# [Tutor] bubble sort function

Steven D'Aprano steve at pearwood.info
Sun Nov 16 05:50:33 CET 2014

```On Sat, Nov 15, 2014 at 04:46:26PM +0000, Spyros Charonis wrote:
> Dear group,
>
>
> I'm having a bit of trouble with understanding why my bubble sort
> implementation doesn't work. I've got the following function to perform a
> bubble sort operation on a list of numbers:

It doesn't work because it is completely wrong. Sorry to be harsh, but
sometimes it is easier to throw broken code away and start again than it
is to try to diagnose the problems with it.

Wikipedia:

https://en.wikipedia.org/wiki/Bubble_sort#Implementation

procedure bubbleSort( A : list of sortable items )
n = length(A)
repeat
swapped = false
for i = 1 to n-1 inclusive do
/* if this pair is out of order */
if A[i-1] > A[i] then
/* swap them and remember something changed */
swap( A[i-1], A[i] )
swapped = true
end if
end for
until not swapped
end procedure

Let's translate that to Python:

def bubbleSort(alist):
n = len(alist)
swapped = True
while swapped:
swapped = False
for i in range (1, n-1):
# if this pair is out of order
if alist[i-1] > alist[i]:
# swap them and remember something changed
alist[i-1], alist[i] = alist[i], alist[i-1]
swapped = True

Let's add something to print the partially sorted list each time we go
through the loop:

def bubbleSort(alist):
print("Unsorted: %r" % alist)
n = len(alist)
swapped = True
count = swaps = 0
while swapped:
count += 1
swapped = False
for i in range (1, n):
# if this pair is out of order
if alist[i-1] > alist[i]:
# swap them and remember something changed
swaps += 1
alist[i-1], alist[i] = alist[i], alist[i-1]
swapped = True
print("Iteration %d, %d swaps; list: %r" % (count, swaps, alist))

And now let's try it:

py> mylist = [2, 4, 6, 8, 1, 3, 5, 7, 9, 0]
py> bubbleSort(mylist)
Unsorted: [2, 4, 6, 8, 1, 3, 5, 7, 9, 0]
Iteration 1, 5 swaps; list: [2, 4, 6, 1, 3, 5, 7, 8, 0, 9]
Iteration 2, 9 swaps; list: [2, 4, 1, 3, 5, 6, 7, 0, 8, 9]
Iteration 3, 12 swaps; list: [2, 1, 3, 4, 5, 6, 0, 7, 8, 9]
Iteration 4, 14 swaps; list: [1, 2, 3, 4, 5, 0, 6, 7, 8, 9]
Iteration 5, 15 swaps; list: [1, 2, 3, 4, 0, 5, 6, 7, 8, 9]
Iteration 6, 16 swaps; list: [1, 2, 3, 0, 4, 5, 6, 7, 8, 9]
Iteration 7, 17 swaps; list: [1, 2, 0, 3, 4, 5, 6, 7, 8, 9]
Iteration 8, 18 swaps; list: [1, 0, 2, 3, 4, 5, 6, 7, 8, 9]
Iteration 9, 19 swaps; list: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Iteration 10, 19 swaps; list: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Now you can inspect the working code and compare it to the non-working
code below and see what is different:

> def bubble_sort_ascending(unsorted):
>       """ Sorts a list of numbers into ascending order """
>        iterations = 0
>         size = len(unsorted) - int(1)
>        for i in range(0, size):
>             unsorted[i] = float(unsorted[i])
>             while unsorted[i] > unsorted[i+1]:
>                   # Use a tuple assignment in order to swap the value of
> two variables
>                   unsorted[i], unsorted[i+1] = unsorted[i+1], unsorted[i]
>                   iterations += 1
>                   sorted_vec = unsorted[:] # copy unsorted which is now
> sorted
>                   print "\nIterations completed: %s\n" %(iterations)
>        return sorted_vec

--
Steven
```