Python: conditionally delete elements from list

Question

Suppose I have a list of tuples:

x = [(1,2), (3,4), (7,4), (5,4)]

Of all tuples that share the second element, I want to preserve the tuple with the largest first element:

y = [(1,2), (7,4)]

What is the best way to achieve this in Python?

---

Thanks for the answers.

• The tuples could be two-element lists instead, if that makes a difference.
• All elements are nonnegative integers.
• I like the current answers. I should really learn more about what collections has to offer!

Answer 1

Similar to Aaron's answer

>>> from collections import defaultdict
>>> x = [(1,2), (3,4), (7,4), (5,4)]
>>> d = defaultdict(int)
>>> for v,k in x:
...   d[k] = max(d[k],v) 
... 
>>> y=[(k,v) for v,k in d.items()]
>>> y
[(1, 2), (7, 4)]

note that the order is not preserved with this method. To preserve the order use this instead

>>> y = [(k,v) for k,v in x if d[v]==k]
>>> y
[(1, 2), (7, 4)]

here is another way. It uses more storage, but has less calls to max, so it may be faster

>>> d = defaultdict(list)
>>> for k,v in x:
...   d[v].append(k)
... 
>>> y = [(max(k),v) for v,k in d.items()]
>>> y
[(1, 2), (7, 4)]

Again, a simple modification preserves the order

>>> y = [(k,v) for k,v in x if max(d[v])==k]
>>> y
[(1, 2), (7, 4)]