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I'm not sure how to write a comparator in Python 3 as the cmp parameter is removed. Considering the following code in Python 3, how do I rewrite the comparator using only key?
import functools
def my_cmp(x, y):
return x*5-y*2
l = [50, 2, 1, 9]
print(sorted(l, key=functools.cmp_to_key(my_cmp)))
thanks.
423
To do this, you must create a Checker class that implements the Comparator interface, then write an int compare(Player a, Player b) method implementing the Comparator. compare(T o1, T o2) method. In short, when sorting in ascending order, a comparator function returns -1 if a<b , 0 if a=b , and 1 if a>b .
A comparator function is used to sort a given data structure along with the help of the sorted() function. In Python 3, we use a key function to carry out a custom sorting process. Here, we use the functools.
Key Functions sort() and sorted() have a key parameter to specify a function (or other callable) to be called on each list element prior to making comparisons. The value of the key parameter should be a function (or other callable) that takes a single argument and returns a key to use for sorting purposes.
The key parameter takes a function as its value, which is applied to each element before sorting, so that the elements are sorted based on the output of this function.
This "comparison" function that you came up with is inconsistent: it should provide a definite (deterministic) order, meaning, if you change the order of the elements in the list and run sorted - you should get the same result!
In your case, the order of the elements effects the sorting:
import functools
def my_cmp(x, y):
return x*5-y*2
l = [50, 2, 1, 9]
print(sorted(l, key=functools.cmp_to_key(my_cmp))) # [2, 1, 9, 50]
l = [50, 1, 2, 9]
print(sorted(l, key=functools.cmp_to_key(my_cmp))) # [1, 2, 9, 50]
which means that your "comparison" function is inconsistent. First provide good ordering function, then it should not be very difficult to convert it to a key function.
Regards the question that you raised in the comments, key accepts a function that takes only a single argument - and returns a "measurement" of "how big is it". The easiest example would be to compare numbers, in that case your key function can simply be: lambda x: x. For any number the lambda expression will returns itself and the comparison is now trivial!
Modifying your example:
def my_key(x):
return x
l = [50, 2, 1, 9]
print(sorted(l, key=my_key)) # [1, 2, 9, 50]
A shorter version of the above would be:
l = [50, 2, 1, 9]
print(sorted(l, key=lambda x: x)) # [1, 2, 9, 50]
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