[Haskell-cafe] Producing MinimumValue

Alexteslin alexteslin at yahoo.co.uk
Thu Jul 19 15:30:06 EDT 2007

I have defined the first line it seems right to me but second line not sure. 
I have True or False and whatever value i give it produces that value.

allEqual :: [Int] -> Bool
allEqual (x1:x2:xs) = (x1 == x2) && allEqual xs
allEqual _          = ???

Steve Schafer wrote:
> On Thu, 19 Jul 2007 10:55:19 -0700 (PDT), you wrote:
>>The question suggests to use some functions defined in the section, and
>>of them is iSort.
> Aha. Well, that one certainly lends itself better to this particular
> proplen than either map or filter.
>>minimumValue :: [Int] -> Int
>>minimumValue ns = head (iSort ns)
> If I were going to use a sort, then yes, that's the way I would do it.
> Of course, sorting isn't the best way to solve the problem, as sorting
> will always be at least O(n * log n), whereas a more straightforward
> algorithm would be O(n).
>>The other question is to test whether the values of allEqual on inputs 0
>>n are all equal.  Again, I defined the method but not sure if its concise?
>>allEqual :: [Int] -> Bool
>>allEqual xs = length xs == length (filter isEqual xs)
>>	where
>>	isEqual n = (head xs) == n
> Simple recursion is probably the most conceptually straightforward
> approach here. One little difficulty with this problem (and with
> minimumValue, too) is that the problem isn't completely specified,
> because we don't know what answer we're supposed to give when the list
> is empty. Let's assume that allEqual is supposed to return True on an
> empty list. In that case, we can write it like this:
>  allEqual :: [Int] -> Bool
>  allEqual (x1:x2:xs) = ???
>  allEqual _          = ???
> where the two ???s are left as an exercise for the reader.
> -Steve
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