How to implement Bubble Sort in Python?

Sorting means arranging any data in an increasing or decreasing order according to some linear relationship among the elements. This article on Bubble Sort in Python will help you understand this concept in detail. 

We’ll be covering the below topics in this blog:

• What is Bubble sort?
• Steps for performing a Bubble Sort 
• Bubble Sort Algorithm 
• Python Program to implement Bubble Sort

What is Bubble Sort?

Bubble sort is also known as sinking sort. It is a simple sorting algorithm that continuously steps through the list to be sorted, comparing each pair of adjacent items and swapping them if they are not in the correct order. The steps are repeated until no more swaps are needed, which is when the list is sorted.

Steps for performing a Bubble Sort

• Compare first and second element in the list and swap if they are in the wrong order.
• Compare second and third element and swap them if they are in the wrong order.
• Proceed similarly until the last element of the list in a similar fashion.
• Keep repeating all of the above steps until the list is sorted.

The above steps will be more clear by the following visualizations –

Bubble Sort Algorithm

Now let us look at the Algorithm behind Bubble Sort. 

First Pass:

( 16,19,11,15,10 ) –> ( 16,19,11,15,10 ) – The algorithm compares first two elements and swaps since 19 > 16

( 16,19,11,15,10 ) –>  ( 16,11,19,15,10 ) – Swap since 19 > 11

( 16,11,19,15,10 ) –>  ( 16,11,15,19,10 ) – Swap since 19 > 15

( 16,11,15,19,10 ) –> ( 16,11,15,10,19 ) – Now, since these elements are already in the correct order (19 > 10), the algorithm does not swap them.

Second Pass:

( 16,11,15,10,19 ) –> ( 11,16,15,10,19)  – Swap since 16>11

( 11,16,15,10,19 ) –> ( 11,15,16,10,19 ) – Swap since 16 > 15

( 11,15,16,10,19 ) –> ( 11,15,10,16,19 ) – Swap since 16>10

( 11,15,10,16,19  ) –> ( 11,15,10,16,19  )

The array is sorted, but our algo does not know if it is completed. Hence, it needs another whole pass without any swap to know it is sorted.

Third Pass:

( 11,15,10,16,19 ) –> ( 11,15,10,16,19 )

( 11,15,10,16,19 ) –> ( 11,10,15,16,19) – Swap since 15>10

( 11,10,15,16,19 ) –> ( 11,10,15,16,19 )

( 11,10,15,16,19 ) –> ( 11,10,15,16,19 )

Fourth Pass:

( 11,10,15,16,19 ) –> ( 10,11,15,16,19) – Swap since 11>10

The final output is  ( 10,11,15,16,19)

Let us now code this up – 

Python Program to implement Bubble Sort

a = [16, 19, 11, 15, 10, 12, 14]

#repeating loop len(a)(number of elements) number of times
for j in range(len(a)):
    #initially swapped is false
    swapped = False
    i = 0
    while i<len(a)-1: #comparing the adjacent elements if a[i]>a[i+1]:
            #swapping
            a[i],a[i+1] = a[i+1],a[i]
            #Changing the value of swapped
            swapped = True
        i = i+1
    #if swapped is false then the list is sorted
    #we can stop the loop
    if swapped == False:
        break
print (a)

OUTPUT: 

In the above code, we compare the adjacent numbers and swap them if they are not in the correct order. Repeat the same process len(a) number of times. We have assigned a variable ‘swapped’ and made it ‘True’ if any two elements are swapped in an iteration. And if there is no interchanging of elements then the list is already sorted and hence, there is no change in the value of the ‘swapped’ and we can break the loop.

With this, we come to an end of the blog titled “How to implement Bubble Sort in Python”. I hope the content added value to your Python knowledge.