O LevelComputer ScienceCambridge (CIE)Revision Notes7. Algorithm Design & Problem-SolvingStandard Methods of a SolutionLinear Search & Bubble Sort

Linear Search & Bubble Sort (Cambridge (CIE) O Level Computer Science)

Revision Note

Robert Hampton

Expertise

Computer Science Content Creator

What is a searching algorithm?

  • Searching algorithms are precise step-by-step instructions that a computer can follow to efficiently locate specific data in massive datasets

  • A linear search starts with the first value in a dataset and checks every value one at a time until all values have been checked

  • A linear search can be performed even if the values are not in order

Step

Instruction

1

Check the first value

2

IF it is the value you are looking for

  • STOP!

3

ELSE move to the next value and check

4

REPEAT UNTIL you have checked all values and not found the value you are looking for

Exam Tip

You will not be asked to perform a linear search on a dataset in the exam, you will be expected to understand how to do it and know the advantages and disadvantages of performing it

A linear search in Python code

# Identify the dataset to search, the target value and set the initial flag
data = [5, 2, 8, 1, 9]
target = 11
found = False

# Loop through each element in the data
for index in range(0,len(data) - 1):
  
  # Check if the current element matches the target
  if data[index] == target:
    
    # If found, output message
    found = True
    print("Target found")

#If the target is not found, output a message
if found is False:
  print("Target not found")

What is a sorting algorithm?

  • Sorting algorithms are precise step-by-step instructions that a computer can follow to efficiently sort data in massive datasets

Bubble Sort

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

7-3-standard-methods-standard-methods-02

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

Exam Tip

In the exam you do not have to show every swap that takes place in a bubble sort. You can show the outcome of a bubble sort at the end of each pass. If you have the outcome of each pass correct then a bubble sort has been implemented correctly and all marks will be given!

A bubble sort in Python code

# 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)

Worked Example

A program uses a file to store a list of words.

A sample of this data is shown

Milk

Eggs

Bananas

Cheese

Potatoes

Grapes

Show the stages of a bubble sort when applied to data shown  [2]

How to answer this question

  • We need to sort the values in to alphabetical order from A-Z

  • You CAN use the first letter of each word to simplify the process

Answer

  • E, B, C, M, G, P (pass 1)

  • B, C, E, G, M, P (pass 2)

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    Robert Hampton

    Author: Robert Hampton

    Rob has over 16 years' experience teaching Computer Science and ICT at KS3 & GCSE levels. Rob has demonstrated strong leadership as Head of Department since 2012 and previously supported teacher development as a Specialist Leader of Education, empowering departments to excel in Computer Science. Beyond his tech expertise, Robert embraces the virtual world as an avid gamer, conquering digital battlefields when he's not coding.