380162267qq@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).
<URL:
https://docs.python.org/3/library/functions.html#sorted>
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.
<URL:
https://docs.python.org/3/library/functools.html#functools.cmp_to_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")
<functools.K object at 0x7f6781ce0980>
>>> key_func("32") < key_func("5")
True
See the Sorting HOWTO <URL:
https://docs.python.org/3/howto/sorting.html>
for this and other tricks.
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Ben Finney
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