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]
def bubble(badList):
length = len(badList) - 1
unsorted = True
while unsorted:
for element in range(0,length):
unsorted = False
if badList[element] > badList[element + 1]:
hold = badList[element + 1]
badList[element + 1] = badList[element]
badList[element] = hold
print badList
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):
if badList[element] > badList[element + 1]:
sorted = False # We found two elements in the wrong order
hold = badList[element + 1]
badList[element + 1] = badList[element]
badList[element] = hold
# 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.
def bubble(bad_list):
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])
).
bad_list[i], bad_list[i+1] = bad_list[i+1], bad_list[i]
Put it all together, and you get this:
my_list = [12, 5, 13, 8, 9, 65]
def bubble(bad_list):
length = len(bad_list) - 1
sorted = False
while not sorted:
sorted = True
for i in range(length):
if bad_list[i] > bad_list[i+1]:
sorted = False
bad_list[i], bad_list[i+1] = bad_list[i+1], bad_list[i]
bubble(my_list)
print my_list
(I removed your print statement too, by the way.)