Use pseudocode (INPUT, WRITE, FOR, WHILE)

Algorithms and Flowcharts (Cambridge IGCSE/A‑Level IT 9626 – Topic 4)

Learning Objectives (Cambridge Assessment Objectives)

1. AO1 – Knowledge & Understanding: recognise and use the standard pseudocode constructs required by the syllabus.
2. AO2 – Application: write correct algorithms and flowcharts, edit given ones and translate between the two representations.
3. AO3 – Analysis: evaluate the suitability of different control structures and identify errors in algorithms/flowcharts.

Key Concepts

• What an algorithm is and the characteristics of a good algorithm.
• Cambridge flow‑chart symbols and the conventions used in examinations.
• Pseudocode conventions – case‑insensitive, indentation for readability, use of ← for assignment.
• Control structures: sequential processing, looping, decision‑making, and sub‑routines.
• Editing and error‑checking techniques for algorithms and flowcharts.

Full Syllabus Coverage (Topic 4)

Syllabus Item	Present in Notes?	Action Required
INPUT / READ	Yes	–
WRITE / PRINT	Yes	–
FOR … NEXT	Yes	–
FOR … NEXT … STEP	No	Added in “Looping Constructs”.
IF … ELSE … ENDIF	No	Added in “Decision‑Making”.
WHILE … END WHILE	Yes	–
REPEAT … UNTIL	No	Added in “Looping Constructs”.
CASE … ENDCASE	No	Added in “Decision‑Making”.
Comparison & arithmetic operators	Implicit	Explicit table added.
Sub‑routines / procedures	No	New “Sub‑routines” section introduced.
Editing a given algorithm / flowchart	No	Spot‑the‑error exercises added.
Nested decisions	No	Example with grading added.

Pseudocode Syntax (Cambridge Standard)

Statement	Purpose	Example
INPUT / READ	Read data from the user or a file	INPUT n
WRITE / PRINT	Display data on the screen	WRITE "Result =", result
FOR i = start TO end	Loop a known number of times (increment = 1)	FOR i = 1 TO n
FOR i = start TO end STEP step	Loop with a user‑defined increment or decrement	FOR i = 10 TO 0 STEP -2
END FOR	Terminate a FOR loop	END FOR
WHILE condition	Repeat while the condition is true (test before first iteration)	WHILE total < limit
END WHILE	Terminate a WHILE loop	END WHILE
REPEAT	Start a loop that executes at least once	REPEAT
UNTIL condition	Terminate a REPEAT loop when the condition becomes true (test after each iteration)	UNTIL password = "Cambridge"
IF condition THEN	Single‑branch decision	IF score ≥ 50 THEN
ELSE	Alternative branch for an IF	ELSE
ENDIF	Terminate an IF … THEN … ELSE … ENDIF block	ENDIF
CASE expression OF	Multi‑way selection	CASE choice OF
WHEN value:	Branch of a CASE	WHEN 1: …
WHEN OTHER:	Default branch of a CASE	WHEN OTHER: …
ENDCASE	Terminate a CASE block	ENDCASE
CALL subroutine (parameters)	Invoke a sub‑routine	CALL Factorial (n, result)
PROCEDURE subroutine (parameters)	Define a sub‑routine	PROCEDURE Factorial (n, result)
END PROCEDURE	Terminate a sub‑routine definition	END PROCEDURE

Comparison & Arithmetic Operators (Cambridge)

Operator	Meaning
=	Equal to
≠ or #	Not equal to
<	Less than
>	Greater than
≤	Less than or equal to
≥	Greater than or equal to
+	Addition
-	Subtraction
*	Multiplication
/	Division
MOD	Remainder after integer division

Flowchart Symbols (Cambridge Standard)

Symbol	Name	Represents
🟠 (Oval)	Terminator	Start or End of the process
▭ (Parallelogram)	Input/Output	INPUT or WRITE
▭ (Rectangle)	Process	Assignment, calculation or sub‑routine call
◇ (Diamond)	Decision	Any conditional test – IF, WHILE, REPEAT…UNTIL, CASE
⇆ (Connector)	Arrow	Shows flow direction
⧉ (Circle)	Sub‑routine	Call and return points for procedures

Algorithm Constructs

1. Input / Output

• Use INPUT (or READ) to obtain data.
• Use WRITE (or PRINT) to display results.

2. Looping Constructs

• FOR … NEXT: when the number of repetitions is known and the step is 1.
• FOR … NEXT … STEP: same as above but with a custom increment or decrement.
• WHILE … END WHILE: test before the first iteration; loop may execute zero times.
• REPEAT … UNTIL: test after the first iteration; loop executes at least once.

3. Decision‑Making

• IF … THEN … ENDIF: single‑branch decision.
• IF … THEN … ELSE … ENDIF: two‑branch decision.
• Nested IF … THEN …: decisions inside decisions.
• CASE … OF … ENDCASE: multi‑way selection; useful for menus.

4. Sub‑routines / Procedures

• Define a reusable block with PROCEDURE … END PROCEDURE.
• Invoke it with CALL.
• In flowcharts a circle (or double‑lined rectangle) denotes a sub‑routine.

5. Editing & Spot‑the‑Error Techniques (AO3)

• Check that every loop has a matching start and end statement.
• Verify that every IF has a corresponding ENDIF (and an ELSE if required).
• Confirm that variable names are used consistently and that assignment uses ←.
• Look for missing or extra arrows in flowcharts, especially at decision diamonds.

6. Translating Between Flowcharts and Pseudocode – Checklist (AO2)

1. Identify each symbol and write the matching pseudocode statement.
2. Follow the arrows to preserve the exact order of execution.
3. For every decision diamond:
   • If the flow returns to the same point → use WHILE or REPEAT…UNTIL.
   • If the flow diverges once → use IF … THEN … ELSE … ENDIF or CASE.
4. Mark the start and end of every loop with FOR … END FOR, WHILE … END WHILE or REPEAT … UNTIL.
5. Ensure every input and output symbol is represented by INPUT or WRITE.
6. When a sub‑routine symbol appears, write a CALL statement and verify that a matching PROCEDURE … END PROCEDURE exists.

Worked Examples

Example 1 – Sum of the First n Natural Numbers (FOR loop)

INPUT n
total ← 0
FOR i = 1 TO n
    total ← total + i
END FOR
WRITE "Sum =", total

Example 2 – Sum of Even Numbers up to n (FOR … STEP)

INPUT n
total ← 0
FOR i = 2 TO n STEP 2
    total ← total + i
END FOR
WRITE "Even sum =", total

Example 3 – Greatest Common Divisor (WHILE loop)

INPUT a
INPUT b
WHILE b ≠ 0
    temp ← b
    b ← a MOD b
    a ← temp
END WHILE
WRITE "GCD =", a

Example 4 – Password Check (REPEAT … UNTIL)

REPEAT
    INPUT password
UNTIL password = "Cambridge"
WRITE "Access granted"

Example 5 – Simple Menu (CASE … ENDCASE)

WRITE "1 – Add   2 – Subtract   3 – Exit"
INPUT choice
CASE choice OF
    WHEN 1:
        INPUT a, b
        result ← a + b
        WRITE "Result =", result
    WHEN 2:
        INPUT a, b
        result ← a - b
        WRITE "Result =", result
    WHEN 3:
        WRITE "Good‑bye"
    WHEN OTHER:
        WRITE "Invalid choice"
ENDCASE

Example 6 – Grading System (Nested IF … THEN …)

INPUT score
IF score ≥ 80 THEN
    grade ← "A"
ELSE
    IF score ≥ 70 THEN
        grade ← "B"
    ELSE
        IF score ≥ 60 THEN
            grade ← "C"
        ELSE
            grade ← "F"
        ENDIF
    ENDIF
ENDIF
WRITE "Grade =", grade

Example 7 – Factorial Using a Sub‑routine (PROCEDURE & CALL)

PROCEDURE Factorial (n, result)
    result ← 1
    FOR i = 2 TO n
        result ← result * i
    END FOR
END PROCEDURE

INPUT n
CALL Factorial (n, fact)
WRITE "Factorial of", n, "is", fact

Spot‑the‑Error Exercises (AO3)

Algorithm A – Incorrect Loop Bounds

INPUT n
total ← 0
FOR i = 1 TO n‑1
    total ← total + i
END FOR
WRITE total

Error: Loop stops at n‑1; the sum of the first n numbers is incomplete.

Correction: Change FOR i = 1 TO n‑1 to FOR i = 1 TO n.

Algorithm B – Mis‑used REPEAT

REPEAT
    INPUT answer
    WRITE "Try again"
UNTIL answer = correct
WRITE "Correct!"

Error: The message “Try again” is printed even when the first answer is correct.

Fix: Move the WRITE statement after the test, or replace the structure with a WHILE loop:

INPUT answer
WHILE answer ≠ correct
    WRITE "Try again"
    INPUT answer
END WHILE
WRITE "Correct!"

Flowchart C – Missing Decision Exit

Correction: Add a second arrow labelled “No” that leads to the WRITE symbol (or the terminator).

Algorithm D – Unmatched ENDIF

INPUT x
IF x > 0 THEN
    WRITE "Positive"
ELSE
    IF x = 0 THEN
        WRITE "Zero"
END IF

Error: Two IF statements but only one ENDIF. The inner IF is not closed.

Fix: Add an additional ENDIF after the “Zero” branch.

Practice Questions

1. Write pseudocode to read a list of 10 integers and display the largest value using a FOR loop.
2. Convert the following flowchart (provided by the exam board) into correct Cambridge‑style pseudocode. Include all required END statements.
3. Design a menu‑driven program that allows the user to:
   • Calculate the area of a rectangle,
   • Calculate the area of a circle,
   • Exit.
   Use CASE … ENDCASE for the menu and appropriate loops to allow repeated use until the user chooses “Exit”.
4. Write a sub‑routine called IsPrime that takes an integer n and returns TRUE if n is a prime number, otherwise FALSE. Show how you would call this procedure from a main algorithm.
5. Given the algorithm below, identify and correct all errors (both logical and syntactic).
   

   INPUT n
   i ← 2
   WHILE i ≤ n
       IF n MOD i = 0 THEN
           WRITE n, "is not prime"
       ENDIF
       i ← i + 1
   END WHILE
   WRITE n, "is prime"
   

Summary Checklist for the Exam

• Know the exact spelling and order of every Cambridge pseudocode keyword.
• Remember that FOR loops always require END FOR; WHILE requires END WHILE; REPEAT pairs with UNTIL.
• All IF structures terminate with ENDIF; include ELSE only when a second branch is required.
• When translating a decision diamond that loops back, use WHILE or REPEAT…UNTIL; when it splits, use IF…ELSE or CASE.
• Check indentation – it makes nested structures clear and helps avoid missing END statements.
• In flowcharts, every diamond must have at least two outgoing arrows (Yes/No or multiple cases).