[Haskell-cafe] loop in C , recursion in haskell
sholderm at students.cs.uu.nl
Wed Mar 24 16:38:43 EST 2004
AG> I am new to haskell and would look to write a function
AG> equivalent to the following loop in C
AG> int value = 500000;
AG> int part_stack;
AG> int *part_ptr = part_stack;
AG> for (; value; value /= 10000)
AG> *part_ptr++ = value % 10000;
What about this?
> part :: Int -> Int -> [Int]
> part k n = unfoldr f k
> where f 0 = Nothing
> f m = Just (m `mod`n, m `div` n)
part 500000 10000, for instance, produces the [0, 50].
Here, unfoldr is the dual of the function foldr from the Prelude.
> unfoldr :: (b -> Maybe (a,b)) -> b -> [a]
> unfoldr f b = case (f b) of
> Nothing -> 
> Just (a,b) -> a : unfoldr f b
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