Using a comparator function to sort

Question

So I'm working with a few pre-existing comparators that compare certain values in two tuples and return true if the first is greater than the second, false if otherwise. Here's the code for one of them:

def cmpValue(subInfo1, subInfo2):
    """
    Returns True if value in (value, work) tuple subInfo1 is GREATER than
    value in (value, work) tuple in subInfo2
    """
    # TODO...
    if subInfo1[0] > subInfo2[0]:
        return True
    else:
        return False

Now, I have a dictionary that has numerous tuple entries of the type being compared above. I want to sort them all in reverse order, but I don't really understand how I would accomplish that. I was thinking something like:

sortedDict = sorted(subjects, key=comparator, reverse = True)

But I don't know what to pass into the comparator because each comparator takes two arguments (subInfo1, subInfo2). I cannot change the comparator functions.

Comparator functions are deprecated in Python; use key functions instead. — Ignacio Vazquez-Abrams, Oct 05 '12 at 15:28
`if condition: return True else: return False` should be `return condition`. — Fred Foo, Oct 05 '12 at 15:30
Dictionaries do not preserve order. If you want a sorted dictionary you should use `OrderedDict` from the collections module. — Matt, Oct 05 '12 at 15:32
@IgnacioVazquez-Abrams : I miss a link to the deprecation declaration of the `cmp` operator. [Here](https://www.python.org/dev/peps/pep-0004/) it is... The [python wiki](https://wiki.python.org/moin/HowTo/Sorting/#The_Old_Way_Using_the_cmp_Parameter) has an article how to convert from `cmp` to `key`. — Pascal, Aug 15 '18 at 15:27

score 55 · Accepted Answer · answered Oct 05 '12 at 15:31

55

You're passing the comparator as the key function. You should be passing it as the cmp, wrapped in some kind of function that turns it into a proper comparator.

def make_comparator(less_than):
    def compare(x, y):
        if less_than(x, y):
            return -1
        elif less_than(y, x):
            return 1
        else:
            return 0
    return compare

sortedDict = sorted(subjects, cmp=make_comparator(cmpValue), reverse=True)

(Although actually, you should be using key functions:

sorted(subjects, operator.itemgetter(0), reverse=True)

Also note that sortedDict will not actually be a dict, so the name is rather confusing.)

answered Oct 05 '12 at 15:31

Fred Foo

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10

Also, the comparator should not return `True` or `False` but rather -1, 0, or 1. – kindall Oct 05 '12 at 15:33
7

Nice work on the wrapper function for the comparator. You might mention `functools.cmp_to_key` also. – kindall Oct 05 '12 at 15:47
6

`functools.cmp_to_key` is available for this sort of thing – Jon Clements Oct 05 '12 at 15:49
5

Note that this is **deprecated**. See @IgnacioVazquez-Abrams or my answer in the comments of the question. – Pascal Aug 15 '18 at 15:29

kaya3 · Answer 2 · 2021-09-11T11:23:54.620

In Python 3 there is no cmp argument for the sorted function (nor for list.sort).

According to the docs, the signature is now sorted(iterable, *, key=None, reverse=False), so you have to use a key function to do a custom sort. The docs suggest:

Use functools.cmp_to_key() to convert an old-style cmp function to a key function.

Here's an example:

>>> def compare(x, y):
...     return x[0] - y[0]
... 
>>> data = [(4, None), (3, None), (2, None), (1, None)]
>>> from functools import cmp_to_key
>>> sorted(data, key=cmp_to_key(compare))
[(1, None), (2, None), (3, None), (4, None)]

However, your function doesn't conform to the old cmp function protocol either, since it returns True or False. For your specific situation you can do:

>>> your_key = cmp_to_key(make_comparator(cmpValue))
>>> sorted(data, key=your_key)
[(1, None), (2, None), (3, None), (4, None)]

using the make_comparator function from @Fred Foo's answer.

score 1 · Answer 3 · answered Nov 23 '21 at 00:39

The answer of @kaya3 is correct. I just propose another implementation in which we can use boolean for the comparator.

class YourTupleComparator(tuple):
    def __lt__(self, other):
        return self[0] < other[0]

sorted(subjects, key=YourTupleComparator)

score 0 · Answer 4 · answered Feb 20 '22 at 12:02

0

We can now use this to sort a 2-D array:

A.sort(key=lambda a: (a[0], -a[1]))

This will sort the 2d-array by ascending order of A[0] and descending order of A[1].

answered Feb 20 '22 at 12:02

Priyanshu Tiwari

187
10

This answer does not seem to be related to the question, which is about using comparator functions to sort. A key function is not a comparator function. It looks like you are trying to answer a different question [like this one](https://stackoverflow.com/questions/5212870/sorting-a-python-list-by-two-fields), though that question already has an answer saying the same thing so there is no need to write it again. – kaya3 Feb 21 '22 at 06:23
This answer is related to this! Now instead of comparator we can use key in Python3 which works as a comparator. – Priyanshu Tiwari Feb 21 '22 at 07:18

Using a comparator function to sort

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