Add dropEnd and takeEnd #130
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test/Test/Data/List.purs
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| @@ -226,6 +226,11 @@ testList = do | |||
| assert $ (take 2 (l [1, 2, 3])) == l [1, 2] | |||
| assert $ (take 1 nil) == nil | |||
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| log "take should keep the specified number of items from the end of an list, discarding the rest" | |||
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takeEnd should keep the specified number of items from the end of an list, discarding the rest
test/Test/Data/List.purs
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| @@ -236,6 +241,11 @@ testList = do | |||
| assert $ (drop 2 (l [1, 2, 3])) == l [3] | |||
| assert $ (drop 1 nil) == nil | |||
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| log "drop should remove the specified number of items from the front of an list" | |||
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dropEnd should remove the specified number of items from the end of an list
| -- | Running time: `O(2n - m)` where `n` is the number of elements in list | ||
| -- | and `m` is number of elements to take. | ||
| takeEnd :: forall a. Int -> List a -> List a | ||
| takeEnd n xs = drop (length xs - n) xs |
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This involves two traversals of the list, which we should try to avoid. Let's try to write these using foldr instead please.
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Is it even possible for this function to have better complexity?
in order to take n elements from the end we need to start iterating the list from the start till we reach length - n position and return the tail. So we require length (whole list iteration) and we need to determine the index we stop on (length - n elements iteration)
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Yeah, ignore me, this is fine as it is. I thought we could do it in one pass with a buffer of size n, but it's probably not worth it. We can revisit later if necessary.
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Alternative implementation will be to first reverse list and then take. it's performance will be n + m. it will be better then current version when m is smaller then n/2.
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first reverse list and then take and then reverse
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oh yeh :D so that will be better when m is less then n/4. so never mind
test/Test/Data/List.purs
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| log "takeWhile should keep all values that match a predicate from the front of an list" | ||
| assert $ (takeWhile (_ /= 2) (l [1, 2, 3])) == l [1] | ||
| assert $ (takeWhile (_ /= 3) (l [1, 2, 3])) == l [1, 2] | ||
| assert $ (takeWhile (_ /= 1) nil) == nil | ||
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| log "drop should remove the specified number of items from the front of an list" | ||
| log "dropEnd should remove the specified number of items from the end of an list" |
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You made this change in for wrong string :p
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Looks good now? |
| log "takeWhile should keep all values that match a predicate from the front of an list" | ||
| assert $ (takeWhile (_ /= 2) (l [1, 2, 3])) == l [1] | ||
| assert $ (takeWhile (_ /= 3) (l [1, 2, 3])) == l [1, 2] | ||
| assert $ (takeWhile (_ /= 1) nil) == nil | ||
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| log "drop should remove the specified number of items from the front of an list" | ||
| log "dropE should remove the specified number of items from the front of an list" |
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drop not dropE :)
But this is fine.
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Great, thanks! |
Similar to the purescript/purescript-arrays#113