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Bubble Sort Homework

Questions:

In class we are doing sorting algorithms and, although I understand them fine when talking about them and writing pseudocode, I am having problems writing actual code for them.

This is my attempt in Python:

mylist = [12, 5, 13, 8, 9, 65]

unsorted = True

while unsorted:
for element in range(0,length):
unsorted = False
else:
unsorted = True

print bubble(mylist)

Now, this (as far as I can tell) sorts correctly, but once it finishes it just loops indefinitely.

How can this code be fixed so the function finishes properly and correctly sorts a list of any (reasonable) size?

P.S. I know I should not really have prints in a function and I should have a return, but I just have not done that yet as my code does not really work yet.

To explain why your script isn’t working right now, I’ll rename the variable unsorted to sorted.

At first, your list isn’t yet sorted. Of course, we set sorted to False.

As soon as we start the while loop, we assume that the list is already sorted. The idea is this: as soon as we find two elements that are not in the right order, we set sorted back to False. sorted will remain True only if there were no elements in the wrong order.

sorted = False  # We haven't started sorting yet

while not sorted:
sorted = True  # Assume the list is now sorted
for element in range(0, length):
sorted = False  # We found two elements in the wrong order
# We went through the whole list. At this point, if there were no elements
# in the wrong order, sorted is still True. Otherwise, it's false, and the
# while loop executes again.

There are also minor little issues that would help the code be more efficient or readable.

• In the for loop, you use the variable element. Technically, element is not an element; it’s a number representing a list index. Also, it’s quite long. In these cases, just use a temporary variable name, like i for “index”.

for i in range(0, length):

• The range command can also take just one argument (named stop). In that case, you get a list of all the integers from 0 to that argument.

for i in range(length):

• The Python Style Guide recommends that variables be named in lowercase with underscores. This is a very minor nitpick for a little script like this; it’s more to get you accustomed to what Python code most often resembles.

• To swap the values of two variables, write them as a tuple assignment. The right hand side gets evaluated as a tuple (say, (badList[i+1], badList[i]) is (3, 5)) and then gets assigned to the two variables on the left hand side ((badList[i], badList[i+1])).

Put it all together, and you get this:

my_list = [12, 5, 13, 8, 9, 65]

sorted = False

while not sorted:
sorted = True
for i in range(length):
sorted = False

bubble(my_list)
print my_list

(I removed your print statement too, by the way.)

Questions:

The goal of bubble sort is to move the heavier items at the bottom in each round, while moving the lighter items up. In the inner loop, where you compare the elements, you don’t have to iterate the whole list in each turn. The heaviest is already placed last. The swapped variable is an extra check so we can mark that the list is now sorted and avoid continuing with unnecessary calculations.

for i in range(0,length):
swapped = False
for element in range(0, length-i-1):
swapped = True
if not swapped: break

unsorted = True
while unsorted:
unsorted = False
for element in range(0,length):
#unsorted = False
unsorted = True
#else:
#unsorted = True

Questions:

This is what happens when you use variable name of negative meaning, you need to invert their values. The following would be easier to understand:

sorted = False
while not sorted:
...

On the other hand, the logic of the algorithm is a little bit off. You need to check whether two elements swapped during the for loop. Here’s how I would write it:

def bubble(values):
length = len(values) - 1
sorted = False
while not sorted:
sorted = True
for element in range(0,length):
if values[element] > values[element + 1]:
hold = values[element + 1]
values[element + 1] = values[element]
values[element] = hold
sorted = False
return values

Questions:

Your use of the Unsorted variable is wrong; you want to have a variable that tells you if you have swapped two elements; if you have done that, you can exit your loop, otherwise, you need to loop again. To fix what you’ve got here, just put the “unsorted = false” in the body of your if case; remove your else case; and put “unsorted = true before your for loop.

Questions:
def bubble_sort(l):
for passes_left in range(len(l)-1, 0, -1):
for index in range(passes_left):
if l[index] < l[index + 1]:
l[index], l[index + 1] = l[index + 1], l[index]
return l

Questions:

#A very simple function, can be optimized (obviously) by decreasing the problem space of the 2nd array. But same O(n^2) complexity.

def bubble(arr):
l = len(arr)
for a in range(l):
for b in range(l-1):
if (arr[a] < arr[b]):
arr[a], arr[b] = arr[b], arr[a]
return arr

Questions:

You’ve got a couple of errors in there. The first is in length, and the second is in your use of unsorted (as stated by McWafflestix). You probably also want to return the list if you’re going to print it:

mylist = [12, 5, 13, 8, 9, 65]

unsorted = True

while unsorted:
for element in range(0,length):
unsorted = False

unsorted = True

print bubble(mylist)

eta: You’re right, the above is buggy as hell. My bad for not testing through some more examples.

swapped = True

while swapped:
swapped = False
for i in range(0, length):

# swap

swapped = True

Questions:

The problem with the original algorithm is that if you had a lower number further in the list, it would not bring it to the correct sorted position. The program needs to go back the the beginning each time to ensure that the numbers sort all the way through.

I simplified the code and it will now work for any list of numbers regardless of the list and even if there are repeating numbers. Here’s the code

mylist = [9, 8, 5, 4, 12, 1, 7, 5, 2]
print mylist

element = 0
while element < length:
element = 0
else:
element = element + 1

print bubble(mylist)

Questions:
def bubble_sort(l):
exchanged = True
iteration = 0
n = len(l)

while(exchanged):
iteration += 1
exchanged = False

# Move the largest element to the end of the list
for i in range(n-1):
if l[i] > l[i+1]:
exchanged = True
l[i], l[i+1] = l[i+1], l[i]
n -= 1   # Largest element already towards the end

print 'Iterations: %s' %(iteration)
return l

Questions:
def bubbleSort(alist):
if len(alist) <= 1:
return alist
for i in range(0,len(alist)):
print "i is :%d",i
for j in range(0,i):
print "j is:%d",j
print "alist[i] is :%d, alist[j] is :%d"%(alist[i],alist[j])
if alist[i] > alist[j]:
alist[i],alist[j] = alist[j],alist[i]
return alist

alist = [54,26,93,17,77,31,44,55,20,-23,-34,16,11,11,11]

print bubbleSort(alist)

Questions:
def bubble_sort(a):
t = 0
sorted = False # sorted = False because we have not began to sort
while not sorted:
sorted = True # Assume sorted = True first, it will switch only there is any change
for key in range(1,len(a)):
if a[key-1] > a[key]:
sorted = False
t = a[key-1]; a[key-1] = a[key]; a[key] = t;
print a

Questions:

A simpler example:

a = len(alist)-1
while a > 0:
for b in range(0,a):
if alist[b]>=alist[b+1]:
#swap both elements
alist[b], alist[b+1] = alist[b+1], alist[b]
a-=1

This simply takes the elements from 0 to a(basically, all the unsorted elements in that round) and compares it with its adjacent element, and making a swap if it is greater than its adjacent element. At the end the round, the last element is sorted, and the process runs again without it, until all elements have been sorted.

There is no need for a condition whether sort is true or not.

Note that this algorithm takes into consideration the position of the numbers only when swapping, so repeated numbers will not affect it.

PS. I know it has been very long since this question was posted, but I just wanted to share this idea.

Questions:

I am a fresh fresh beginner, started to read about Python yesterday.
Inspired by your example I created something maybe more in the 80-ties style, but nevertheless it kinda works

lista1 = [12, 5, 13, 8, 9, 65]

i=0
while i < len(lista1)-1:
if lista1[i] > lista1[i+1]:
x = lista1[i]
lista1[i] = lista1[i+1]
lista1[i+1] = x
i=0
continue
else:
i+=1

print(lista1)

Questions:
def bubble_sort(li):
l = len(li)
tmp = None
sorted_l = sorted(li)
while (li != sorted_l):
for ele in range(0,l-1):
if li[ele] > li[ele+1]:
tmp = li[ele+1]
li[ele+1] = li [ele]
li[ele] = tmp
return li

Questions:

Answers provided by the-fury and Martin Cote fixed the problem of the infinite loop, but my code would still not work correctly (for a larger list, it would not sort correctly.). I ended up ditching the unsorted variable and used a counter instead.

n = 0
for element in range(0,length):