Re: How to sort this without 'cmp=' in python 3?

[Chris Angelico via Python-list]

On Wed, 25 Oct 2023 at 13:02, Mike H via Python-list
 wrote:
> Is it possible to use lambda expression instead of defining a `Key` class? 
> Something like `sorted(my_list, key = lambda x, y: x+y > y+x)`?

Look up functools.cmp_to_key.

ChrisA
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Re: How to sort this without 'cmp=' in python 3?

[Mike H via Python-list]

On Saturday, October 15, 2016 at 12:27:42 AM UTC-7, Peter Otten wrote:
> 38016...@gmail.com wrote: 
> 
> > nums=['3','30','34','32','9','5'] 
> > I need to sort the list in order to get the largest number string: 
> > '953433230' 
> > 
> > nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True) 
> > 
> > But how to do this in python 3? 
> > 
> > Thank you
> While cmp_to_key is neat doing it by hand should also be instructive. 
> Essentially you move the comparison into a method of the key: 
> 
> $ cat translate_cmp.py 
> class Key(str): 
> def __lt__(a, b): 
> return a + b < b + a 
> 
> nums = ['3','30','34','32','9','5'] 
> print(nums) 
> nums.sort(key=Key, reverse=True) 
> print(nums) 
> print("".join(nums)) 
> 
> $ python3 translate_cmp.py
> ['3', '30', '34', '32', '9', '5']
> ['9', '5', '34', '3', '32', '30']
> 953433230 
> 
> The above works because in CPython list.sort() currently uses only the < 
> operator; adding __gt__() and __eq__() to make this portable is 
> straightforward even if you do not use the functools.total_ordering class 
> decorator.
Is it possible to use lambda expression instead of defining a `Key` class? 
Something like `sorted(my_list, key = lambda x, y: x+y > y+x)`?
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Re: How to sort this without 'cmp=' in python 3?

[380162267qq]

在 2016年10月14日星期五 UTC-4下午7:35:08，38016...@gmail.com写道：
> nums=['3','30','34','32','9','5']
> I need to sort the list in order to get the largest number string: '953433230'
> 
> nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True)
> 
> But how to do this in python 3?
> 
> Thank you

!I learn more new tricks in Python. Thank you all of you guys.
You are all very kind helpful and knowledgeable.
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Re: How to sort this without 'cmp=' in python 3?

[Peter Otten]

38016226...@gmail.com wrote:

> nums=['3','30','34','32','9','5']
> I need to sort the list in order to get the largest number string:
> '953433230'
> 
> nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True)
> 
> But how to do this in python 3?
> 
> Thank you

While cmp_to_key is neat doing it by hand should also be instructive. 
Essentially you move the comparison into a method of the key:

$ cat translate_cmp.py
class Key(str):
  def __lt__(a, b):
  return a + b < b + a

nums = ['3','30','34','32','9','5']
print(nums)
nums.sort(key=Key, reverse=True)
print(nums)
print("".join(nums))

$ python3 translate_cmp.py 
['3', '30', '34', '32', '9', '5']
['9', '5', '34', '3', '32', '30']
953433230

The above works because in CPython list.sort() currently uses only the < 
operator; adding __gt__() and __eq__() to make this portable is 
straightforward even if you do not use the functools.total_ordering class 
decorator.


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Re: How to sort this without 'cmp=' in python 3?

[breamoreboy]

On Saturday, October 15, 2016 at 12:53:48 AM UTC+1, sohca...@gmail.com wrote:
> On Friday, October 14, 2016 at 4:35:08 PM UTC-7, 38016...@gmail.com wrote:
> > nums=['3','30','34','32','9','5']
> > I need to sort the list in order to get the largest number string: 
> > '953433230'
> > 
> > nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True)
> > 
> > But how to do this in python 3?
> > 
> > Thank you
> 
> You don't need a lambda in this case.
> 
> Sort the strings in reverse order:
> nums.sort(reverse=True)
> 
> Create a string:
> biggestNum = ''.join(nums)
> 
> Or in a single line, which doesn't change the original value of nums:
> biggestNum = ''.join(sorted(nums, reverse=True))

No, you've made exactly the same mistake that I did :(

>>> nums=['3','30','34','32','9','5']
>>> wants='953433230'
>>> nums.sort(reverse=True)
>>> result = ''.join(nums)
>>> result == wants
False
>>> result
'953432303'
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Re: How to sort this without 'cmp=' in python 3?

[Ned Batchelder]

On Friday, October 14, 2016 at 7:49:33 PM UTC-4, Robin Koch wrote:
> Am 15.10.2016 um 01:33 schrieb 38016226...@gmail.com:
> > nums=['3','30','34','32','9','5']
> > I need to sort the list in order to get the largest number string: 
> > '953433230'
> >
> > nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True)
> >
> > But how to do this in python 3?
> 
> https://docs.python.org/3/library/functools.html#functools.cmp_to_key
> 
> | Transform an old-style comparison function to a key function.

cmp_to_key is a neat magic trick of a function, because it's seemingly
doing the impossible: how can a two-argument function be turned into 
a one-argument function?

Studying the code turns up a few clever Python tricks.

--Ned.
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Re: How to sort this without 'cmp=' in python 3?

[Ben Finney]

38016226...@gmail.com writes:

> nums=['3','30','34','32','9','5']
> I need to sort the list in order to get the largest number string: '953433230'
> nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True)

For demonstration, I'll re-write this such that the names and output
make more sense::

$ python2
Python 2.7.12+ (default, Sep  1 2016, 20:27:38)
[GCC 6.2.0 20160927] on linux2
Type "help", "copyright", "credits" or "license" for more information.

>>> digits = ['3','30','34','32','9','5']
>>> sorted(
... digits,
... cmp=(lambda a, b: cmp(a+b, b+a)),
... reverse=True)
['9', '5', '34', '3', '32', '30']

> But how to do this in python 3?

The Python 3 sorting functions take a ‘key’ parameter:

`key` specifies a function of one argument that is used to extract a
comparison key from each list element: `key=str.lower`. The default
value is `None` (compare the elements directly).



The `functools.cmp_to_key` helper is designed to help you transition to
that style:

functools.cmp_to_key(func)

Transform an old-style comparison function to a key function. Used
with tools that accept key functions (such as sorted(), min(),
max(), heapq.nlargest(), heapq.nsmallest(), itertools.groupby()).
This function is primarily used as a transition tool for programs
being converted from Python 2 which supported the use of comparison
functions.

A comparison function is any callable that accept two arguments,
compares them, and returns a negative number for less-than, zero for
equality, or a positive number for greater-than. A key function is a
callable that accepts one argument and returns another value to be
used as the sort key.



This works in the latest Python 2 and Python 3.

>>> import functools
>>> digits = ['3','30','34','32','9','5']
>>> sorted(
... digits,
... key=functools.cmp_to_key(lambda a, b: cmp(a+b, b+a)),
... reverse=True)
['9', '5', '34', '3', '32', '30']

The trick is done by creating a key function that takes an item for
comparison, and returns a custom object, which knows how to compare
itself as specified by your comparison function.

>>> key_func = functools.cmp_to_key(lambda a, b: cmp(a+b, b+a))
>>> key_func("32")

>>> key_func("32") < key_func("5")
True

See the Sorting HOWTO 
for this and other tricks.

-- 
 \  “I used to be an airline pilot. I got fired because I kept |
  `\   locking the keys in the plane. They caught me on an 80 foot |
_o__)stepladder with a coathanger.” —Steven Wright |
Ben Finney

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Re: How to sort this without 'cmp=' in python 3?

[sohcahtoa82]

On Friday, October 14, 2016 at 4:35:08 PM UTC-7, 38016...@gmail.com wrote:
> nums=['3','30','34','32','9','5']
> I need to sort the list in order to get the largest number string: '953433230'
> 
> nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True)
> 
> But how to do this in python 3?
> 
> Thank you

You don't need a lambda in this case.

Sort the strings in reverse order:
nums.sort(reverse=True)

Create a string:
biggestNum = ''.join(nums)

Or in a single line, which doesn't change the original value of nums:
biggestNum = ''.join(sorted(nums, reverse=True))

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Re: How to sort this without 'cmp=' in python 3?

[Robin Koch]

Am 15.10.2016 um 01:33 schrieb 38016226...@gmail.com:

nums=['3','30','34','32','9','5']
I need to sort the list in order to get the largest number string: '953433230'

nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True)

But how to do this in python 3?


https://docs.python.org/3/library/functools.html#functools.cmp_to_key

| Transform an old-style comparison function to a key function.

--
Robin Koch



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How to sort this without 'cmp=' in python 3?

[380162267qq]

nums=['3','30','34','32','9','5']
I need to sort the list in order to get the largest number string: '953433230'

nums.sort(cmp=lambda a,b: cmp(a+b, b+a), reverse=True)

But how to do this in python 3?

Thank you
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