CODEWITHSUNDEEP
pythonstringcomparesortcomparefunction
wherby
wherby

Reputation: 784

Python sort lib different behavior with different comparing functions, some comparing function will not work

I want get a max alphabetically order by sub string, so I need to have a comparing method. For the comparing function function

from functools import cmp_to_key
def compare(a,b):
    print(a,b, a+b,b+a,a+b>b+a) 
    return (a+b)>(b+a) # first comparing function

def sortArr(ls):
    ls = sorted(ls,key =cmp_to_key(compare),reverse=True)
    return "".join(ls)

ls = ["8","81","82","829"]
print(sortArr(ls))

Which will output as:

82 829 82829 82982 False
81 82 8182 8281 False
8 81 881 818 True
88182829

But using comparing function as:

from functools import cmp_to_key
def compare(a,b):
    print(a,b, a+b,b+a,a+b>b+a)
    return int(a+b) -int(b+a) # second comparing function

def sortArr(ls):
    ls = sorted(ls,key =cmp_to_key(compare),reverse=True)
    return "".join(ls)

ls = ["8","81","82","829"]
print(sortArr(ls))

the output is

82 829 82829 82982 False
81 82 8182 8281 False
8 81 881 818 True
8 82 882 828 True
8 829 8829 8298 True
88298281

Apparently the second sort function is what's needed ( the final string 88182829 < 88298281 ). the first function didn't do all comparing use first element.

So why the first comparing function didn't work?

Upvotes: -7

Views: 80

Answers (1)

Tim Peters
Tim Peters

Reputation: 70715

It's unclear why you might expect the first function to work. An old-style comparison function has to work like so:

  • cmp(x, y) is negative if and only if x < y.
  • cmp(x, y) is 0 if and only if x == y.
  • cmp(x, y) is strictly positive (> 0) if and only if x > y.

Given that, your return (a+b)>(b+a) doesn't appear to make sense. It can only return True or False, which act like 1 and 0, respectively. So it's impossible for it to return a result which will be interpreted as meaning a is less than b.

Upvotes: 1

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