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

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[] -> error "removeAt: index too large" |
[] -> error "removeAt: index too large" |
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x:rest -> (x, front ++ rest) |
x:rest -> (x, front ++ rest) |
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− | where (front, back) = splitAt k xs |
+ | where (front, back) = splitAt (k - 1) xs |

</haskell> |
</haskell> |
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− | Simply use the <hask>splitAt</hask> to split after k elements. |
+ | Simply use the <hask>splitAt</hask> to split after k - 1 elements. |

− | If the original list has fewer than k |
+ | If the original list has fewer than k elements, the second list will be empty, and there will be no element to extract. |

− | Note that |
+ | Note that we treat 1 as the first element in the list. |

or |
or |
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<haskell> |
<haskell> |
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− | removeAt n xs = (xs!!n,take n xs ++ drop |
+ | removeAt n xs = (xs !! (n - 1), take (n - 1) xs ++ drop n xs) |

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

+ | or, a safe version of that |
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+ | |||

+ | <haskell> |
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+ | removeAt n xs | n > 0 && n <= length xs = (Just (xs !! index), take index xs ++ drop n xs) |
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+ | | otherwise = (Nothing, xs) |
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+ | where index = n - 1 |
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+ | </haskell> |
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+ | |||

+ | or, a list comprehension based one: |
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+ | |||

+ | <haskell> |
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+ | removeAt :: (Enum a) => Int -> [a] -> (a, [a]) |
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+ | removeAt n xs = ((xs !! (n-1)), [ x | (i, x) <- zip [1..] xs, i /= n ]) |
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+ | </haskell> |
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+ | |||

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

+ | And yet another solution (without error checking): |
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+ | |||

+ | <haskell> |
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+ | removeAt :: Int -> [a] -> (a, [a]) |
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+ | removeAt n xs = let (front, back) = splitAt n xs in (last front, init front ++ back) |
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+ | </haskell> |
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+ | |||

+ | Similar, point-free style: |
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+ | |||

+ | <haskell> |
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+ | removeAt n = (\(a, b) -> (head b, a ++ tail b)) . splitAt (n - 1) |
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+ | </haskell> |
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+ | |||

+ | A simple recursive solution: |
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+ | |||

+ | <haskell> |
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+ | removeAt 1 (x:xs) = (x, xs) |
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+ | removeAt n (x:xs) = (l, x:r) |
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+ | where (l, r) = removeAt (n - 1) xs |
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+ | </haskell> |
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+ | |||

+ | |||

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

## Latest revision as of 11:56, 26 April 2016

(*) 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 - 1) xs
```

Simply use the `splitAt`

to split after k - 1 elements.
If the original list has fewer than k elements, the second list will be empty, and there will be no element to extract.
Note that we treat 1 as the first element in the list.

or

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

or, a safe version of that

```
removeAt n xs | n > 0 && n <= length xs = (Just (xs !! index), take index xs ++ drop n xs)
| otherwise = (Nothing, xs)
where index = n - 1
```

or, a list comprehension based one:

```
removeAt :: (Enum a) => Int -> [a] -> (a, [a])
removeAt n xs = ((xs !! (n-1)), [ x | (i, x) <- zip [1..] xs, i /= n ])
```

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
```

And yet another solution (without error checking):

```
removeAt :: Int -> [a] -> (a, [a])
removeAt n xs = let (front, back) = splitAt n xs in (last front, init front ++ back)
```

Similar, point-free style:

```
removeAt n = (\(a, b) -> (head b, a ++ tail b)) . splitAt (n - 1)
```

A simple recursive solution:

```
removeAt 1 (x:xs) = (x, xs)
removeAt n (x:xs) = (l, x:r)
where (l, r) = removeAt (n - 1) xs
```