sorting tuples in python with a custom key

Question

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 tuple if a[1] == b[0]
* a tuple is greater than b tuple if a[0] == b[1]

I've implemented a cmp function like this:

def tcmp(a, b):
    if a[1] == b[0]:
       return -1
    elif a[0] == b[1]:
       return 1
    else:
       return 0

but sorting the list:

lt.sort(tcmp)

lt show me:

lt = [(2, 4), (4, 5), (5, 2)]

What am I doing wrong?

Answer 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:

def sort_last(tuples):

  def last_value_tuple(t):
    return t[-1]

  return sorted(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.

Answer 2

You could also write your code using lambda

def sort(tuples):
  return sorted (tuples,key=lambda last : last[-1])

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

Answer 3

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

Ex.

def sort_last(tuples):

    return sorted(tuples, key=last)

def last(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)]

Answer 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.