Manipulate list, one element at a time
Hello all, How can I do something to each element of a list, but only modify one element at at time, so the result is a list of lists, where each sublist is the original list with one element altered. Something like type Each a = (a->a) -> [a] -> [[a]] I came up with: each :: Each a each f [] = [] each f (x:xs) = (f x : xs) : (map (x:) $ each f xs) λ> each (*10) [1..3] [[10,2,3],[1,20,3],[1,2,30]] but I wonder if there is a more standard way of doing this
On 2017-10-24 12:18, martin wrote:
How can I do something to each element of a list, but only modify one element at at time, so the result is a list of lists, where each sublist is the original list with one element altered.
Something like
[..]
λ> each (*10) [1..3] [[10,2,3],[1,20,3],[1,2,30]]
You could make use of the 'inits' and 'tails' functions from Data.List: λ: inits [1..3] [[],[1],[1,2],[1,2,3]] λ: tails [1..3] [[1,2,3],[2,3],[3],[]] Using this, you could build a little list comprehension which takes every head and every tail and produces a list in which the two are concatenated, except that the first element of the tail has some function applied to it: λ: let each f xs = [i ++ f t:ts | (i, t:ts) <- zip (inits xs) (tails xs)] λ: :t each each :: (a -> a) -> [a] -> [[a]] λ: each (*10) [1..3] [[10,2,3],[1,20,3],[1,2,30]] -- Frerich Raabe - raabe@froglogic.com www.froglogic.com - Multi-Platform GUI Testing
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martin