2.3.1 Variables, Constants & Assignment — Cambridge 9618

Variables

A variable is a named location in memory that stores a value which can change during program execution. Every variable in Cambridge 9618 pseudocode must be declared before use using the DECLARE keyword.

// Declaring variables with DECLARE
DECLARE age : INTEGER
DECLARE name : STRING
DECLARE price : REAL
DECLARE flag : BOOLEAN
DECLARE initial : CHAR

Assignment — the ← operator

In Cambridge 9618, assignment uses the ← operator (left-pointing arrow), not =. The = symbol is reserved for comparison (testing equality).

// Assign values using ←
age ← 17
name ← "Alice"
price ← 9.99
flag ← TRUE
initial ← 'A'

// Reassignment — overwrite previous value
age ← age + 1 // age becomes 18
price ← price * 1.2 // price becomes 11.988

Constants

A constant is a named value that is set once and cannot be changed during the program. Use CONSTANT (not DECLARE) and = (not ←) to define constants:

CONSTANT PI = 3.14159
CONSTANT MAX_SIZE = 100
CONSTANT SCHOOL_NAME = "Greenwood Academy"
CONSTANT VAT_RATE = 0.20

Variable

• Declared with DECLARE
• Value assigned with ←
• Value can change at runtime
• Example: DECLARE count : INTEGER
• Example use: loop counters, user inputs, running totals

Constant

• Declared with CONSTANT
• Value set with = (not ←)
• Value cannot change
• Example: CONSTANT PI = 3.14159
• Example use: mathematical constants, tax rates, maximum sizes

Why Use Constants?

Readability: price * VAT_RATE is clearer than price * 0.20
Maintainability: Changing the VAT rate only requires changing one CONSTANT line
Reliability: Prevents accidental modification of critical values

Scope — Local and Global

The scope of a variable determines where in the program it can be accessed.

Global Scope

DECLARE total : INTEGER

CONSTANT MAX = 100

Accessible anywhere in the program — in the main body, and inside procedures/functions.

Local Scope (inside a procedure)

DECLARE temp : INTEGER

DECLARE i : INTEGER

Declared inside a PROCEDURE or FUNCTION. Exists only while that block is executing. Cannot be accessed from outside.

DECLARE total : INTEGER  // global
total ← 0

PROCEDURE AddToTotal(value : INTEGER)
  DECLARE temp : INTEGER  // local — only exists inside this procedure
  temp ← value * 2
  total ← total + temp  // global can be accessed here
ENDPROCEDURE

// 'temp' does NOT exist here — local to AddToTotal

Arithmetic Operators

Operator	Meaning	Example	Result
`+`	Addition	5 + 3	8
`-`	Subtraction	10 - 4	6
`*`	Multiplication	6 * 7	42
`/`	Division (REAL result)	7 / 2	3.5
`DIV`	Integer quotient	7 DIV 2	3
`MOD`	Remainder	7 MOD 2	1
`&`	String concatenation	"Hi" & " there"	"Hi there"

Comparison Operators (used in conditions)

Operator	Meaning	Example
`=`	Equal to	IF x = 5 THEN
`<>`	Not equal to	WHILE count <> 0 DO
`<`	Less than	IF age < 18 THEN
`>`	Greater than	IF score > 100 THEN
`<=`	Less than or equal	IF mark <= 40 THEN
`>=`	Greater than or equal	IF grade >= 7 THEN

Critical exam point: Cambridge 9618 uses ← for assignment and = for comparison. Writing x = 5 means "does x equal 5?" (comparison). Writing x ← 5 means "set x to 5" (assignment). Mixing these up is the most common error on Cambridge papers for this topic.

DIV vs /: 7 / 2 = 3.5 (REAL result). 7 DIV 2 = 3 (integer quotient, drops the remainder). 7 MOD 2 = 1 (the remainder). Checking whether a number is even: n MOD 2 = 0.

⚠️ Common Mistakes

Using = for assignment instead of ← (Cambridge will mark this wrong)
Using CONSTANT x ← value — CONSTANT uses =, not ←
Using a variable before declaring it with DECLARE
Trying to reassign a CONSTANT — PI ← 3.0 would be an error
Confusing DIV (integer quotient) with / (real division): 7 / 2 = 3.5, not 3
Confusing local/global scope — a local variable declared inside a procedure cannot be accessed outside it

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Worksheet — 2.3.1 Variables, Constants & Assignment

8 questions · Cambridge 9618 standard

Q1Write Cambridge 9618 pseudocode to declare a variable called temperature of type REAL and assign it the value 36.6.[2]

✅ Mark scheme

DECLARE temperature : REAL [1]; temperature ← 36.6 [1].

Q2State the difference between DECLARE and CONSTANT in Cambridge 9618. Write an example of each.[4]

✅ Mark scheme

DECLARE declares a variable whose value can change [1]; example e.g. DECLARE count : INTEGER [1]. CONSTANT declares a named value that cannot be changed [1]; uses = not ←, example CONSTANT MAX = 100 [1].

Q3What is the result of each of the following? (a) 15 DIV 4 (b) 15 MOD 4 (c) 15 / 4[3]

✅ Mark scheme

(a) 15 DIV 4 = 3 [1]; (b) 15 MOD 4 = 3 [1]; (c) 15 / 4 = 3.75 [1].

Q4Identify the error in each statement and write the corrected version.
(a) DECLARE total = 0
(b) CONSTANT rate ← 0.2[4]

✅ Mark scheme

(a) Error: using = for assignment instead of ← [1]; corrected: DECLARE total : INTEGER (then assign with ←: total ← 0) [1]. (b) Error: using ← for constant instead of = [1]; corrected: CONSTANT rate = 0.2 [1].

Q5Explain the difference between local and global scope using an example.[4]

✅ Mark scheme

Global variable: declared outside procedures, accessible everywhere in the program [1]; example DECLARE total : INTEGER at top level [1]. Local variable: declared inside a procedure/function, only exists while that block runs [1]; example DECLARE temp : INTEGER inside a PROCEDURE body — cannot be accessed from main program [1].

Q6Give two reasons why using a CONSTANT for a value like VAT rate (0.20) is better than writing 0.20 directly in every calculation.[2]

✅ Mark scheme

Any 2 of: Readability — VAT_RATE is self-documenting, 0.20 is a magic number [1]; Maintainability — changing the rate only requires editing one CONSTANT line rather than every occurrence [1]; Reliability — prevents the value being accidentally overwritten [1].

Q7A program uses a 2D array Grid[1:4, 1:4] of INTEGER. Write pseudocode to: (a) initialise all elements to 0, (b) set diagonal elements (where row index = column index) to 1, (c) output the entire grid row by row. Use nested FOR loops throughout.[6]

✅ Mark scheme

(a) Nested FOR loops for row and column from 1 to 4: Grid[r,c] ← 0 [1]; (b) FOR r ← 1 TO 4: Grid[r,r] ← 1 [1]; (c) Nested FOR loops for r and c: OUTPUT Grid[r,c] (optionally with newline after each row) [1]; Correct CAIE pseudocode syntax throughout (DECLARE, FOR/NEXT or ENDFOR, array declaration) [1]; Array declaration: DECLARE Grid : ARRAY[1:4, 1:4] OF INTEGER [1]; Correct 2D indexing Grid[r,c] used throughout [1]. Award max 6.

Q8Explain the difference between a static array and a dynamic array. State one advantage and one disadvantage of each, and explain why CAIE pseudocode uses static arrays (with fixed bounds declared at compile time).[5]

✅ Mark scheme

Static array: size is fixed at declaration / compile time and cannot change during execution [1]; advantage: simple, predictable memory allocation — no overhead from resizing [1]; disadvantage: may waste memory if fewer elements are used than declared, or fail if more are needed than allocated [1]; Dynamic array: size can grow or shrink at runtime [1]; advantage: memory-efficient — only allocates what is needed; disadvantage: resizing incurs overhead (copying elements to a new block of memory) [1]; CAIE pseudocode uses static arrays because it is a teaching language designed for clarity — dynamic memory management adds complexity not needed for algorithm design at this level [1]. Award max 5.

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Term	Definition

Variables

Assignment — the ← operator

Constants

Why Use Constants?

Scope — Local and Global

Arithmetic Operators

Comparison Operators (used in conditions)

Worksheet — 2.3.1 Variables, Constants & Assignment

Mini Test — 2.3.1 Variables & Constants