Sorting list of strings using L.sort with key multiple parameters

Question

Pretty new to Python and have been playing around with lists and the sort member. My question is if I have a list of random strings (all strings of equal length):

li=["hgjtud", "iernfd", "scoeps", "stiiss", "stripe"]

and now I want to sort that list according to some rank which I define with the following function:

def rank_string(str1,str2):
    rank=0
    for i in range(len(str1)): #all strings will be checked to be equal length
        if (str1[i]==str2[i]):
            rank += 1
    return rank

and now I want to sort my list using this function together with a target string, so I try the following:

target_string="stripe"
df = lambda x:rank_string(x,target_string)
sorted_list = resultlist.sort(key=df)

I was of the impression that all list values will be given a key after one pass of the ranking function and thereafter be sorted according to this? This runs, but sorted_list has value None. I assume then that being a n00b, I am missing something fundamental. What? :-)

Thanks for the help in advance.

Martijn Pieters · Accepted Answer · 2012-09-26 14:02:47Z

1

The .sort() method sorts in-place and doesn't return anything.

Use sorted() instead if you want the sorted list returned and the original input list left untouched.

>>> a = [2, 3, 1]
>>> a.sort()
>>> a
[1, 2, 3]
>>> b = [2, 3, 1]
>>> sorted(b)
[1, 2, 3]
>>> b
[2, 3, 1]

answered Sep 26, 2012 at 14:02

Martijn Pieters

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Comments

eumiro · Accepted Answer · 2012-09-26 14:07:59Z

1

As other said, sorted() will return a new list. Your code could look like this:

li = ["hgjtud", "iernfd", "scoeps", "stiiss", "stripe"]
target_string = "stripe"

sorted_list = sorted(li, key=lambda x: sum(1 for c1,c2 in zip(x, target_string) if c1==c2))

returns ['hgjtud', 'iernfd', 'scoeps', 'stiiss', 'stripe']

answered Sep 26, 2012 at 14:07

eumiro

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1 Comment

Englishbob Over a year ago

I'm beginning to realize that terseness is very pythonesque :-) and is the way to go. Thanks.

pillmuncher · Accepted Answer · 2012-09-27 03:37:13Z

1

Like the others said, use sorted(). Also you could make things shorter:

from functools import partial

def rank_string(str1, str2):
    return sum(a == b for a, b in zip(str1, str2))

li = ["hgjtud", "iernfd", "scoeps", "stiiss", "stripe"]
sorted_list = sorted(li, key=partial(rank_string, "stripe"))

[EDIT]

from operator import eq

def rank_string(str1, str2):
    return sum(map(eq, str1, str2))

:-)

[/EDIT]

edited Sep 27, 2012 at 3:37

answered Sep 26, 2012 at 14:29

pillmuncher

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1 Comment

Englishbob Over a year ago

again, I guess the terseness comes with the familiarity .-)

ch3ka · Accepted Answer · 2012-09-26 14:03:01Z

0

resultlist.sort is a method of list, sorts resultlist in place, and returns None.

sorted_list = sorted(li, key=df) should do the trick.

answered Sep 26, 2012 at 14:03

ch3ka

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