functools.cmp_to_key clear solution for Bigger Together by Phil15 - python coding challenges

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functools.cmp_to_key solution in Clear category for Bigger Together by Phil15

from functools import cmp_to_key
str_compare = lambda s1, s2: int(s1 + s2) - int(s2 + s1)

def bigger_together(ints):
    ints = sorted(map(str, ints), key = cmp_to_key(str_compare))
    return int(''.join(reversed(ints))) - int(''.join(ints))

Sept. 24, 2018

Comments:

hbczmxy on Jan. 5, 2022, 7:10 a.m.
Hello, under your introduction last time, I already know functools.cmptokey, but I don't fully understand it. Last introduction: <a href="https://py.checkio.org/forum/post/14931/cannot-solve-nearest-value-with-cmp_to_key/">"cannot solve 'Nearest Value' with cmptokey"</a> In the official documentation of Python <blockquote> In Py2.x, sort allowed an optional function which can be called for doing the comparisons.That function should take two arguments to be compared and then return a negative value for less-than, return zero if they are equal, or return a positive value for greater-than. </blockquote> In the process of solving this problem, the comparison function I wrote seemed clumsy and bloated. This is the comparison function I wrote:<a href="https://py.checkio.org/mission/bigger-together/publications/hbczmxy/python-3/first-use-comparisons-between-strings-and-functoolscmp_to_key/">mine</a> But for the way you use functools.cmptokey, I still don't quite understand how it works? <pre class='brush: python'>str_compare = lambda s1, s2: int(s1 + s2) - int(s2 + s1) </pre> Especially when s1 is equal to the string '11' and s2 is equal to the string '1', that is, when the expression value is zero, what will happen to your function? How will your function handle this situation?

Phil15 on Jan. 5, 2022, 9:39 a.m.
I agree that "cmptokey" can be a bit tricky at first. But other things in "functools" too. But "cmptokey" is really appropriate here. Feel free to ask any question. For the particular example you took: `str_compare('11', '1') == 0`. So for me, "11" are "1" considered equal. Because it does not matter if "1" ends up before or after "11" as results would be "1 11" or "11 1" (both "111"). So yeah I don't sort them. If 11 and 1 are the only elements in "ints", then ["11", "1"] or ["1", "11"] does not matter. If there are other elements, other comparisons will do the sort.

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