Why doesn't this sorting method work in Python?

Question

I've got this code to sort a list without using the sorted function in python. The output is the opposite to what is expected (largest to smallest rather than smallest to largest)

Changing the < to a > seems to help but im not sure why this is

lista=[2,1,5,1,3,6]

for i in range(len(lista)):
  for l in range(len(lista)):
    if i==l:
      continue
    if lista[l]<lista[i]:
      temp=lista[i]
      lista[i]=lista[l]
      lista[l]=temp


print(lista)

expected output list that is smalled to largest, but getting the opposite unless I change the < sign to a > sign, but I'm not sure why this is?

mini · Accepted Answer · 2019-06-09 16:41:47Z

3

Try to write on paper every iteration of your algorithm:

i = 0: 2 1 5 1 3 6
i = 1: 1 2 5 1 3 6 
i = 2: 2 1 5 1 3 6

your issue is that the inner loop for l in range(len(lista)): start every time from 0, but instead you have to start from position i. When you finish the inner loop you increment i of 1 and everything that is before i is already sorted. If the inner loop restart from the begin like in this case you will have that 1 is less than 2 (on i = 1) you swap it again.

lista=[2,1,5,1,3,6]

for i in range(len(lista)):
   for l in range(i, len(lista)):
       if i==l:
           continue
       if lista[l]<lista[i]:
           temp=lista[i]
           lista[i]=lista[l]
           lista[l]=temp

I recommend you to read about insertion sort and selection sort to learn better this algorithm.

edited Jun 9, 2019 at 16:41

answered Jun 9, 2019 at 16:14

mini

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aiguofer · Accepted Answer · 2019-06-09 16:14:10Z

0

The problem is that the second loop goes through the entire list again. What you'd want to do is to look at the remaining elements instead:

lista = [2, 1, 5, 1, 3, 6]

for i in range(len(lista)):
    # change range to start at i
    for l in range(i, len(lista)):
        if i == l:
            continue
        if lista[l] < lista[i]:
            temp = lista[i]
            lista[i] = lista[l]
            lista[l] = temp

edit: To be more specific, think about what happens on the last iteration of the outer loop. lista[i] will be the last spot on the list, and you're swapping each time lista[l] is smaller, so in the end you have the smallest number as the last one.

edited Jun 9, 2019 at 16:14

answered Jun 9, 2019 at 16:02

aiguofer

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