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ojuarez@mtecv2.mty.itesm.mx (Octavio Juarez Espinosa) writes:

>I am very interested in to receive some version for funtion middle.
>
>(middle list)  return nil if is even number of elements.
>		return the element if is odd the number of elements
>
>Constrains:
>Only one recursion call and only with the CRD of argument.

I don't quite understand why you want to return "nil" when the
list has an even number of elements. If you are applying the
function to a list of numbers, then "nil" is not a number, so
it won't work for strongly typed languages; if you are applying
the function to a list of lists, then you can't tell whether
the function worked or not. Much better to return either a tagged
value (Ok val | Error message) or to raise an exception for even
length lists. The following code completely ignores the issue for
simplicity.

NB. all these functions are "primitive recursive" in the original poster's
sense.

tail' [] = []
tail' (x:xs) = xs

middle_odd [] ys = tail' ys
middle_odd (x:xs) ys = middle_odd (tail' xs) (tail' ys)

middle l = middle_odd ([] : l) l

-- 
Fergus Henderson             fjh@munta.cs.mu.OZ.AU      
This .signature virus is a self-referential statement that is true - but 
you will only be able to consistently believe it if you copy it to your own
.signature file!

In article <6006@mtecv2.mty.itesm.mx>, ojuarez@mtecv2.mty.itesm.mx (Octavio Juarez Espinosa) writes:
>I am very interested in to receive some version for funtion middle.
>
>(middle list)  return nil if is even number of elements.
>		return the element if is odd the number of elements

In article <9224705.24159@mulga.cs.mu.OZ.AU>, fjh@munta.cs.mu.OZ.AU (Fergus James HENDERSON) writes:
...
|> the function worked or not. Much better to return either a tagged
|> value (Ok val | Error message) or to raise an exception for even
|> length lists. The following code completely ignores the issue for
|> simplicity.
|> 
|> NB. all these functions are "primitive recursive" in the original poster's
|> sense.
|> 
|> tail' [] = []
|> tail' (x:xs) = xs
|> 
|> middle_odd [] ys = tail' ys
|> middle_odd (x:xs) ys = middle_odd (tail' xs) (tail' ys)
|> 
|> middle l = middle_odd ([] : l) l
|> 

Sorry, but I tried out this function and it is not correct.
My attempt is (I return a singleton list on success):

middle l = even l l

even halfSpeed []     = []
even halfSpeed (x:xs) = odd halfSpeed xs

odd (midValue:halfSpeed) []     = [midValue]
odd (void    :halfSpeed) (x:xs) = even halfSpeed xs

Test:
Miranda [middle [1..n]|n<-[0..20]]
[[],[1],[],[2],[],[3],[],[4],[],[5],[],[6],[],[7],[],[8],[],[9],[],[10],[]]
Miranda ^D

By the way, I did not understand the constraint completely,
what means CRD?  Do you mean CDR, or as we LISP-haters say: tail?
You have to know the length of the list before you can decide what
element to return.

Mark Ramaer