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So I am new to python and I have a project which requires us to go through a really long tuple list and we have to order the list in descending and ascending order. However, for both my functions I always get ascending order, WHAT IS WRONG? someone please help im really stressed out 

def bubblesort_descending(tuple_list):
    j = len(tuple_list)
    made_swap = True
    swaps = 0
    while made_swap:
        made_swap = False
        for cnt in range (j-1):
            if tuple_list[cnt] < tuple_list[cnt+1]:
                tuple_list[cnt], tuple_list[cnt+1] = tuple_list[cnt+1], tuple_list[cnt]
                made_swap = True
                swaps = swaps + 1
    return swaps

Main Program:

elif choice ==  'd':
    unsorted = range(len(numbers))
    shuffle(unsorted)
    print ("Randomised tuple list generated:")
    print
    print (unsorted)

    swaps = bubblesort_descending (unsorted)
    print
    print ("heres the sorted list")
    print
    print (unsorted)
    print
    print (swaps, "swap(s) made")
    print
James Mills
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user3200834
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2 Answers2

2

The basic difference between ascending and descending sorting order is in the comparison: Here is a bubble sort implementation as taken from http://www.codecodex.com/wiki/Bubble_sort#Python:

def bubble_sort(lst, asc=True):
    lst = list(lst)  # copy collection to list
    for passesLeft in range(len(lst)-1, 0, -1):
        for i in range(passesLeft):
            if asc:
                if lst[i] > lst[i + 1]:
                    lst[i], lst[i + 1] = lst[i + 1], lst[i]
            else:
                if lst[i] < lst[i + 1]:
                    lst[i], lst[i + 1] = lst[i + 1], lst[i]
    return lst

Note: The difference based on the asc parameter?

Example:

>>> xs = [1, 2, 9, 4, 0]
>>> bubble_sort(xs, asc=True)
[0, 1, 2, 4, 9]
>>> bubble_sort(xs, asc=False)
[9, 4, 2, 1, 0]

So in effect swapping your logical operator < to > reverses the sorting order.

James Mills
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1

You need to convert that iterator to a list. 

unsorted = range(10)
unsorted_list = list(unsorted)

After this, your code will sort in descending order because you are making a swap if tuple_list[cnt] is less than tuple_list[cnt+1]. If you change the logical operator from "<"to ">" you will get ascending order because after changing, you will make swaps if tuple_list[cnt] is greater than tuple_list[cnt+1]

By naming your list as tuple_list, it is kind of confusing. Because in python lists and tuples are different.
What's the difference between lists and tuples?

Lafexlos
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  • Actually ``range()`` in Python 2 gives you a list. In Python 3 it gives you an iterator of ````. – James Mills Jan 16 '14 at 03:57
  • since he is using prints as functions (print()) i assumed he is not using python 2. So he is not creating a list. For python 3, I didn't know that. Thanks for info. Updating my answer according to that. – Lafexlos Jan 16 '14 at 04:00
  • You cannot make that assumption. ``from __future__ import print_function``. – James Mills Jan 16 '14 at 04:02
  • "print_function 2.6.0a2 3.0 PEP 3105: Make print a function" from http://docs.python.org/2/library/__future__.html -- It's been in Python 2.6 for quite some time. – James Mills Jan 16 '14 at 04:04
  • Ok. Another unknown. Thanks again. =) Code should work in python 2.6+ because range() would have created list. Since it is not working, it is another thing that makes me assuming he is using python 3. (seems only "right" thing) – Lafexlos Jan 16 '14 at 04:11
  • Thanks for updating your answer :) Also update it to place more emphasis on the *logical operator*. This was/is (*I believe*) the OP's basic problem. – James Mills Jan 16 '14 at 04:19
  • I tried to make it clear why I changed the _logical operator_. I hope it is clear enough. :) – Lafexlos Jan 16 '14 at 04:28