Bubble Sort (AQA GCSE Computer Science)

What is a bubble sort?

• A bubble sort is a simple sorting algorithm that starts at the beginning of a dataset and checks values in 'pairs' and swaps them if they are not in the correct order
• One full run of comparisons from beginning to end is called a 'pass', a bubble sort may require multiple 'passes' to sort the dataset
• The algorithm is finished when there are no more swaps to make

How do you perform a bubble sort?

Step	Instruction
1	Compare the first two values in the dataset
2	IF they are in the wrong order... Swap them
3	Compare the next two values
4	REPEAT step 2 & 3 until you reach the end of the dataset (pass 1)
5	IF you have made any swaps... REPEAT from the start (pass 2,3,4...)
6	ELSE you have not made any swaps... STOP! the list is in the correct order

Example

• Perform a bubble sort on the following dataset

5

2

4

1

6

3

Step	Instruction
1	Compare the first two values in the dataset 5 2 4 1 6 3
2	IF they are in the wrong order... Swap them 2 5 4 1 6 3
3	Compare the next two values 2 5 4 1 6 3
4	REPEAT step 2 & 3 until you reach the end of the dataset 5 & 4 SWAP! 2 4 5 1 6 3 5 & 1 SWAP! 2 4 1 5 6 3 5 & 6 NO SWAP! 2 4 1 5 6 3 6 & 3 SWAP! 2 4 1 5 3 6 End of pass 1
5	IF you have made any swaps... REPEAT from the start End of pass 2 (swaps made) 2 1 4 3 5 6 End of pass 3 (swaps made) 1 2 3 4 5 6 End of pass 4 (no swaps) 1 2 3 4 5 6
6	ELSE you have not made any swaps... STOP! the list is in the correct order

A bubble sort in python

# unsorted dataset
nums=[66, 7, 69, 50, 42, 80, 71, 321, 67, 8, 39]

# count the length of the dataset
numlength = len(nums)

# sets a flag to initiate the loop
swaps = True

while swaps == True : 
    swaps = False
    # loop through the dataset
    for y in range(numlength-1) :
        # if the first number is bigger than the second number
        if nums[y] > nums[y+1] :
            # swap the numbers using a temporary variable
            temp = nums[y]
            nums[y] = nums[y+1]
            nums[y+1] = temp
            # sets the flag to true
            swaps = True

# prints the sorted list
print (nums)