Selection And Iteration Code Guides: Avoiding Infinite Loops In Exam Questions (Copy)

Avoiding Infinite Loops In Exam Questions

What An Infinite Loop Is

• An infinite loop is a loop that never stops executing
• Happens when:
  • The loop condition never becomes FALSE (WHILE)
  • The exit condition never becomes TRUE (REPEAT UNTIL)
• In exam questions:
  • Even one infinite loop = logic considered unsafe
  • Can cost method marks even if output looks correct

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Core Examiner Rule (Non-Negotiable)

• Every loop must have a clear and reachable exit condition
• Examiners look for:
  • A variable in the condition
  • That variable being updated correctly inside the loop
  • Update moving the logic towards termination

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Main Causes Of Infinite Loops (Big Picture)

Cause	Loop Type	Why It Becomes Infinite
Condition variable never changes	WHILE / REPEAT	Condition never flips
Wrong comparison operator	WHILE / REPEAT	Condition always TRUE/FALSE
Counter update missing	All	No progress towards exit
Wrong STEP direction	FOR	Counter moves away from end
Sentinel never reached	WHILE / REPEAT	Exit value never met
Logical operator misuse (AND / OR)	WHILE / REPEAT	Condition permanently satisfied

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change AS Level Computer Science Full Scale Course

Infinite Loops In FOR Loops (Yes, They Happen Too)

Cause 1: Wrong STEP Direction

• Example (infinite)

FOR i ← 10 TO 1 STEP 1
  OUTPUT i
NEXT i

• Why infinite
  • i increases instead of decreasing
• Correct

FOR i ← 10 TO 1 STEP -1
  OUTPUT i
NEXT i

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Cause 2: Changing Counter Inside Loop

• Example (unpredictable / unsafe)

FOR i ← 1 TO 10
  i ← i - 1
NEXT i

• Why dangerous
  • Counter never reaches end
• Exam rule
  • Never manually change FOR counter

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Cause 3: End Value Never Reached

• Example

FOR i ← 1 TO 10 STEP 2
  OUTPUT i
NEXT i

• Is this infinite?
  • No, but end value 10 is never reached
• Examiner note
  • Loop still terminates safely
  • Do not confuse with infinite loops

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Infinite Loops In WHILE Loops (Most Common Exam Error)

Cause 1: Control Variable Never Updated

• Example (classic infinite loop)

i ← 1
WHILE i <= 5 DO
  OUTPUT i
ENDWHILE

• Why infinite
  • i never changes
• Correct

i ← 1
WHILE i <= 5 DO
  OUTPUT i
  i ← i + 1
ENDWHILE

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Cause 2: Condition Always TRUE

• Example

WHILE 1 = 1 DO
  OUTPUT "Hello"
ENDWHILE

• Why infinite
  • Condition is permanently TRUE
• Examiner warning
  • Treated as invalid logic

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Cause 3: Wrong Comparison Operator

• Example

i ← 1
WHILE i >= 1 DO
  OUTPUT i
  i ← i + 1
ENDWHILE

• Why infinite
  • i keeps increasing
  • Condition never becomes FALSE
• Correct

WHILE i <= 5 DO

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change AS Level Computer Science Full Scale Course

Infinite Loops In REPEAT UNTIL Loops

Cause 1: Exit Condition Never Becomes TRUE

• Example

REPEAT
  OUTPUT "X"
UNTIL 2 < 1

• Why infinite
  • Condition is permanently FALSE
• Correct
  • Condition must depend on a changing variable

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Cause 2: Variable Not Updated Inside Loop

• Example

count ← 0
REPEAT
  OUTPUT count
UNTIL count = 5

• Why infinite
  • count never changes
• Correct

count ← 0
REPEAT
  OUTPUT count
  count ← count + 1
UNTIL count = 5

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Cause 3: Wrong Logical Operator

• Example (always FALSE)

UNTIL num < 0 AND num > 10

• Why infinite
  • Condition can never be TRUE
• Correct

UNTIL num < 0 OR num > 10

(if stopping on invalid input)

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Sentinel-Controlled Infinite Loop Errors

Sentinel Loop Pattern (Safe)

INPUT num
WHILE num <> -1 DO
  <process>
  INPUT num
ENDWHILE

Common Infinite Version

INPUT num
WHILE num <> -1 DO
  <process>
ENDWHILE

• Why infinite
  • num never changes
• Fix
  • Re-input inside loop

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change AS Level Computer Science Full Scale Course

Logical Operator Traps (AND / OR)

AND Trap

WHILE num >= 0 AND num <= 100 DO

• Loop runs only while input is valid
• If no re-input inside loop → infinite

OR Trap

WHILE num < 0 OR num > 100 DO

• Correct for validation only if input is updated

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Early-Exit Logic That Prevents Infinite Loops

Using Flags Safely

found ← FALSE
index ← 1
WHILE index <= n AND found = FALSE DO
  IF arr[index] = target THEN
    found ← TRUE
  ELSE
    index ← index + 1
  ENDIF
ENDWHILE

• Why safe
  • Two independent exit routes
• Examiner likes
  • Defensive logic

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Dry-Run Technique To Detect Infinite Loops

Step-by-Step Safety Check

• Identify loop type
• Locate loop condition
• Identify all variables in condition
• Check:
  • Are they initialised?
  • Are they updated?
  • Do updates move towards exit?

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Examiner Red Flags (Instant Mark Loss)

• Loop condition uses constant values only
• Loop variable updated outside loop
• No update for condition variable
• Counter moves in wrong direction
• Sentinel not re-input

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Safe Loop Writing Checklist (Memorise This)

• Loop condition uses variable(s)
• Variable(s) initialised before loop
• Variable(s) updated inside loop
• Update moves towards termination
• Logical operators make sense
• Loop type matches problem

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Mini Exam Practice: Infinite Or Safe?

Practice 1

i ← 1
WHILE i < 5 DO
  OUTPUT i
  i ← i + 1
ENDWHILE

• Safe

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Practice 2

REPEAT
  OUTPUT "A"
UNTIL 5 = 10

• Infinite

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Practice 3

FOR i ← 1 TO 10 STEP -1
  OUTPUT i
NEXT i

• Infinite

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Practice 4

INPUT num
WHILE num <> 0 DO
  INPUT num
ENDWHILE

• Safe

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Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change AS Level Computer Science Full Scale Course

One-Line Exam Rules To Remember

• No update = infinite
• Wrong direction = infinite
• Constant condition = infinite
• Sentinel not re-input = infinite
• If you can’t explain how it stops → it probably doesn’t