[Haskell-cafe] Foldl
Marco Comini
comini at dimi.uniud.it
Fri Nov 24 16:27:28 EST 2006
Donald Bruce Stewart:
> zacara:
> > i'd like to write a function that given a list like [1,2,3,4...]
> > returns a list of couple where the first element is the corresponding
> > element of the string, and the second is the sum of the previous
> > elements.
> Looks like a scanl (a prefix fold) -- the "of the previous
> elements" is a clue that you're describing a scan:
>
> Prelude> :t scanl
> scanl :: (a -> b -> a) -> a -> [b] -> [a]
>
> Prelude> scanl (+) 0 [1..4]
> [0,1,3,6,10]
>
> You should be able to work out the rest from here (or rewrite it as a
> fold).
I agree Don,
certainly with scanl one can solve the problem elegantly.
[For ClareZako: it could be something like
f (x:xs) = scanl acc (x,0) xs
where
acc (x,s) y = (y, s+x)
]
On the contrary a solution in terms of a fold is indeed more tricky.
Probably ClareZako was actually looking for a solution in term of a
scan instead of a fold... otherwise would be interesting to know why
he's willing to solve this problem in such a twisted way... don't you
agree?
Cheers,
--
Marco.
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