# 99 questions/Solutions/15

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< 99 questions | Solutions(Difference between revisions)

m (Improved my version a little) |
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repli :: [a] -> Int -> [a] | repli :: [a] -> Int -> [a] | ||

repli xs n = concatMap (take n . repeat) xs | repli xs n = concatMap (take n . repeat) xs | ||

+ | </haskell> | ||

+ | |||

+ | or, using the list monad: | ||

+ | <haskell> | ||

+ | repli :: [a] -> Int -> [a] | ||

+ | repli xs n = xs >>= replicate n | ||

+ | </haskell> | ||

+ | |||

+ | or, a more verbose solution without the use of <hask>replicate</hask>: | ||

+ | <haskell> | ||

+ | repli :: [a] -> Int -> [a] | ||

+ | repli xs n = foldl (\acc e -> acc ++ repli' e n) [] xs | ||

+ | where | ||

+ | repli' _ 0 = [] | ||

+ | repli' x n = x : repli' x (n-1) | ||

+ | </haskell> | ||

+ | |||

+ | or, a version that does not use list concatenation: | ||

+ | <haskell> | ||

+ | repli :: [a] -> Int -> [a] | ||

+ | repli [] _ = [] | ||

+ | repli (x:xs) n = foldr (const (x:)) (repli xs n) [1..n] | ||

</haskell> | </haskell> |

## Revision as of 16:55, 19 November 2012

(**) Replicate the elements of a list a given number of times.

repli :: [a] -> Int -> [a] repli xs n = concatMap (replicate n) xs

or, in Pointfree style:

repli = flip $ concatMap . replicate

replicate

repli :: [a] -> Int -> [a] repli xs n = concatMap (take n . repeat) xs

or, using the list monad:

repli :: [a] -> Int -> [a] repli xs n = xs >>= replicate n

replicate

repli :: [a] -> Int -> [a] repli xs n = foldl (\acc e -> acc ++ repli' e n) [] xs where repli' _ 0 = [] repli' x n = x : repli' x (n-1)

or, a version that does not use list concatenation:

repli :: [a] -> Int -> [a] repli [] _ = [] repli (x:xs) n = foldr (const (x:)) (repli xs n) [1..n]