Bubble sort

Hi there. I am a bit confused about this problem. I have looked through the other posts on the forum about it but I am still not getting it.

My function below does sort the input array as needed, but I don't understand what we need to caclulate for passes. At first I thought it was the number of times an pair is not switched. Then I realized it was the number of times all elements in asubarray was not switched (I think), and I tried to apply this to my code, but I can't seem to get it. I don't understand why my below code is not giving the correct value for passed when the example input data is used ([3, 1, 4, 1, 5, 9, 2, 6]).

def bub_sort(arr):

"""Takes an array of numbers and sorts them in non-decreasing order,
    return the number of passes (When nothing in the current subarray is swapped)
    and the number of swaps made in total"""

passes = 0
total_swaps = 0

# Use for loop to run while loop for each wanted value of k
for k in range(1,len(arr)):
    j = 0
    current_array_swaps = 0
    # Checks every pair (including previous pairs after a switch) in each sublist of arr 
    while j < k:

        current_left = arr[k-j-1]
        current_right = arr[k-j]

            # Second element larger

        if current_right >= current_left:
            j+=1

        else:
             # Switch elements

            arr[k-j] = current_left
            arr[k-j-1] = current_right
            total_swaps += 1
            current_array_swaps +=1
            j+=1

# If no swaps when go through all of current subarray, we add to pass

        if current_array_swaps == 0:
            passes += 1

print(arr,passes,total_swaps)

Thank you, the step by step example was very useful. I have the code working perfectly now.

Would I be able to get some more help?

def bub_sort(arr):

    not_swapped = 0
    swapped = 0
    passes = 0
    while True:
        for index,num in enumerate(arr[:-1]):

            if num > arr[index+1]: 
                arr[index],arr[index+1] = arr[index+1],arr[index]
                swapped +=1
            else:
                not_swapped += 1

        passes += 1

        # Seeing if no swaps in entrie array 
        if not_swapped == len(arr)-1:
            break
        else:
            not_swapped = 0


    return(arr,passes,swapped)

So I made the above code. It was correct for the example input data [3, 1, 4, 1, 5, 9, 2, 6]. I then tried the problem data [20, 16, 17, 4, 12, 1 ,5 ,19, 7, 10, 13, 6, 9, 11 ,15, 14, 3 ,2, 21, 8, 18] and my passes was one off, I had 18, the answer was 17. I couldn't see anything wrong with my code, so I passed throuhg another of the problem data for more numbers [6 ,18, 10 ,16 ,3 ,19 ,5 ,21, 7, 4 ,2 ,8 ,9, 12, 17 ,20, 13 ,11, 1 ,14, 15]. This was correct, and I can't seem to figure out why. It seems my passes is correct sometimes but not all the time.

I don't think I am changing arr when ittertating over it. In python I thought that taking a sliced version of list creates a new list in itself. So my intention was to itterate over the new list arr[:-1], which goes up to the last element of arr. Then apply bubble sort if needs be.

Do you thing I should spend more time trying to figure out why my code is not working for [20, 16, 17, 4, 12, 1 ,5 ,19, 7, 10, 13, 6, 9, 11 ,15, 14, 3 ,2, 21, 8, 18], or just contitnue with the other problems?

I got it working. And I think I understand the issue now. I thought that the subarry arr[:-1] would also update as I change arr. But it is a completly different list in memory, so I was checking the wrong number at times, and since the program would keep running until I got the correct array order I would have a different value for passes sometimes.

Thank you for the help :)