2.1.2 Pseudocode and Trace Tables — Cambridge 9618

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Cambridge 9618 Pseudocode Reference

Cambridge 9618 has a very specific pseudocode syntax that must be used in Paper 2. Deviation from these conventions will lose marks. The complete syntax is covered below.

Variables and Assignment

Declaring Variables

DECLARE x : INTEGER
DECLARE name : STRING
DECLARE score : REAL
DECLARE flag : BOOLEAN
DECLARE ch : CHAR

Assignment

x ← 10
name ← "Alice"
score ← 98.5
flag ← TRUE
ch ← 'A'

Input and Output

Input

INPUT x
INPUT name

Output

OUTPUT x
OUTPUT "Hello ", name
OUTPUT "Score: ", score

Selection — IF and CASE

IF score >= 80
  THEN OUTPUT "Distinction"
  ELSE OUTPUT "Pass"
ENDIF

// CASE statement
CASE grade OF
  'A' : OUTPUT "Excellent"
  'B' : OUTPUT "Good"
  OTHERWISE OUTPUT "Try harder"
ENDCASE

Iteration — FOR, WHILE, REPEAT-UNTIL

// FOR loop (count-controlled)
FOR i ← 1 TO 10
  OUTPUT i
NEXT i

// WHILE loop (condition checked before body)
WHILE count < 10 DO
  count ← count + 1
ENDWHILE

// REPEAT-UNTIL (condition checked after body)
REPEAT
  INPUT num
UNTIL num > 0

Procedures and Functions

PROCEDURE Greet(name : STRING)
OUTPUT "Hello, ", name
ENDPROCEDURE

CALL Greet("Alice")

// Function — MUST use RETURNS keyword
FUNCTION Square(n : INTEGER) RETURNS INTEGER
RETURN n * n
ENDFUNCTION

result ← Square(5)

Trace Tables

A trace table is a manual simulation of algorithm execution. You create a column for each variable and track how values change line-by-line. Cambridge exam questions ask you to complete trace tables — marks are lost for missing rows or wrong variable values.

Key rules for trace tables:

Create a column for every variable that changes
Also add a column for any OUTPUT values
Show the value of a variable only when it changes (leave blank otherwise)
Show each iteration of a loop as a separate row
If an array is involved, track the specific element (e.g., A[2])

Worked Example — Trace this algorithm:

DECLARE x : INTEGER
DECLARE y : INTEGER
x ← 1
y ← 0
WHILE x <= 4 DO
y ← y + x
x ← x + 1
ENDWHILE
OUTPUT y

Completed trace table:

x	y	x <= 4?	OUTPUT
1	0
		TRUE
	1
2
		TRUE
	3
3
		TRUE
	6
4
		TRUE
	10
5
		FALSE
			10

The algorithm sums 1+2+3+4 = 10. The trace shows y accumulating the sum, and x incrementing until x=5 makes the WHILE condition false.

Complete Pseudocode Quick Reference

Construct	Cambridge 9618 Syntax	Notes
Assignment	`x ← 5`	Use ← not = or :=
Comparison	`x = 5`	= used for equals comparison in conditions
Not equal	`x <> 5`	Not != or ≠
AND/OR/NOT	`AND OR NOT`	Always uppercase keywords
String concat	`str1 & str2`	Ampersand for concatenation
Integer divide	`x DIV y`	Truncates to integer
Modulo	`x MOD y`	Remainder after division
For loop	`FOR i ← 1 TO 10 ... NEXT i`	NEXT not ENDFOR
For with step	`FOR i ← 10 TO 1 STEP -2`	STEP can be negative
While loop	`WHILE cond DO ... ENDWHILE`	Condition checked before body
Repeat loop	`REPEAT ... UNTIL cond`	Condition checked after body (runs at least once)
IF statement	`IF cond THEN ... ELSE ... ENDIF`	ELSE is optional
CASE statement	`CASE var OF 'A': ... ENDCASE`	OTHERWISE for default
Array	`DECLARE A : ARRAY[1:10] OF INTEGER`	1-indexed by default
2D array	`DECLARE M : ARRAY[1:3,1:3] OF REAL`	Row, Column indexing
Procedure	`PROCEDURE name(p:T) ... ENDPROCEDURE`	Called with CALL
Function	`FUNCTION f(p:T) RETURNS T ... ENDFUNCTION`	Must have RETURNS keyword

Exam tip: Cambridge mark schemes are strict — using = instead of ← for assignment, or ENDFOR instead of NEXT i, will lose the mark for that line. Trace tables must show every row — if a variable's value doesn't change in a row, leave that cell blank (don't copy the previous value).

⚠️ Common Mistakes

Using = for assignment — always use ←
Writing ENDFOR — Cambridge uses NEXT i
Confusing WHILE (pre-check) with REPEAT-UNTIL (post-check) — REPEAT always executes body at least once
Functions vs procedures: functions use RETURNS type and RETURN value; procedures don't return a value
In trace tables: filling in unchanged values in every row — only fill when a variable changes
Forgetting to declare variables with DECLARE name : TYPE

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Worksheet — 2.1.2 Pseudocode and Trace Tables

6 questions · instantly marked · Cambridge 9618 standard

Q1Write Cambridge 9618 pseudocode to declare a variable total of type REAL, assign it the value 0, then add 3.5 to it three times using a WHILE loop.[5]

✅ Mark scheme

DECLARE total : REAL [1]; total ← 0 [1]; DECLARE count : INTEGER; count ← 0; WHILE count < 3 DO [1]; total ← total + 3.5 [1]; count ← count + 1; ENDWHILE [1].

Q2State the difference between a WHILE loop and a REPEAT-UNTIL loop in Cambridge 9618 pseudocode, with respect to when the condition is checked.[2]

✅ Mark scheme

WHILE loop: condition checked before the loop body executes — the body may never execute if the condition is initially false [1]; REPEAT-UNTIL: condition checked after the body executes — the body always executes at least once [1].

Q3Write a Cambridge 9618 FUNCTION called Cube that takes an integer parameter n and returns n³.[3]

✅ Mark scheme

FUNCTION Cube(n : INTEGER) RETURNS INTEGER [1]; RETURN n * n * n [1]; ENDFUNCTION [1]. (Deduct 1 if RETURNS keyword missing or ENDFUNCTION missing.)

Q4Complete the trace table for this algorithm with inputs 7, 3:

INPUT a
INPUT b
WHILE a > b DO
  a ← a - b
ENDWHILE
OUTPUT a

[4]

✅ Mark scheme

Initial: a=7, b=3 [1]; Check: 7>3=TRUE, a←7-3=4 [1]; Check: 4>3=TRUE, a←4-3=1 [1]; Check: 1>3=FALSE, EXIT loop; OUTPUT 1 [1]. (This computes 7 MOD 3 = 1.)

Q5In Cambridge 9618 pseudocode, what is the difference between using DIV and MOD? Give an example of each using the values 17 and 5.[4]

✅ Mark scheme

DIV gives the integer quotient (whole number result) of integer division [1]; 17 DIV 5 = 3 [1]; MOD gives the remainder after integer division [1]; 17 MOD 5 = 2 [1]. (17 = 5×3 + 2, so quotient is 3, remainder is 2.)

Q6Write a Cambridge 9618 pseudocode PROCEDURE called PrintSquares that takes a parameter n : INTEGER and outputs the square of each integer from 1 to n.[4]

✅ Mark scheme

PROCEDURE PrintSquares(n : INTEGER) [1]; FOR i ← 1 TO n [1]; OUTPUT i * i [1]; NEXT i [1]; ENDPROCEDURE. (Award 3 if ENDPROCEDURE missing.)

Q7Write pseudocode for a function CountVowels(Word : STRING) RETURNS INTEGER that counts and returns the number of vowels (A, E, I, O, U — case-insensitive) in the input string. Use a FOR loop and an appropriate string function.[5]

✅ Mark scheme

FUNCTION CountVowels(Word : STRING) RETURNS INTEGER [1]; DECLARE Count : INTEGER ← 0 and DECLARE Ch : CHAR [1]; FOR loop iterating from 1 to LENGTH(Word) [1]; Ch ← UCASE(MID(Word, i, 1)) or equivalent extraction of each character [1]; IF Ch = 'A' OR Ch = 'E' OR Ch = 'I' OR Ch = 'O' OR Ch = 'U' THEN Count ← Count + 1 [1]; NEXT i / ENDFOR; RETURN Count / ENDFUNCTION [1 bonus]. Award max 5.

Q8Explain the difference between a FUNCTION and a PROCEDURE in CAIE pseudocode. State one situation where a FUNCTION is more appropriate than a PROCEDURE, and one where a PROCEDURE is more appropriate. Give a one-line pseudocode declaration for each.[5]

✅ Mark scheme

FUNCTION returns a single value to the calling code; PROCEDURE performs an action but does not return a value [1]; FUNCTION appropriate: calculating and returning a result e.g. square root, total of array — the result is used directly in an expression [1]; example: FUNCTION Square(n : INTEGER) RETURNS INTEGER [1]; PROCEDURE appropriate: performing an output operation or modifying multiple variables via BYREF — no return value needed [1]; example: PROCEDURE PrintReport(Data : ARRAY) [1]. Award max 5.

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Term	Definition

Pro Content

Cambridge 9618 Pseudocode Reference

Variables and Assignment

Declaring Variables

Assignment

Input and Output

Input

Output

Selection — IF and CASE

Iteration — FOR, WHILE, REPEAT-UNTIL

Procedures and Functions

Trace Tables

Key rules for trace tables:

Worked Example — Trace this algorithm:

Complete Pseudocode Quick Reference

Worksheet — 2.1.2 Pseudocode and Trace Tables

Mini Test — 2.1.2 Pseudocode