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Steve Tjoa
Steve Tjoa

Reputation: 61044

Python: conditionally delete elements from list

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.

Upvotes: 9

Views: 2156

Answers (5)

aaronasterling
aaronasterling

Reputation: 71004

use collections.defaultdict

import collections

max_elements = collections.defaultdict(tuple)

for item in x:
    if item > max_elements[item[1]]:
        max_elements[item[1]] = item

y = max_elements.values()

Upvotes: 5

gotgenes
gotgenes

Reputation: 40029

If you can make the assumption that tuples with identical second elements appear in contiguous order in the original list x, you can leverage itertools.groupby:

import itertools
import operator

def max_first_elem(x):
    groups = itertools.groupby(x, operator.itemgetter(1))
    y = [max(g[1]) for g in groups]
    return y

Note that this will guarantee preservation of the order of the groups (by the second tuple element), if that is a desired constraint for the output.

Upvotes: 2

pillmuncher
pillmuncher

Reputation: 10162

>>> from collections import defaultdict
>>> d = defaultdict(tuple)
>>> x = [(1,2), (3,4), (7,4), (5,4)]
>>> for a, b in x:
...     d[b] = max(d[b], (a, b))
...
>>> d.values()
[(1, 2), (7, 4)

Upvotes: 0

John La Rooy
John La Rooy

Reputation: 304147

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)]

Upvotes: 5

Steve Tjoa
Steve Tjoa

Reputation: 61044

My own attempt, slightly inspired by aaronsterling:

(oh yeah, all elements are nonnegative)

def processtuples(x):
    d = {}
    for item in x:
        if x[0] > d.get(x[1],-1):
            d[x[1]] = x[0]

    y = []
    for k in d:
        y.append((d[k],k))
    y.sort()
    return y

Upvotes: 0

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