Pseudocode Core Structures (Cambridge Standard): WHILE Loops And Condition Placement Errors (Copy)

WHILE Loops And Condition Placement Errors (Cambridge Standard – O Level 2210 + IGCSE 0478)

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Purpose Of WHILE Loops In Cambridge Pseudocode

• WHILE loops are used when:
  • The number of repetitions is not known in advance
• Cambridge uses WHILE loops to assess:
  • Condition-controlled repetition
  • Correct placement of conditions
  • Safe termination of loops
• WHILE loops are commonly tested in:
  • Input validation
  • Sentinel-controlled input
  • Searching algorithms
  • Repeating processes until a condition changes

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Core WHILE Loop Structure (Cambridge Standard)

• WHILE condition DO
  • statements
• ENDWHILE

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Meaning Of Each Part

• condition
  • A Boolean expression
  • Evaluated before every iteration
• statements
  • Executed only if the condition is TRUE
• ENDWHILE
  • Marks the end of the loop body

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Execution Rule (Examiner-Critical)

• The condition is checked:
  • Before the first iteration
  • Before every subsequent iteration
• If the condition is FALSE initially:
  • The loop body does not execute at all

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WHILE Loop Vs FOR Loop (Examiner Perspective)

WHILE Loop	FOR Loop
Condition-controlled	Count-controlled
Unknown repetitions	Known repetitions
High infinite-loop risk	Fixed boundaries
Used for validation/search	Used for arrays/fixed data

• Cambridge expects:
  • WHILE loops only when repetition count is unknown
  • FOR loops when iteration count is known

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Correct Use Cases For WHILE Loops

• Validating user input
• Re-entering data until valid
• Reading input until a sentinel value is entered
• Searching until a value is found or data ends
• Repeating a process until a condition becomes FALSE

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Standard Input Validation Pattern (Cambridge-Safe)

• INPUT value
• WHILE value < minimum OR value > maximum DO
  • OUTPUT “Invalid input”
  • INPUT value
• ENDWHILE

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Examiner Expectations

• Initial INPUT must occur before the WHILE loop
• Re-input must occur inside the loop
• Condition must clearly describe invalid state

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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 O Level And IGCSE Computer Science Full Scale Course

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Condition Placement Rules (High-Risk Area)

Rule 1: Condition Is Checked Before Loop Body

• WHILE is a pre-test loop
• The loop body:
  • May execute zero times
• Never assume:
  • At least one execution

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Rule 2: Condition Variables Must Be Initialised Before WHILE

• All variables used in the condition must:
  • Have defined values before the loop starts

Incorrect:

• WHILE value <> -1 DO
  • INPUT value
• ENDWHILE

Correct:

• INPUT value
• WHILE value <> -1 DO
  • PROCESS value
  • INPUT value
• ENDWHILE

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Rule 3: Condition Variables Must Be Updated Inside Loop

• At least one variable in the condition must:
  • Change inside the loop
• Otherwise:
  • Infinite loop occurs

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Rule 4: Condition Must Represent “Continue Looping”

• The condition should describe:
  • When the loop continues
• Avoid inverted or confusing logic

Better:

• WHILE value < min OR value > max DO

Not:

• WHILE NOT (value >= min AND value <= max) 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 O Level And IGCSE Computer Science Full Scale Course

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Common WHILE Loop Condition Placement Errors

Error 1: Using Condition Variable Before Initial INPUT

• Condition checks an uninitialised variable
• Results in:
  • Undefined logic
  • Automatic mark loss

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Error 2: Condition Variable Never Changes

Incorrect:

• WHILE count < 10 DO
  • total ← total + value
• ENDWHILE

Why it fails:

• count never updates
• Loop never terminates

Correct:

• WHILE count < 10 DO
  • total ← total + value
  • count ← count + 1
• ENDWHILE

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Error 3: Updating Condition Variable Outside The Loop

Incorrect:

• WHILE score < 50 DO
  • OUTPUT “Retry”
• ENDWHILE
• score ← score + 10

Why it fails:

• score never changes while looping

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Error 4: Wrong Condition Direction

Incorrect:

• WHILE valid = FALSE DO
  • PROCESS
• ENDWHILE

When valid never changes:

• Infinite loop

Correct:

• WHILE NOT valid DO
  • INPUT value
  • valid ← value >= min AND value <= max
• ENDWHILE

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WHILE Loops And Sentinel Values

Sentinel-Controlled Input Pattern

• INPUT value
• WHILE value <> sentinel DO
  • PROCESS value
  • INPUT value
• ENDWHILE

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Examiner Expectations For Sentinel Loops

• Sentinel value:
  • Must be clearly defined
• Sentinel must:
  • Not be processed
• INPUT must appear:
  • Before the loop
  • At the end of loop body

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Common Sentinel Errors

• Processing the sentinel value
• Forgetting final INPUT
• Using sentinel without explanation

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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 O Level And IGCSE Computer Science Full Scale Course

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WHILE Loops And Searching Algorithms

Standard Linear Search Pattern

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

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Examiner Checks

• Loop condition includes:
  • Found flag
  • Boundary condition
• Index increments only when not found
• Search terminates safely

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Search Loop Traps

• Missing boundary condition
• Forgetting to increment index
• Infinite loop when target not found

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WHILE Loops And Trace Tables

Trace Table Expectations

• Trace tables must show:
  • Condition evaluation per iteration
  • Changes in variables used in condition
• Typical traced variables:
  • Condition variables
  • Counters
  • Flags

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Common Trace Errors

• Assuming loop runs at least once
• Skipping the terminating condition check
• Missing final iteration where condition becomes FALSE

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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 O Level And IGCSE Computer Science Full Scale Course

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WHILE Loops Vs REPEAT UNTIL (Cambridge Context)

WHILE	REPEAT UNTIL
Pre-test loop	Post-test loop
May run zero times	Runs at least once
Preferred by Cambridge	Rarely used
Safer for validation	Used only when required

• In Paper 2:
  • WHILE is the expected default

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WHILE Loops In Section B Modifications

• Section B commonly asks to:
  • Add extra condition
  • Restrict loop execution
  • Prevent infinite loops
• Correct approach:
  • Modify condition only
  • Keep loop structure intact

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Illegal WHILE Loop Practices (Guaranteed Mark Loss)

• Missing ENDWHILE
• Using uninitialised condition variables
• No update to condition variables
• Infinite loops
• INPUT only inside loop without initial INPUT

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Best-Practice WHILE Loop Strategy For Paper 2

• Initialise all condition variables
• Ensure condition can become FALSE
• Update condition variables inside loop
• Keep conditions clear and readable
• Prefer WHILE only when repetition count is unknown

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Final Quality Checklist

• Condition evaluated before loop
• Condition variables initialised
• Condition variables updated
• Termination guaranteed
• ENDWHILE present
• Logic traceable step-by-step

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Final Lock-In Rules

• WHILE loops are condition-controlled
• Condition placement decides correctness
• Infinite loops lose marks immediately
• Validation and searching depend on WHILE
• Clean WHILE logic = safe Paper 2 marks

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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 O Level And IGCSE Computer Science Full Scale Course