Python 2: Sorting algorithm does not work

Question

I am working on a program that takes a list and sorts it from highest to lowest [Yes I know I could use .sort()]. The program is the following:

UnList = raw_input("Enter a list of numbers seperated by commas") #collects the users list
List = UnList.split(",") #Turns the users numbers into a list
for x in range(len(List)): #number of time to sort
  Switcher = 0 #the switcher (also will reset it later)
  for z in range(len(List)-1): #number of time the switcher runs
    if List[Switcher] > List[(Switcher + 1)]: #if one number is bigger than the other
      List[Switcher],List[(Switcher+1)] = List[(Switcher+1)],List[Switcher] #this switches those 2 numbers
    Switcher += 1 #adds to the switcher
print "The new list is:", List #prints the output

Sometimes it works, as in with the example "1,7,4,6,3" Other times, such as with "-10,5,4,32,4,-40,2" it gives the completely incorrect output "['-10', '-40', '2', '32', '4', '4', '5']"

Your values are strings and not integers and therefore -10 appears before -40 — Alex
– Alex, Commented Mar 4, 2016 at 14:22
Nitpicky thing unrelated to the issue, convention is to start your variables with a lowercase, rather than capitalizing them — StephenTG
– StephenTG, Commented Mar 4, 2016 at 14:25

StephenTG · Accepted Answer · 2016-03-04 14:26:05Z

5

Based on the order you're getting, I think the issue may be that the sort is going in lexicographic order, rather than numeric order. Ensure that all the elements are being compared as integers rather than strings

As Johny suggests, List = [int(x) for x in UnList.split(",")] would be one way to convert to a list of integers

edited Mar 4, 2016 at 14:26

answered Mar 4, 2016 at 14:20

StephenTG

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3 Comments

TheRIProgrammer Over a year ago

When I changed it to List = int(UnList.split(",")) I got the error: TypeError: int() argument must be a string or a number, not 'list' on line 9 in main.py obviously I am doing something wrong, but I am not sure exactly what. Thank you.

Johny Over a year ago

List = [int(x) for x in UnList.split(",")]

StephenTG Over a year ago

@TheRIProgrammer The way you were doing it, you tried to convert the entire list to a single integer. Johny's way should convert each element individually

Alex · Accepted Answer · 2016-03-04 14:24:49Z

You need to convert your values to integers, otherwise they do not get sorted correctly. You could do this by modifying the line of code that splits the strings:

List = map(int,UnList.split(",")) #Turns the users numbers into a list

After this change, the output is:

[-40, -10, 2, 2, 3, 4, 4, 5]

This is my full code:

UnList = '-10, -40, 2, 2, 3, 4, 4, 5'
List = map(int,UnList.split(",")) #Turns the users numbers into a list

for x in range(len(List)): #number of time to sort
  Switcher = 0 #the switcher (also will reset it later)
  for z in range(len(List)-1): #number of time the switcher runs
    if List[Switcher] > List[(Switcher + 1)]: #if one number is bigger than the other
      List[Switcher],List[(Switcher+1)] = List[(Switcher+1)],List[Switcher] #this switches those 2 numbers
    Switcher += 1 #adds to the switcher
print "The new list is:", List #prints the output

Mahendra Gaur · Accepted Answer · 2016-03-04 14:26:18Z

1

You are getting wrong output because you are using a list of string and not a list of integers. Convert your list of string to list of integers before processing the list.

answered Mar 4, 2016 at 14:26

Mahendra Gaur

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