Programming

Paper 2 ends with a 15-mark programming scenario. Everything here, plus the runner below, helps you write it confidently.

1. Variables and constants

A variable is a named memory location whose value can change while the program runs. A constant is a named value that cannot change, which makes programs easier to update and read.

DECLARE PlayerName : STRING
DECLARE Score : INTEGER
CONSTANT MaxLives ← 3
PlayerName ← "Ela"
Score ← 0

2. Data types

Data type	Stores	Example
INTEGER	Whole numbers	Score ← 42
REAL	Numbers with a decimal part	Price ← 3.99
CHAR	A single character	Grade ← ‘A’
STRING	Text (zero or more characters)	Name ← “Ela”
BOOLEAN	TRUE or FALSE	GameOver ← FALSE

3. Input and output

Use OUTPUT to display values and prompts, and INPUT to read a value from the user into a variable.

OUTPUT "Enter your age"
INPUT Age
OUTPUT "In 5 years you will be ", Age + 5

4. Sequence and selection

Sequence means statements run one after another in order. Selection chooses which statements run, using IF or CASE. The example below also shows nested selection: an IF inside another IF.

IF Temperature > 30
  THEN
    OUTPUT "Hot"
  ELSE
    IF Temperature > 15
      THEN
        OUTPUT "Warm"
      ELSE
        OUTPUT "Cold"
    ENDIF
ENDIF

CASE OF Choice
  1 : OUTPUT "Play"
  2 : OUTPUT "Settings"
  3 : OUTPUT "Quit"
  OTHERWISE OUTPUT "Invalid choice"
ENDCASE

5. Iteration

Loop	Pseudocode	When to use it
Count-controlled	FOR … TO … NEXT	When you know how many times to repeat
Pre-condition	WHILE … DO … ENDWHILE	When the condition is checked before each repeat; it may run zero times
Post-condition	REPEAT … UNTIL	When the code must run at least once, e.g. validating input

Totalling (Total ← Total + Value) and counting (Count ← Count + 1) are done inside loops; see Unit 7 for full examples.

6. String handling

Word ← "Computer"
OUTPUT LENGTH(Word)              // 8
OUTPUT SUBSTRING(Word, 1, 4)     // "Comp"
OUTPUT UCASE(Word)               // "COMPUTER"
OUTPUT LCASE(Word)               // "computer"

In Cambridge pseudocode the first character is at position 1. Some programming languages start at 0.

7. Operators

Type	Operators
Arithmetic	+ − * / ^ (power), MOD (remainder), DIV (whole-number division)
Relational	= < <= > >= <> (not equal to)
Logical	AND, OR, NOT

8. Procedures, functions and parameters

A procedure is a named block of code that performs a task; it is run with CALL. A function is similar but always returns a value, so it is used inside an expression. Parameters pass values into them (up to three in the exam).

PROCEDURE ShowLine(Size : INTEGER)
  FOR Count ← 1 TO Size
    OUTPUT "-"
  NEXT Count
ENDPROCEDURE

FUNCTION Area(Length : REAL, Width : REAL) RETURNS REAL
  RETURN Length * Width
ENDFUNCTION

CALL ShowLine(10)
OUTPUT Area(4.5, 2)

A local variable exists only inside the procedure or function where it is declared. A global variable can be used anywhere in the program.

9. Library routines

OUTPUT MOD(17, 5)                // 2: the remainder
OUTPUT DIV(17, 5)                // 3: whole-number division
OUTPUT ROUND(3.14159, 2)         // 3.14
Dice ← ROUND(RANDOM() * 5, 0) + 1  // a whole number from 1 to 6

10. Maintainable programs

• Meaningful identifiers for variables, constants, arrays, procedures and functions, e.g. TotalScore, not T.
• Comments (// in pseudocode) that explain what sections of code do.
• Procedures and functions that break the program into reusable parts.

11. Arrays

An array stores many values of the same data type under one name. Each value is accessed by its index, which can be a variable. A 1D array is a list; a 2D array is a table of rows and columns. Use loops (and nested loops for 2D) to read and write values.

DECLARE Marks : ARRAY[1:5] OF INTEGER
FOR Index ← 1 TO 5
  INPUT Marks[Index]
NEXT Index

DECLARE Board : ARRAY[1:3, 1:3] OF CHAR
FOR Row ← 1 TO 3
  FOR Column ← 1 TO 3
    Board[Row, Column] ← '-'
  NEXT Column
NEXT Row

12. File handling

Data stored in a file is kept after the program ends, so it can be used again later or by another program.

DECLARE LineOfText : STRING
OPENFILE "Scores.txt" FOR WRITE
WRITEFILE "Scores.txt", "Ela 85"
CLOSEFILE "Scores.txt"

OPENFILE "Scores.txt" FOR READ
READFILE "Scores.txt", LineOfText
OUTPUT LineOfText
CLOSEFILE "Scores.txt"

13. Run your own pseudocode

14. Examiner tips

1. Write pseudocode, not program code, except in the 15-mark scenario question.
2. Use meaningful names and comment your code in longer answers.
3. Functions RETURN a value; procedures don’t. Call a procedure with CALL.
4. Initialise totals and counters before the loop, and update them inside it.
5. Close every structure (ENDIF, ENDCASE, NEXT, ENDWHILE, UNTIL, ENDPROCEDURE, ENDFUNCTION).
6. MOD gives the remainder; DIV gives the whole-number part.
7. Arrays: check the index range and use a loop to process every element.
8. Files: OPENFILE before reading or writing, and CLOSEFILE afterwards.

15. Practice questions

16. Topic test: predict your grade