Get maximum tuples in list

Question

I have a list of tuples that can be understood as key-value pairs, where a key can appear several times, possibly with different values, for example

[(2,8),(5,10),(2,5),(3,4),(5,50)]

I now want to get a list of tuples with the highest value for each key, ie

[(2,8),(3,4),(5,50)]

The order of the keys is irrelevant.

How do I do that in an efficient way?

Answer 1

Sort them and then cast to a dictionary and take the items again from it:

l = [(2,8),(5,10),(2,5),(3,4),(5,50)]
list(dict(sorted(l)).items()) #python3, if python2 list cast is not needed
[(2, 8), (3, 4), (5, 50)]

The idea is that the key-value pairs will get updated in ascending order when transforming to a dictionary filtering the lowest values for each key, then you just have to take it as tuples.

Answer 2

At its core, this problem is essentially about grouping the tuples based on their first element and then keeping only the maximum of each group.

Grouping can be done easily with a defaultdict . A detailed explanation of grouping with defaultdicts can be found in my answer here . In your case, we group the tuples by their first element and then use the max function to find the tuple with the largest number.

import collections

tuples = [(2,8),(5,10),(2,5),(3,4),(5,50)]

groupdict = collections.defaultdict(list)
for tup in tuples:
    group = tup[0]
    groupdict[group].append(tup)

result = [max(group) for group in groupdict.values()]
# result: [(2, 8), (5, 50), (3, 4)]

In your particular case, we can optimize the code a little bit by storing only the maximum 2nd element in the dict, rather than storing a list of all tuples and finding the maximum at the end:

tuples = [(2,8),(5,10),(2,5),(3,4),(5,50)]

groupdict = {}
for tup in tuples:
    group, value = tup

    if group in groupdict:
        groupdict[group] = max(groupdict[group], value)
    else:
        groupdict[group] = value

result = [(group, value) for group, value in groupdict.items()]

This keeps the memory footprint to a minimum, but only works for tuples with exactly 2 elements.

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This has a number of advantages over Netwave's solution :

• It's more readable. Anyone who sees a defaultdict being instantiated knows that it'll be used to group data, and the use of the max function makes it easy to understand which tuples are kept. Netwave's one-liner is clever, but clever solutions are rarely easy to read.
• Since the data doesn't have to be sorted, this runs in linear O(n) time instead of O(n log n).