On 5/28/11 8:31 AM, Daniel Fischer wrote:
On Saturday 28 May 2011 14:19:18, Dmitri O.Kondratiev wrote:
Thanks for simple and beautiful code to get all pairs.
Yet, I need to get to the next step - from all pairs to build all
chains, to get as a result a list of lists:
[[abcde, acde, ade, ae,]
[bcde, bde, be,]
[cde, cd, ce,]
de]]
This is where I got stuck.
-- instead of pairing with a single later element, we cons with the
-- tail beginning at that later element
chainsFromFirst :: [a] -> [[a]]
chainsFromFirst (x:xs) = [x:ys | ys<- init (tails xs)]
chainsFromFirst [] = []
-- we need init (tails xs) becuase we don't want the final empty tail
allChains :: [a] -> [[a]]
allChains xs = tails xs>>= chainsFromFirst
-- we could use init (tails xs) here too, but that is not necessary
The variant I came up with was:
allChains xs = do
y:ys <- tails xs
zs@(_:_) <- tails ys
return (y:zs)
Which is essentially the same. I only very rarely use list
comprehensions, but if you prefer that syntax you can always do:
allChains xs = [ (y:zs) | y:ys <- tails xs, zs@(_:_) <- tails ys ]
--
Live well,
~wren
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