Question

I have the following list in Python:

l = [[2], [3], [2, 2], [5], [2], [3], [7], [2, 2, 2], [3, 3], [2], [5], [11], [2, 2], [3], [13], [2], [7], [3], [5], [2, 2, 2, 2], [17], [2], [3, 3], [19], [2, 2], [5]]

I want to write a function that will return the uniquely-valued sublists of maximum length. In this case, the function would return:

l = [[5], [7], [3, 3], [11], [13], [2, 2, 2, 2], [17], [19]]

I am still a beginner at python, and I have very little idea as to how to write such a function, however. The furthest I got was figuring out that I could iterate over the sublists by using nested loops. But from what I’ve seen of Python, it seems like there must be some simpler way to return the list I am looking for than using loops.

Update:

Here’s what I was doing with the code: solving project euler #5, the non-brute force way!

I’m sure this code could be refactored, but whatever.

Thanks for your help, guys. itemgetter was just what I needed.

#!/usr/bin/python
# coding = UTF-8

import argparse, sys, math
from itertools import groupby
from collections import defaultdict
from operator import itemgetter

parser = argparse.ArgumentParser()
parser.add_argument('filename', nargs='?')
args = parser.parse_args()
if args:
   intinput = int(sys.argv[1])
elif not sys.stdin.isatty():
    intinput = int(sys.stdin.read())
else:
    parser.print_help()

def prime_factorize(n):
    factors = []
    number = math.fabs(n)

    while number > 1:
        factor = get_next_prime_factor(number)
        factors.append(factor)
        number /= factor

    if n < -1: 
        factors[0] = -factors[0]

    return factors

def get_next_prime_factor(n):
    if n % 2 == 0:
        return 2

    for x in range(3, int(math.ceil(math.sqrt(n)) + 1), 2):
        if n % x == 0:
            return x

    return int(n)


def mkfactors(n):
  tpf = []
  for i in range(n+1):
    tpf.extend(prime_factorize(i))
  return tpf

l = [list(g) for k,g in groupby(mkfactors(intinput))]

m = [max(g) for _,g in groupby(sorted(l,key=itemgetter(0)),key=itemgetter(0))]


prod = 1

for list in m:
  for element in list:
    prod *= element

print prod

Answer 1

from itertools import groupby
from operator import itemgetter

[max(g) for _,g in groupby(sorted(l),key=itemgetter(0))]

out:

[[2, 2, 2, 2], [3, 3], [5], [7], [11], [13], [17], [19]]