CAIE 9618 · Paper 2 · Topic 2.1.2

Pseudocode &
Programming Constructs

CAIE Pseudocode Standard · Sequence · Selection · Iteration

CSZone Cambridge International AS & A Level Computer Science 9618

CAIE Pseudocode Basics

Assignment, Input & Output

ASSIGNMENT

x <- 5
name <- "Ali"
total <- price * qty

INPUT / OUTPUT

INPUT age
OUTPUT "Hello", name

VARIABLES & DECLARATIONS

DECLARE score : INTEGER
DECLARE name : STRING
DECLARE flag : BOOLEAN

CONCATENATION

message <- "Hi " & name
OUTPUT message

Selection

IF…THEN…ELSE and CASE

IF…THEN…ELSE…ENDIF

IF score >= 50
THEN
OUTPUT "Pass"
ELSE
OUTPUT "Fail"
ENDIF

CASE…OF…OTHERWISE…ENDCASE

CASE OF grade
'A' : OUTPUT "Excellent"
'B' : OUTPUT "Good"
'C' : OUTPUT "Pass"
OTHERWISE : OUTPUT "Fail"
ENDCASE

Use CASE when checking one variable against multiple discrete values. Use IF-ELSE-IF when conditions involve comparisons or boolean expressions.

Iteration

FOR, WHILE and REPEAT

FOR…NEXT (count-controlled)

FOR i <- 1 TO 10
OUTPUT i
NEXT i

WHILE…DO…ENDWHILE (pre-condition)

WHILE x > 0 DO
OUTPUT x
x <- x - 1
ENDWHILE

REPEAT…UNTIL (post-condition)

REPEAT
INPUT answer
UNTIL answer = "yes"

Key difference: WHILE tests condition before entering loop (may not execute at all). REPEAT tests condition after (always executes at least once).

Boolean Logic in Pseudocode

Operators and Expressions

COMPARISON OPERATORS

= equal to
<> not equal to
< less than
> greater than
<= less than or equal
>= greater than or equal

LOGICAL OPERATORS

AND, OR, NOT
// e.g.:
IF age >= 18 AND hasID = TRUE
THEN OUTPUT "Access granted"
ENDIF

DIV AND MOD

17 DIV 5 = 3 // integer division
17 MOD 5 = 2 // remainder

Exam Practice

Cambridge-style questions

Question 1

Write pseudocode that reads 10 numbers from the user and outputs the largest value entered. Use a FOR loop.

4 marks

DECLARE largest, num, i : INTEGER
INPUT largest // seed with first value
FOR i <- 2 TO 10
INPUT num
IF num > largest THEN
largest <- num
ENDIF
NEXT i
OUTPUT "Largest: ", largest

Common Mistakes

Don't lose easy marks

1

Using = for assignment in pseudocode — CAIE pseudocode uses <- (left arrow) for assignment, and = for comparison. Writing x = 5 instead of x <- 5 is an error.

2

Forgetting to close loops and conditionals — every FOR needs a NEXT, every WHILE needs an ENDWHILE, every IF needs ENDIF, every CASE needs ENDCASE. Missing these will lose marks.

3

Choosing the wrong loop type — use FOR when you know how many iterations. Use WHILE when condition must be checked before entering (may not run). Use REPEAT-UNTIL when loop body must run at least once (e.g. input validation).

Topic Summary — 2.1.2

What You Need to Know

ASSIGNMENT & I/O

x <- value (not =)
INPUT / OUTPUT keywords
DECLARE name : TYPE

SELECTION

IF-THEN-ELSE-ENDIF
CASE OF-OTHERWISE-ENDCASE
Operators: = <> < > <= >=

ITERATION

FOR i <- 1 TO n ... NEXT i
WHILE cond DO ... ENDWHILE
REPEAT ... UNTIL cond
AND/OR/NOT · DIV/MOD

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2.1.3

Searching Algorithms

Linear Search · Binary Search · Comparison

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