
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+ | where splitted = splitAt nr xs |
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</haskell> |
</haskell> |

## Revision as of 15:34, 18 August 2010

(*) Remove the K'th element from a list.

```
removeAt :: Int -> [a] -> (a, [a])
removeAt k xs = case back of
[] -> error "removeAt: index too large"
x:rest -> (x, front ++ rest)
where (front, back) = splitAt k xs
```

Simply use the `splitAt`

to split after k elements.
If the original list has fewer than k+1 elements, the second list will be empty, and there will be no element to extract.
Note that the Prolog and Lisp versions treat 1 as the first element in the list, and the Lisp version appends NIL elements to the end of the list if k is greater than the list length.

or

```
removeAt n xs = (xs!!n,take n xs ++ drop (n+1) xs)
```

Another solution that avoids throwing an error and using ++ operators. Treats 1 as the first element in the list.

```
removeAt :: Int -> [a] -> (Maybe a, [a])
removeAt _ [] = (Nothing, [])
removeAt 1 (x:xs) = (Just x, xs)
removeAt k (x:xs) = let (a, r) = removeAt (k - 1) xs in (a, x:r)
```

Another solution that also uses Maybe to indicate failure:

```
removeAt :: Int -> [a] -> (Maybe a, [a])
removeAt _ [] = (Nothing, [])
removeAt 0 xs = (Nothing, xs)
removeAt nr xs | nr > length xs = (Nothing, xs)
| otherwise = (Just (xs !! nr), fst splitted ++ (tail . snd) splitted)
where splitted = splitAt nr xs
```