[Python-Dev] [Python-ideas] minmax() function returning (minimum, maximum) tuple of a sequence

[Steven D'Aprano]

On Mon, 11 Oct 2010 05:57:21 am Paul McGuire wrote:
> Just as an exercise, I wanted to try my hand at adding a function to
> the compiled Python C code.  An interesting optimization that I read
> about (where? don't recall) finds the minimum and maximum elements of
> a sequence in a single pass, with a 25% reduction in number of
> comparison operations:
> - the sequence elements are read in pairs 
> - each pair is compared to find smaller/greater
> - the smaller is compared to current min
> - the greater is compared to current max
>
> So each pair is applied to the running min/max values using 3
> comparisons, vs. 4 that would be required if both were compared to
> both min and max.
>
> This feels somewhat similar to how divmod returns both quotient and
> remainder of a single division operation.
>
> This would be potentially interesting for those cases where min and
> max are invoked on the same sequence one after the other, and
> especially so if the sequence elements were objects with expensive
> comparison operations.

Perhaps more importantly, it is ideal for the use-case where you have an 
iterator. You can't call min() and then max(), as min() consumes the 
iterator leaving nothing for max(). It may be undesirable to convert 
the iterator to a list first -- it may be that the number of items in 
the data stream is too large to fit into memory all at once, but even 
if it is small, it means you're now walking the stream three times when 
one would do.

To my mind, minmax() is as obvious and as useful a built-in as divmod(), 
but if there is resistance to making such a function a built-in, 
perhaps it could go into itertools. (I would prefer it to keep the same 
signature as min() and max(), namely that it will take either a single 
iterable argument or multiple arguments.)

I've experimented with minmax() myself. Not surprisingly, the 
performance of a pure Python version doesn't even come close to the 
built-ins.

I'm +1 on the idea.

Presumably follow-ups should go to python-ideas.



-- 
Steven D'Aprano