min max from tuples in list

[Jussi Piitulainen]

Robert Voigtländer writes:

> Hi,
> 
> I have a list like this:

# shortened:
a = [(52, 193), (52, 193), (52, 192),
     (51, 193), (51, 191), (51, 190), (51, 189), (51, 188),
     (50, 194)]

> I need to find a -performant- way to transform this into a list with
> tuples (a[0],[a[0][1]min],[a[0][1]max]).
> 
> Hard to explaint what I mean .. [0] of the first three tuples is
> 52. [1] is 193,193 and 192.  What I need as result for these three
> tuples is: (52,192,193).
> 
> For the next five tuples it is (51,188,193).

I think you want [(50, 194, 194), (51, 188, 193), (52, 192, 193)] for
the shortened list, in some order. Then you might be happy with a dict
instead. If I misunderstand, the following will be irrelevant.

> Extra challenges:
> - This list is sorted. For performance reasons I would like to keep
>   it unsorted.
> - There may be tuples where min=max.
> - There my be tupples where [0] only exists once. So mix is
>   automatically max

Scan the list and keep the running min and max for each key in a dict.

mix = dict()
for k,v in a:
   mix[k] = ( min(mix.get(k, (v,v))[0], v),
              max(mix.get(k, (v,v))[1], v) )

# mix == {50: (194, 194), 51: (188, 193), 52: (192, 193)}
# As list: [ (k,v[0],v[1]) for k,v in mix.items() ]