Selection And Iteration Code Guides: WHILE Loops: Entry-Controlled Iteration (Copy)

WHILE Loops: Entry-Controlled Iteration

What A WHILE Loop Is (Entry-Controlled Loop)

• A WHILE loop repeats a block of code as long as a condition is TRUE
• The condition is checked before the loop body runs
• If the condition is FALSE at the start:
  • The loop does not run at all
• Best used when:
  • The number of repetitions is unknown
  • The loop depends on user input, data validity, or dynamic conditions

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Standard WHILE Loop Structure (Pseudocode)

WHILE <condition> DO
  <statements>
ENDWHILE

• Execution order
  • Check condition
  • If TRUE → run body
  • Update variables
  • Re-check condition
• Critical rule
  • The condition must eventually become FALSE or the loop becomes infinite

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WHILE Loop Vs FOR Loop (Quick Decision Table)

Situation	Use WHILE	Use FOR
Number of repetitions unknown	Yes	No
Input validation	Yes	No
Repeat until user chooses to stop	Yes	No
Fixed number of repetitions	No	Yes
Traversing array with known size	Possible but not ideal	Yes

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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

Structure Components (What Each Part Does)

Condition

• A Boolean expression
• Must evaluate to TRUE or FALSE
• Examples
  • num < 0
  • choice <> "Q"
  • count <= 10
• Examiner trap
  • Writing a condition that never changes

Loop Body

• Code that runs repeatedly
• Must include:
  • Input
  • Processing
  • Update to a variable used in the condition

Update Statement (Exit Control)

• Changes a variable involved in the condition
• Without it → infinite loop

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Core WHILE Loop Patterns (High-Frequency Exam Use)

Pattern 1: Input Validation

• Scenario
  • Keep asking user until they enter a positive number

INPUT num
WHILE num <= 0 DO
  OUTPUT "Invalid input"
  INPUT num
ENDWHILE

• Why WHILE fits
  • Condition checked before accepting input
• Pitfall
  • Forgetting second INPUT num inside loop

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Pattern 2: Unknown Number Of Inputs (Sentinel-Controlled)

• Scenario
  • Input numbers until user enters -1, then output total

total ← 0
INPUT num
WHILE num <> -1 DO
  total ← total + num
  INPUT num
ENDWHILE
OUTPUT total

• Key idea
  • -1 is the sentinel value
• Pitfall
  • Adding sentinel to total (logic order mistake)

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Pattern 3: Menu-Controlled Program

• Scenario
  • Keep showing menu until user chooses exit option

INPUT choice
WHILE choice <> "X" DO
  IF choice = "A" THEN
    OUTPUT "Option A selected"
  ELSE
    OUTPUT "Invalid choice"
  ENDIF
  INPUT choice
ENDWHILE

• Examiner focus
  • Loop condition
  • Correct re-input placement

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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

WHILE Loop With Counters (Controlled Exit)

Pattern 4: Count Up To A Limit

• Scenario
  • Output numbers from 1 to 5 using WHILE

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

• Difference from FOR
  • Counter update is manual
• Pitfall
  • Forgetting i ← i + 1

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Pattern 5: Count Down

• Scenario
  • Output numbers from 5 down to 1

i ← 5
WHILE i >= 1 DO
  OUTPUT i
  i ← i - 1
ENDWHILE

• Pitfall
  • Wrong condition (i <= 1) causes zero iterations

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Using WHILE With Arrays

Pattern 6: Traverse Array Until Condition Met

• Scenario
  • Find first value greater than 50 in array arr[1..n]

index ← 1
WHILE index <= n AND arr[index] <= 50 DO
  index ← index + 1
ENDWHILE

IF index <= n THEN
  OUTPUT arr[index]
ELSE
  OUTPUT "Not found"
ENDIF

• Why WHILE is ideal
  • Loop stops early once condition is met
• Pitfall
  • Accessing arr[index] when index > n

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Pattern 7: Searching With Flag Variable

• Scenario
  • Search for value target in array

found ← FALSE
index ← 1

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

IF found = TRUE THEN
  OUTPUT "Found"
ELSE
  OUTPUT "Not found"
ENDIF

• Examiner likes
  • Correct flag usage
  • Safe condition ordering

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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

Tracing A WHILE Loop (Step-By-Step Method)

Tracing Checklist

• Write columns for:
  • Condition result (TRUE/FALSE)
  • Control variable value
  • Output
• Check condition before entering loop
• Update variable at correct point

Worked Trace Example

i ← 1
WHILE i < 4 DO
  OUTPUT i
  i ← i + 1
ENDWHILE

Step	i	Condition (i < 4)	Output
Start	1	TRUE	—
Loop 1	1	TRUE	1
Loop 2	2	TRUE	2
Loop 3	3	TRUE	3
End	4	FALSE	—

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Common Examiner Pitfalls (And How To Avoid Them)

Pitfall 1: Infinite Loop

• Cause
  • Condition never becomes FALSE
• Example

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

• Fix
  • Add i ← i + 1

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Pitfall 2: Condition Never True

• Example

i ← 10
WHILE i < 5 DO
  OUTPUT i
  i ← i + 1
ENDWHILE

• Result
  • Loop never runs
• Fix
  • Correct initial value or condition

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Pitfall 3: Wrong Condition Direction

• Example
  • Using = instead of < or <>
• Fix
  • Re-read condition in plain English:
    • “Repeat while …”

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Pitfall 4: Forgetting Initial Input

• Example (sentinel loop)

WHILE num <> -1 DO
  INPUT num
ENDWHILE

• Problem
  • num not initialised
• Fix
  • Always input once before WHILE

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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

Comparing Entry-Controlled And Exit-Controlled Loops

Feature	WHILE Loop	REPEAT UNTIL Loop
Condition checked	Before loop	After loop
Minimum runs	0	1
Input validation	Good	Risky
Sentinel loops	Very suitable	Less common

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Copy-Paste WHILE Loop Templates (Exam-Safe)

Template A: Input Validation

INPUT value
WHILE value < 0 DO
  INPUT value
ENDWHILE

Template B: Sentinel-Controlled Total

total ← 0
INPUT num
WHILE num <> -1 DO
  total ← total + num
  INPUT num
ENDWHILE
OUTPUT total

Template C: Count-Controlled WHILE

i ← 1
WHILE i <= n DO
  <process>
  i ← i + 1
ENDWHILE

Template D: Search With Early Exit

found ← FALSE
index ← 1
WHILE index <= n AND found = FALSE DO
  <check>
ENDWHILE

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“Spot The Bug” Mini Practice

Bug 1

WHILE count < 5 DO
  OUTPUT count
ENDWHILE

• Error
  • count never changes
• Fix
  • Add count ← count + 1

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Bug 2

WHILE num <> -1 DO
  total ← total + num
  INPUT num
ENDWHILE

• Error
  • num not initialised
• Fix
  • Add INPUT num before loop

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Bug 3

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

• Error
  • Condition wrong
• Fix
  • WHILE i >= 1 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