O Level Computer Science: Programming — Revision Notes

Topic 8 of Cambridge O Level Computer Science (2210) is tested on Paper 2 by asking you to read, complete and write pseudocode. Most lost marks come from a few small decisions: the wrong data type, the wrong loop, a misread SUBSTRING, a total reset in the wrong place or a file opened in the wrong mode. These notes are arranged as decision guides and quick drills so you can make each choice correctly under time pressure.

Condensed for the final weeks. For full explanations built around one example program, use the Programming study guide; for exam questions, use the Programming practice questions.

Syllabus coverage (2210, 2026–2028)

Ref	What you must be able to do
8.1.1–8.1.3	Variables and constants; integer, real, char, string, Boolean; input and output
8.1.4	Sequence; IF and CASE; count-controlled, pre-condition and post-condition loops; totalling and counting; string handling (length, substring, upper, lower); arithmetic, relational and logical operators
8.1.5	Nested selection and iteration (up to three levels)
8.1.6	Procedures, functions, parameters (up to three); local and global variables
8.1.7	Library routines MOD, DIV, ROUND, RANDOM
8.1.8	Maintainable programs: meaningful identifiers, comments, subroutines
8.2	1D and 2D arrays; variables as indexes; reading and writing with (nested) iteration
8.3	Purpose of files; open, close, read and write single items and lines of text

Decision guide: which data type?

Ask yourself	Choose
Only TRUE/FALSE or yes/no?	BOOLEAN
Exactly one character?	CHAR
Text, or digits never used in arithmetic (phone number, code with leading zero)?	STRING
A count or whole number used in calculations?	INTEGER
Money, measurements or anything with decimals?	REAL

Decision guide: which loop?

Situation	Loop
You know how many times (30 students, 12 months)	FOR ... TO ... NEXT
It might not need to run at all	WHILE ... DO ... ENDWHILE
It must run at least once (menu, input then check)	REPEAT ... UNTIL

Decision guide: IF or CASE?

Use CASE OF when one variable is compared with several fixed values; put OTHERWISE last. Use IF for ranges or conditions combining several variables.

Drill: string functions

Code ← "MB-2210-OL" (10 characters)

Expression	Value	Why
LENGTH(Code)	10	Count every character, including hyphens
SUBSTRING(Code, 4, 4)	“2210”	Start at position 4, take 4 characters
SUBSTRING(Code, 9, 2)	“OL”	Positions 9 and 10
LCASE(SUBSTRING(Code, 1, 2))	“mb”	Innermost function first

Drill: library routines

Expression	Value
DIV(250, 60)	4
MOD(250, 60)	10
ROUND(3.14159, 2)	3.14
ROUND(Total / Count, 1) with Total = 47, Count = 6	7.8
MOD(Number, 2) = 0	TRUE when Number is even

Order matters: ROUND(Total / Count, 1) divides first, then rounds; ROUND(Total, 1) / Count does not round the answer.

Subroutines in four lines

• Procedure: performs a task; run with CALL Name(arguments).
• Function: returns a value with RETURN; header ends RETURNS <type>; used inside an expression.
• Parameter: a value passed in, declared with its data type, e.g. (Width : REAL).
• Local variable: declared inside a subroutine, exists only there. Global: available to the whole program.

Arrays at a glance

DECLARE Rainfall : ARRAY[1:12] OF REAL
DECLARE Grid : ARRAY[1:5, 1:5] OF CHAR

• Use the loop counter as the index: Rainfall[Month], not Rainfall[1], inside a loop.
• 2D arrays need nested loops — outer for rows, inner for columns.
• The first index can be 0 or 1; check the declaration in the question.

File commands at a glance

Command	Use
OPENFILE "Log.txt" FOR READ	Open to read existing data
OPENFILE "Log.txt" FOR WRITE	Create a new file (existing contents are lost)
READFILE "Log.txt", LineOfText	Read one item or line into a variable
WRITEFILE "Log.txt", LineOfText	Write one item or line
CLOSEFILE "Log.txt"	Close the file when finished

Purpose of files: data is kept after the program ends and can be reused or shared.

Maintainability checklist

Meaningful identifiers · comments that explain sections · procedures and functions to avoid repeated code · constants for fixed values.

2210 and its sibling syllabus

Topic 8 in 2210 has the same content and the same pseudocode guide as Cambridge IGCSE Computer Science (0478).

Exam traps

Drawn from our practice page, which paraphrases the Cambridge 2210 June 2025 mark schemes and lists where marks are usually lost:

• Writing ROUND(Total, 2) / 6 so rounding happens before division, or leaving out the number of decimal places.
• Reading the third SUBSTRING parameter as an end position rather than a length, or counting from 0 when the question uses 1.
• Using a fixed index such as Scores[1] inside a loop instead of the loop counter.
• Starting a “highest” variable at a large value (or “lowest” at a small one), so it is never replaced.
• Putting the final OUTPUT inside the loop, so it prints on every pass.

Frequently asked questions

How many parameters can a 2210 procedure or function have? The syllabus says procedures and functions may have up to three parameters in the exam.

Is RANDOM() a whole number? No. In Cambridge pseudocode RANDOM() returns a number between 0 and 1 inclusive; multiply and ROUND it to get a whole number in a range.

Can I use a WHILE loop for validation? Yes. Input once, then WHILE the value is invalid, output an error and input again. A REPEAT loop also works, because the input must happen at least once.

Self-test

1. Choose a data type for a postcode, a temperature in °C to one decimal place and a yes/no answer.
2. State DIV(77, 10) and MOD(77, 10).
3. What does SUBSTRING("Peshawar", 2, 4) return?
4. Which loop always executes its body at least once?
5. Why should a file be closed after use?

Answers: 1. STRING; REAL; BOOLEAN. 2. 7 and 7. 3. “esha”. 4. The post-condition loop, REPEAT … UNTIL. 5. So the data is saved correctly and the file is released for other use; the syllabus pseudocode closes every file once it is no longer needed.