[Haskell-cafe] working with lists of couples

Henning Thielemann lemming at henning-thielemann.de
Fri Nov 17 13:39:44 EST 2006

On Fri, 17 Nov 2006, Clara Zamolo wrote:

> Hello,
> 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.
> An example:
> input: [1,2,3,4]
> output: [(1,0)(2,1)(3,3)(4,6)]
> The problem could seem trivial, and here's a first solution:
> buildCouples = snd . foldl op (0,[])
>        where
>        op (v,xs) x = (v+x,xs++[(x,v)])
> The problem is that this solution is quadratic, and I want a solution
> that take time n (2n is not good either).
> Any suggestion?

I suggest using 'scanl' and then 'zip' the result together with the
original list.

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