[Haskell-beginners] Removing the biggest element from a list - maybe slow?

Markus Läll markus.l2ll at gmail.com
Mon May 24 10:15:32 EDT 2010

On a list, that is internally too a list, the complexity of the
operation is always at least O(n), because to find the maximum, you
have to look through all the list -- i.e do n comparisons.

If you don't have any other variables like pointers to the previous
and next elements of the maximum, when you are going trough the list
to find it, then after finding the maximum value, you have to go
through the list again, to find its position and remove it...

> delete (maximum xs) xs
the complexity is O(n + m), where m is the index of the maximum. This
goes through the list to find the maximum, and then goes through it
again up  until it finds the first occurrance of it.

> init $ sort xs
it is (n log n) + n, where (n log n) is the complexity of the sorting
algorithm (it might be something else depending on the algorithm), and
(+ n) is because the init funcion has to go through  the list again to
find its last element and remove it.

Although Haskell is lazy, and when sorting a list, it might not get
fully sorted until you request the last element from the sorted list,
the init function still forces it to do so.

I don't know how one would do the removing of the maximum by going
through the list and whenever finding a bigger element, then saving
the positions of its predecessor and successor, so it could reassemble
the list in the middle, when it has done looking through it. Anyone
have ideas?

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