[Haskell-cafe] How to improve its performance ?

[zaxis]

`allPairs list = [(x,y) | x <- list, y <- list]  ` is not what `combination`
does !
>let allPairs list = [(x,y) | x <- list, y <- list] 
>allPairs [1,2,3]
[(1,1),(1,2),(1,3),(2,1),(2,2),(2,3),(3,1),(3,2),(3,3)]

>combination [1,2,3]
[[1,2,3],[2,3],[1,3],[3],[1,2],[2],[1],[]]


Alexander Solla-2 wrote:
> 
> 
> On Mar 17, 2010, at 6:14 PM, Daniel Fischer wrote:
> 
>> I found it surprisingly not-slow (code compiled with -O2, as usual).
>> There are two points where you waste time.
> 
> 
> I found one big point where time is wasted:  in computing the powerset  
> of a list.  He's making 2^n elements, and then iterating through them  
> all and filtering, but only needs n^2 or n `choose` 2 of the  
> (depending on the semantics for his "groups").
> 
> The answer is to do something like:
> 
> allPairs list = [(x,y) | x <- list, y <- list]
> 
> to get it done in n^2 time.
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> 
> 


-----
fac n = let {  f = foldr (*) 1 [1..n] } in f 
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