A LevelComputer ScienceCambridge (CIE)Revision Notes21. Exam skillsKey Exam SkillsWriting Pseudocode in the Exam

Writing Pseudocode in the Exam (Cambridge (CIE) A Level Computer Science): Revision Note

Exam code: 9618

Written by: Robert Hampton

Reviewed by: Bridgette Barrett

Updated on 16 October 2026

Why Cambridge pseudocode matters

What is Cambridge pseudocode?

Cambridge pseudocode is the standard way of writing algorithms in 9618 exams
Its rules are set out in the Pseudocode Guide for Teachers, published by Cambridge for each syllabus period
Questions in Paper 2 and Paper 3 ask for answers in pseudocode, and any pseudocode printed in those papers is written in this style (Paper 3 also prints assembly language and declarative code, which follow their own rules)
Examiners state that following the recommended pseudocode lets them follow the structure and logic of an answer and credit it; answers written in a programming language are not accepted when a question asks for pseudocode
For how to work through Paper 2 itself, including using the Insert, see the Paper 2 guide

Why do Python users lose marks?

Most students practise programming in Python (or Java or Visual Basic) for Paper 4
Examiners warn that the syntax of the language you have studied is not acceptable in a pseudocode answer
The table shows the most common Python habits and the Cambridge equivalent

Python habit	Cambridge pseudocode
`x = 5`	`X ← 5`
`if x == 5:`	`IF X = 5 THEN`
`!=`	`<>`
`elif`	no `ELSEIF`: use a nested `IF` inside `ELSE`, or a `CASE` statement
`"Hi " + name`	`"Hi " & Name`
`//` and `%`	`DIV` and `MOD`
`for i in range(1, 11):`	`FOR Index ← 1 TO 10` ... `NEXT Index`
`for item in items:`	no equivalent: loop through the index with a `FOR` loop
`len(s)`, `s.upper()`	`LENGTH(S)`, `TO_UPPER(S)` (from the Insert)
`print()` and `input()`	`OUTPUT` and `INPUT`
`return` inside `def`	`RETURN` inside `FUNCTION ... RETURNS <data type>`
`# comment`	`// comment`

For changing case, use TO_UPPER and TO_LOWER from the Insert; the Pseudocode Guide lists UCASE and LCASE instead, but the Insert is what you have in the exam, so use its function names

Conventions examiners check

How do you set out pseudocode?

Keywords are in upper case: IF, WHILE, PROCEDURE
Identifiers (names of variables, constants and modules) are in mixed case, such as NumberOfPlayers
- They may only contain letters, digits and underscores, and must start with a letter
- They must never be keywords, so String, Input and Integer are not valid names
- They are not case sensitive, so Total and TOTAL are the same identifier
Indent the contents of every IF, loop and module (usually by three spaces)
Comments start with //

Declarations and assignment

Declare each variable with its data type: INTEGER, REAL, CHAR, STRING, BOOLEAN or DATE
Constants use = and can only be given a literal value
Arrays use square brackets and state the lower and upper bound
Assignment always uses the arrow ←; the = sign is only for comparison (and constants)
On Paper 3, define user-defined types with TYPE: a record with TYPE ... ENDTYPE, and an enumerated type with TYPE Season = (Spring, Summer, ...); examiners report students using ENUM or DECLARE where TYPE was needed

DECLARE Count : INTEGER
DECLARE Average : REAL
CONSTANT MaxStudents = 30
DECLARE Names : ARRAY[1:30] OF STRING
DECLARE Grid : ARRAY[1:3, 1:3] OF CHAR
Count ← 0
Names[1] ← "Ali"

String values go in double quotes ("Ali") and single characters in single quotes ('A')
To access a field of a record stored in an array, put the index on the array, then use dot notation: Batch[Index].Weight

Operators

Arithmetic: +, -, *, /, DIV (whole-number quotient) and MOD (remainder)
- / always gives a REAL result, even if both values are integers
Comparison: =, <>, <, >, <=, >=
Logic: AND, OR, NOT; each side of AND and OR must be a complete condition
Concatenation: &
Use brackets to make the order of operations clear

Selection

IF Score >= 50 THEN
   OUTPUT "Pass"
ELSE
   IF Score >= 40 THEN
      OUTPUT "Resit"
   ELSE
      OUTPUT "Fail"
   ENDIF
ENDIF

CASE OF Grade
   'A' : Points ← 10
   'B' : Points ← 8
   OTHERWISE : Points ← 0
ENDCASE

Every IF needs THEN and a matching ENDIF
CASE clauses are checked in order, and only the first one that matches runs; OTHERWISE must come last
A CASE clause can test a range, such as 1 TO 10 : ...

Iteration

FOR Index ← 1 TO 10
   Total ← Total + Marks[Index]
NEXT Index

REPEAT
   INPUT Password
UNTIL Password = "Secret"

WHILE Number > 9
   Number ← Number - 9
ENDWHILE

FOR loops include the ← and repeat the loop variable after NEXT; add STEP to count in steps other than 1 (including negative steps)
REPEAT ... UNTIL always runs at least once
WHILE ... ENDWHILE may run zero times, because the condition is tested first

Procedures and functions

PROCEDURE ShowTotal(Total : INTEGER)
   OUTPUT "The total is ", Total
ENDPROCEDURE

FUNCTION Max(Number1 : INTEGER, Number2 : INTEGER) RETURNS INTEGER
   IF Number1 > Number2 THEN
      RETURN Number1
   ELSE
      RETURN Number2
   ENDIF
ENDFUNCTION

CALL ShowTotal(25)
Biggest ← Max(Score1, Score2)

A procedure is called with CALL, as a statement on its own
A function is never called with CALL; it is used inside an expression, such as an assignment, an IF condition or an OUTPUT
When you call a module, write only its name and the values, not the full header with data types
Parameters are passed by value unless BYREF is written before them; once written, BYREF also applies to the parameters that follow it in the same header

File handling

OPENFILE "Stock.txt" FOR READ
WHILE NOT EOF("Stock.txt")
   READFILE "Stock.txt", LineOfText
   OUTPUT LineOfText
ENDWHILE
CLOSEFILE "Stock.txt"

Open a file in READ, WRITE or APPEND mode before using it; WRITE creates a new file and wipes any existing data
Use READFILE and WRITEFILE (not READ or WRITE on their own) and close every file with CLOSEFILE
The file name must be in quotes when it is written directly, and must be included in EOF() and CLOSEFILE

Mistakes that cost marks

Which pseudocode errors do examiners see most?

Cambridge examiner reports list invalid pseudocode like the examples below; each one loses marks

What students wrote	What is wrong	Correct pseudocode
`FOR X 1 TO 100`	The assignment arrow is missing	`FOR X ← 1 TO 100`
`FOR Index ← 1 TO EOF(Filename)`	`EOF` returns `TRUE` or `FALSE`, not a number of lines	`WHILE NOT EOF(Filename)`
`IF X = 3 OR 4 OR 5 THEN`	Each side of `OR` must be a full comparison	`IF X = 3 OR X = 4 OR X = 5 THEN`
`WHILE Number >= 100 AND <= 200`	The second condition has no left-hand side	`WHILE Number >= 100 AND Number <= 200`
`IF Num ← 43 THEN`	`←` assigns a value; it does not compare	`IF Num = 43 THEN`
`DECLARE String : STRING`	A keyword is used as an identifier	`DECLARE UserName : STRING`
`CALL Valid(MyString)` then `IF RESULT = TRUE THEN`	Functions are not called with `CALL`, and `RESULT` does not exist	`IF Valid(MyString) = TRUE THEN`
`Batch.Weight(Index)`	The index belongs to the array of records, in square brackets	`Batch[Index].Weight`
`OPENFILE Stock.txt FOR READ`	A file name written directly needs quotes	`OPENFILE "Stock.txt" FOR READ`
`OUTPUT "Total: " & Total` (where `Total` is an `INTEGER`)	`&` only joins strings, so an integer must be converted or separated with a comma	`OUTPUT "Total: ", Total` or `OUTPUT "Total: " & NUM_TO_STR(Total)`
`FOR WebName in Secret`	This is programming language syntax	Loop through the indexes with `FOR Index ← 1 TO ...`

To practise finding and correcting errors like these in a given algorithm, see Test Data & Finding Errors

Missing end statements

Examiners report that many marks are lost through small omissions, such as:
- a correct procedure or function header with no ENDPROCEDURE or ENDFUNCTION
- an IF with no ENDIF
- a loop with no ENDWHILE, UNTIL or NEXT

Worked Example

A global 1D array Marks stores the test marks of 30 students. The lower bound of the array is 1.

A student was asked to write pseudocode for a function Average() that returns the mean of the marks in Marks. This is their answer:

def Average():
   total = 0
   for i in range(30):
      total += Marks[i]
   print(total / 30)

Rewrite the answer as valid Cambridge pseudocode.

[5]

Answer:

FUNCTION Average() RETURNS REAL
   DECLARE Total : INTEGER
   DECLARE Index : INTEGER
   Total ← 0
   FOR Index ← 1 TO 30
      Total ← Total + Marks[Index]
   NEXT Index
   RETURN Total / 30
ENDFUNCTION

Function header with RETURNS REAL, and ENDFUNCTION [1 mark]
Declare and initialise Total using ← [1 mark]
FOR loop from 1 to 30 with NEXT Index [1 mark]
Add each element to Total using ← inside the loop [1 mark]
RETURN the total divided by 30 [1 mark]

range(30) counts from 0 to 29, so with a lower bound of 1 it would try to read Marks[0], which does not exist, and would miss Marks[30]
print outputs the value instead of returning it to the calling code

Examiner Tips and Tricks

Write the closing keyword (ENDIF, NEXT, ENDWHILE, ENDFUNCTION) straight after the opening line, then fill in the body, so you never lose a mark for a missing end statement
In a function, finish with RETURN, not OUTPUT: examiners regularly see OUTPUT used where the value should be returned, and it does not gain the mark

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