Sorting Tuples In Python With A Custom Key

Hi: I'm trying to sort a list of tuples in a custom way: For example: lt = [(2,4), (4,5), (5,2)] must be sorted: lt = [(5,2), (2,4), (4,5)] Rules: * b tuple is greater than a

Solution 1:

Sounds a lot to me you are trying to solve one of the Google's Python class problems, which is to sort a list of tuples in increasing order based on their last element.

This how I did it:

defsort_last(tuples):

  deflast_value_tuple(t):
    return t[-1]

  returnsorted(tuples, key=last_value_tuple)

EDIT: I didn't read the whole thing, and I assumed it was based on the last element of the tuple. Well, still I'm going to leave it here because it can be useful to anyone.

Solution 2:

You could also write your code using lambda

defsort(tuples):
  returnsorted (tuples,key=lambda last : last[-1])

so sort([(1, 3), (3, 2), (2, 1)]) would yield [(2, 1), (3, 2), (1, 3)]

Solution 3:

You can write your own custom key function to specify the key value for sorting.

Ex.

defsort_last(tuples):

    returnsorted(tuples, key=last)

deflast(a):
    return a[-1]

tuples => sorted tuple by last element

• [(1, 3), (3, 2), (2, 1)] => [(2, 1), (3, 2), (1, 3)]
• [(1, 7), (1, 3), (3, 4, 5), (2, 2)] => [(2, 2), (1, 3), (3, 4, 5), (1, 7)]

Solution 4:

I'm not sure your comparison function is a valid one in a mathematical sense, i.e. transitive. Given a, b, c a comparison function saying that a > b and b > c implies that a > c. Sorting procedures rely on this property.

Not to mention that by your rules, for a = [1, 2] and b = [2, 1] you have both a[1] == b[0] and a[0] == b[1] which means that a is both greater and smaller than b.

Solution 5:

Your ordering specification is wrong because it is not transitive.

Transitivity means that if a < b and b < c, then a < c. However, in your case:

(1,2) < (2,3)
(2,3) < (3,1)
(3,1) < (1,2)