Writing & Amending Algorithms - Cambridge IGCSE Computer Science 0478

Learning Objectives

analyse a problem statement and identify inputs, processing and outputs
break a problem into clear algorithmic steps
create algorithms using Cambridge IGCSE pseudocode
represent algorithms using flowcharts
amend an existing algorithm when a requirement changes
select sequence, selection and iteration appropriately
trace an existing solution before changing it
test newly written and amended algorithms with suitable data
check that a change has not broken required existing behaviour
use clear identifiers and readable algorithm structure

Key Terms

Term	Simple definition
Algorithm	A sequence of steps used to solve a problem.
Requirement	What the problem says the solution must do.
Input	Data supplied to the algorithm.
Processing	Calculations or decisions performed on data.
Output	Information produced by the algorithm.
Pseudocode	A structured text description of an algorithm.
Flowchart	A visual description of an algorithm using symbols and arrows.
Amend	Change an existing algorithm to meet a new or changed requirement.
Selection	Choosing between different paths using a condition.
Iteration	Repeating a statement or group of statements.
Dry-run	Manually following an algorithm using test data.
Logic error	The solution behaves incorrectly because the logic is wrong.

1. Understand the Problem Before Writing

Before writing an algorithm, turn the question into a small set of precise requirements.

Ask	Example: student result system
What is the input?	Student mark
What is the processing?	Compare the mark with the pass threshold
What is the output?	Pass or Fail

Identify the data entering the solution.
Identify calculations, comparisons and decisions.
Identify exactly what must be displayed or returned.
Look for words such as IF, OTHERWISE, FOR, EACH, REPEAT and UNTIL.
Break complex requirements into smaller steps before coding.

Real-life example:A school attendance program takes attendance data, processes it and displays a result. That same input → processing → output thinking helps when writing an algorithm.

Activity 1A: Identify IPO

Classroom / homework activity

A program accepts a mark and displays Pass when the mark is 50 or above.

Part	Answer
Input	Mark
Process	Compare mark with 50
Output	Pass or Fail

Activity 1B: Convert the requirement

Classroom / homework activity

Requirement: enter two numbers, add them and display the result.

Input the first number.
Input the second number.
Add the numbers.
Output the result.

Check Your Understanding

1. Why identify input, processing and output first?

It clarifies what data is available.
It identifies what the algorithm must do.
It identifies the required result.
It reduces the chance of missing a requirement.

2. Which words can suggest selection?

If can indicate a decision.
Otherwise can indicate an alternative path.
A condition normally controls which branch is followed.
In pseudocode this is commonly represented by IF.

3. Which words can suggest iteration?

For can indicate a known number of repetitions.
Each can indicate repeated processing.
Repeat and until can indicate condition-controlled repetition.
Recognising these words helps select a loop structure.

4. Why break a large problem into smaller steps?

Smaller steps are easier to understand.
Missing actions are easier to identify.
Each step can be checked.
The steps can then be represented in pseudocode or a flowchart.

5. Why should the output be clear before writing the algorithm?

It defines what the solution must produce.
It helps identify the required processing.
It helps identify the final output statement.
It provides something concrete to test.

2. Create an Algorithm from a Requirement

Creating an algorithm means converting the requirements into a logical sequence of instructions.

Stage	What to decide
Inputs	What values are needed?
Variables	What values need to be stored?
Processing	What calculations or checks are required?
Control flow	Is sequence, selection or iteration needed?
Output	What result must be produced?

Cambridge IGCSE pseudocode

DECLARE Mark1 : REAL
DECLARE Mark2 : REAL
DECLARE Average : REAL

INPUT Mark1
INPUT Mark2

Average ← (Mark1 + Mark2) / 2

OUTPUT "Average = ", Average

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Activity 2A: Write a rectangle algorithm

Classroom / homework activity

Input Width and Height, calculate Area and output Area.

Model answer

DECLARE Width : REAL
DECLARE Height : REAL
DECLARE Area : REAL

INPUT Width
INPUT Height
Area ← Width * Height
OUTPUT Area

Activity 2B: Add selection

Classroom / homework activity

Amend the rectangle solution so Area greater than 100 outputs Large; otherwise output Small.

Model amendment

IF Area > 100
  THEN
    OUTPUT "Large"
  ELSE
    OUTPUT "Small"
ENDIF

Check Your Understanding

6. What should be decided before detailed pseudocode is written?

Identify the required input.
Identify the processing.
Identify the output.
Choose suitable control structures.

7. Why use meaningful identifiers?

They describe the stored value.
They make the algorithm easier to read.
They reduce ambiguity.
They help another programmer understand the solution.

8. What is the Cambridge assignment operator?

It is ←.
It assigns a value to a variable or data item.
It can assign the result of an expression.
For example, Total ← Total + Number.

9. Why is indentation useful in pseudocode?

It shows which statements are contained inside another statement.
It makes IF structures clearer.
It makes loops clearer.
It reduces ambiguity in nested structures.

10. What is the benefit of writing plain-language steps first?

The logic can be checked before syntax.
Missing steps are easier to find.
The plan can be translated into pseudocode.
The same logic can be represented by a flowchart.

3. Amend an Existing Algorithm

When a requirement changes, the existing algorithm may need to be amended rather than completely rewritten.

Read the complete existing algorithm first.
State exactly what has changed.
Locate the affected part.
Keep correct existing logic whenever possible.
Make the smallest suitable change.
Trace and test the entire amended solution.

Original

INPUT Mark
IF Mark >= 50
  THEN
    OUTPUT "Pass"
  ELSE
    OUTPUT "Fail"
ENDIF

Amended

INPUT Mark
IF Mark >= 50
  THEN
    IF Mark >= 80
      THEN
        OUTPUT "Distinction"
      ELSE
        OUTPUT "Pass"
    ENDIF
  ELSE
    OUTPUT "Fail"
ENDIF

Real-life example:A school grading program already working for Pass/Fail may be amended to add a Distinction band. You do not need to redesign the entire grading system; add the new decision where it belongs.

The completed flowchart shows the original Pass/Fail decision and the new Distinction decision added only to the Pass branch.

Interactive amendment checker

Activity 3A: Change the pass mark

Classroom / homework activity

Change the pass threshold from 50 to 60.

Model amendment

INPUT Mark
IF Mark >= 60
  THEN
    OUTPUT "Pass"
  ELSE
    OUTPUT "Fail"
ENDIF

Activity 3B: Add another grade

Classroom / homework activity

Add Excellent when the mark is 90 or above.

Model amendment

INPUT Mark
IF Mark >= 60
  THEN
    IF Mark >= 90
      THEN
        OUTPUT "Excellent"
      ELSE
        OUTPUT "Pass"
    ENDIF
  ELSE
    OUTPUT "Fail"
ENDIF

Check Your Understanding

11. Why read the whole algorithm before changing it?

You need to understand the existing logic.
The change may depend on earlier steps.
You might remove correct logic accidentally.
You need to know which existing cases should continue working.

12. Why make the smallest necessary change?

It reduces the risk of breaking working logic.
The amendment is easier to understand.
Testing becomes easier.
The solution remains closer to the original.

13. What does the amended grading algorithm add?

It keeps the original Pass/Fail decision.
It adds a second condition.
The second condition is inside the Pass branch.
Marks of 80 or more produce Distinction.

14. Why test the complete solution after an amendment?

A change may introduce a new logic error.
Old cases may still be required.
New cases need to demonstrate the changed requirement.
Testing the whole solution checks for unintended effects.

15. What is an unintended side effect?

A change causes another part of the solution to stop working correctly.
The new requirement may work while an old case fails.
The output may become incorrect for previous inputs.
Careful testing can reveal it.

4. Flowcharts — Create and Amend Visually

The flowchart below is the visual version of the same Pass/Fail decision.

Symbol	Meaning
Terminator	START / END
Rectangle	Process / calculation
Parallelogram	INPUT / OUTPUT
Diamond	Decision / condition
Arrow	Direction of flow

START

↓

INPUT Mark

↓

Mark ≥ 50?

YES
OUTPUT "Pass"

NO
OUTPUT "Fail"

Activity 4A: Plan the flowchart

Classroom / homework activity

For a program that validates a mark from 0 to 100, identify the major flowchart stages.

Stage	Action
1	START
2	INPUT Mark
3	Decision: valid?
4	Output/process the selected path
5	END or repeat if required

Proper Flowchart — Pass / Fail

A decision diamond creates the two paths. Both paths eventually rejoin at END.

Activity 4B: Add a new branch

Classroom / homework activity

A Pass/Fail flowchart now needs Distinction at 80 or above.

Add a second decision on the existing Pass branch:Mark ≥ 80?. This is the flowchart equivalent of the nested IF.

Check Your Understanding

16. What does a diamond represent?

It represents a decision condition.
It creates different branches.
Each branch represents an outcome.
The selected branch determines the next step.

17. Why must arrows be clear?

They show execution order.
They connect the stages.
They show where each branch goes.
They make the control flow understandable.

18. How can a flowchart be amended safely?

Identify the changed requirement.
Find the affected symbol or branch.
Add or replace only the necessary part.
Check all arrows after the change.

19. How is a flowchart decision related to IF?

Both represent a condition.
Both can produce different paths.
A diamond is the visual form.
IF is a pseudocode form.

20. Why can a flowchart help before coding?

It gives a visual overview.
Branches and loops are easy to see.
Missing paths may be detected early.
The logic can then be translated into pseudocode.

5. Choose the Correct Control Structure

Structure	Use it for	Example
Sequence	Steps that always happen in order	INPUT → calculate → OUTPUT
Selection	A decision changes the path	IF Mark >= 50 THEN ...
Iteration	The same work must repeat	FOR Counter ← 1 TO 10

An algorithm can contain all three structures. When amending it, identify which structure the new requirement actually changes.

Activity 5A: Choose a structure

Classroom / homework activity

Choose the best structure for: calculate once; choose Pass/Fail; process ten scores.

Task	Structure
Calculate once	Sequence
Pass/Fail	Selection
Process ten scores	Iteration

Activity 5B: Change the repetition

Classroom / homework activity

A program processes 5 scores. The requirement changes to 10 scores.

Change the FOR loop boundary from1 TO 5to1 TO 10, provided the rest of the algorithm remains correct.

Check Your Understanding

21. When is sequence appropriate?

All actions happen in order.
No decision changes the path.
No repeated block is required.
It is the simplest control structure.

22. When is selection appropriate?

A condition changes what happens next.
Different cases can have different actions.
IF is commonly used in pseudocode.
A flowchart diamond can represent it.

23. When is iteration appropriate?

The same work repeats.
FOR suits a known number of repetitions.
WHILE or REPEAT can suit condition-controlled repetition.
The loop needs a suitable stopping rule.

24. Can one algorithm contain all three?

Yes.
Sequence can contain selection.
Loops can contain selection.
The structures can be combined to solve larger problems.

25. How do you know what part to amend?

Compare the old and new requirements.
Identify the affected control structure.
Change the necessary part.
Test the whole solution afterwards.

6. Test the New or Amended Algorithm

Test	Purpose	Example
Normal	Check an ordinary valid case	65 for Pass at 50
Boundary	Check the exact limit and nearby values	49, 50, 51
Abnormal	Check invalid/out-of-range input	-5 or 105 for marks 0–100
Changed requirement	Check new behaviour	79, 80, 81 for Distinction at 80

Worked test set

Mark	Expected output
45	Fail
50	Pass
79	Pass
80	Distinction
95	Distinction

Exam tip

When a rule uses a boundary such as 50 or 80, test the boundary itself and values immediately below and above it. This helps detect mistakes such as > instead of >=.

Activity 6A: Select boundary tests

Classroom / homework activity

For Pass if Mark >= 50, choose three useful tests.

Input	Reason
49	Below boundary
50	At boundary
51	Above boundary

Activity 6B: Test the new Distinction rule

Classroom / homework activity

For Distinction at 80 or above, choose three boundary tests.

Input	Expected
79	Pass
80	Distinction
81	Distinction

Check Your Understanding

26. Why test a boundary value?

It is where a decision can change branch.
It checks the comparison operator.
It confirms the threshold is handled correctly.
Nearby values add further evidence.

27. What is a normal test?

An ordinary valid input.
It checks expected everyday behaviour.
It normally follows the main path.
It helps confirm the solution works normally.

28. What is an abnormal test?

An invalid or out-of-range input.
It checks validation or rejection.
It can reveal missing conditions.
The chosen value should match the limits in the requirement.

29. Why test existing cases after an amendment?

Old behaviour may still be required.
The amendment could break a previously correct case.
New cases must show the changed requirement works.
Testing both groups checks the complete solution.

30. How can tracing help after an amendment?

It records changing variable values.
It shows which branches are followed.
It can reveal where new logic becomes incorrect.
It supports checking the final output.

7. Exam-Style Writing & Amending Practice

Worked Example — Create then amend

Original:input five scores and output the total.

CIE pseudocode

DECLARE Total : INTEGER
DECLARE Score : INTEGER

Total ← 0

FOR Counter ← 1 TO 5
    INPUT Score
    Total ← Total + Score
NEXT Counter

OUTPUT Total

Changed requirement:also output "High" when Total is 240 or more; otherwise "Standard".

CIE pseudocode amendment

DECLARE Total : INTEGER
DECLARE Score : INTEGER

Total ← 0

FOR Counter ← 1 TO 5
    INPUT Score
    Total ← Total + Score
NEXT Counter

OUTPUT Total

IF Total >= 240
  THEN
    OUTPUT "High"
  ELSE
    OUTPUT "Standard"
ENDIF

Flowchart Version — Total of Five Scores

The loop in the pseudocode becomes a repeated flowchart path back to INPUT Score.

Activity 7A: 5-mark algorithm

Classroom / homework activity

Write an algorithm to input five numbers and output the total.

Model answer

DECLARE Total : INTEGER
DECLARE Number : INTEGER
Total ← 0

FOR Counter ← 1 TO 5
    INPUT Number
    Total ← Total + Number
NEXT Counter

OUTPUT Total

Activity 7B: 4-mark amendment

Classroom / homework activity

Add an output for Total >= 100.

Keep the existing total calculation.
Add a condition testing Total >= 100.
Output the required message when TRUE.
Provide the specified alternative behaviour when FALSE.

Check Your Understanding

31. What clue suggests selection is needed?

A condition changes the action.
Words such as if or otherwise are clues.
Different cases require different behaviour.
Use an IF or a flowchart decision.

32. What clue suggests iteration is needed?

Work is repeated.
Words such as each or every are common clues.
A FOR loop can handle a known repetition count.
A condition-controlled loop needs a stopping condition.

33. Why preserve correct logic when amending?

It already satisfies part of the requirement.
Unnecessary changes increase the risk of errors.
The amendment is easier to review.
Testing becomes more focused.

34. Why should pseudocode and flowchart versions agree?

They represent the same underlying algorithm.
The processing should remain equivalent.
The same conditions should produce the same branches.
Only the representation changes.

Key Takeaways

Understand the requirement before writing or changing the algorithm.
Identify input, processing and output.
Break complex requirements into clear steps.
Use sequence, selection and iteration to match the task.
Use Cambridge IGCSE pseudocode conventions for pseudocode answers.
Read the existing solution before amending it.
Make the smallest necessary change and preserve correct logic.
Use flowchart symbols and arrows to show the same underlying logic.
Test normal, boundary and abnormal cases where appropriate.
Test both the new behaviour and important existing behaviour.
Use tracing/dry-runs to check changing values and decisions.

Question Bank

1. Describe a systematic method for creating an algorithm from a problem specification. [5 marks]

Identify the required inputs.
Identify the processing or calculations.
Identify the required outputs.
Choose suitable sequence, selection and/or iteration.
Write the solution and test it.

2. Explain how to amend an existing algorithm safely. [4 marks]

Read the original algorithm.
Identify the changed requirement.
Change the necessary part while keeping correct logic.
Test the complete amended solution.

3. Explain how pseudocode and flowcharts can represent the same algorithm. [5 marks]

Pseudocode describes the logic with structured text.
A flowchart describes the logic with symbols and arrows.
Both can represent sequence, selection and iteration.
The control flow should be equivalent.
The representation changes, but the underlying logic remains the same.

4. Explain how to test an amended algorithm. [5 marks]

Use normal valid data.
Test boundary values.
Test abnormal or invalid data when appropriate.
Check expected and actual outputs.
Retest important existing cases as well as new cases.

5. Explain sequence, selection and iteration. [4 marks]

Sequence executes steps in order.
Selection chooses a path based on a condition.
Iteration repeats a statement or block.
Choose the structure that matches the requirement.

6. A Pass/Fail solution gains a Distinction rule at 80 or above. Explain how you would amend it. [5 marks]

Keep the existing input.
Keep the outer pass condition.
Add a second condition for 80 or above.
Output Distinction when the second condition is TRUE and Pass otherwise.
Test values below 50, 50–79 and 80 or above.

7. Why are meaningful identifiers and indentation important in Cambridge pseudocode? [4 marks]

Meaningful identifiers make data and purpose clear.
Cambridge uses mixed/Pascal case identifiers.
Indentation shows contained statements.
Clear structure makes the algorithm easier to follow.

8. Describe how to convert a written requirement into a flowchart. [5 marks]

Identify START and END.
Add input and output symbols.
Add process steps for actions/calculations.
Add decision symbols and connect each branch.
Check that the arrows give a complete control flow.