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)

---

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 test:
  • Condition-controlled repetition
  • Correct condition placement
  • Logical termination
• WHILE loops commonly appear in:
  • Input validation
  • Searching
  • Sentinel-controlled processing

---

Core WHILE Loop Structure (Cambridge Standard)

Basic Structure

• WHILE condition DO
  • statements
• ENDWHILE

---

Meaning Of Each Part

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

---

Execution Rule (Critical)

• The condition is checked:
  • Before the first iteration
  • Before every subsequent iteration
• If the condition is FALSE initially:
  • The loop body never executes

---

WHILE Loop Vs FOR Loop (Core Difference)

WHILE Loop	FOR Loop
Condition-based	Count-based
Unknown repetitions	Known repetitions
Risk of infinite loop	Fixed iterations
Requires careful condition	Boundaries explicit

• Cambridge expects:
  • WHILE loops only when necessary
  • FOR loops when counts are fixed

---

Correct Use Cases For WHILE Loops

• Input validation
• Reading data until a sentinel value
• Searching until found
• Repeating until a condition becomes FALSE

---

Example: Input Validation

• INPUT score
• WHILE score < 0 OR score > 100 DO
  • OUTPUT “Invalid input”
  • INPUT score
• ENDWHILE
• Key examiner checks:
  • INPUT before WHILE
  • Re-input inside WHILE

---

Condition Placement Rules (Very High-Risk Area)

Rule 1: Condition Must Be Checked Before Loop Body

• WHILE loops are pre-test loops
• Do NOT assume loop body runs at least once

---

Rule 2: Condition Must Be Updateable Inside Loop

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

---

Rule 3: Condition Must Reflect Termination Clearly

• Condition should describe:
  • When looping continues
• Avoid vague or inverted logic

---

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

---

Common WHILE Loop Condition Placement Errors

Error 1: INPUT Inside Loop Without Initial INPUT

Incorrect:

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

Why it fails:

• value is undefined at first condition check

Correct:

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

---

Error 2: Condition Uses Variable Never Updated

Incorrect:

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

Why it fails:

• count never changes
• Infinite loop

Correct:

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

---

Error 3: Updating Condition Variable Outside Loop

Incorrect:

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

Why it fails:

• score never changes during looping

---

Error 4: Reversing Condition Logic

Incorrect:

• WHILE valid = FALSE DO
  • PROCESS
• ENDWHILE

When valid is never updated:

• Infinite loop

Correct:

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

---

WHILE Loops And Sentinel Values

Sentinel-Controlled Loop Pattern

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

---

Examiner Expectations

• Sentinel:
  • Must be clearly stated
• Sentinel value:
  • Must not be processed
• INPUT must occur:
  • Before loop
  • At end of loop body

---

Common Sentinel Errors

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

---

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

---

WHILE Loops And Searching

Search Loop 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

---

Examiner Checks

• Condition uses:
  • found flag
  • boundary control
• index increment:
  • Must occur if not found

---

Search Loop Trap

• Forgetting boundary condition
• Results in:
  • Array out-of-bounds
  • Infinite loop

---

WHILE Loops And Validation Logic

Correct Validation Pattern

• INPUT value
• WHILE value < min OR value > max DO
  • OUTPUT “Invalid input”
  • INPUT value
• ENDWHILE

---

Incorrect Validation Patterns

• Using IF instead of WHILE
• Re-input without loop
• No re-input at all

---

WHILE Loops And Trace Tables

Trace Table Requirements

• Trace tables must show:
  • Condition evaluation
  • Variable changes per iteration
• Variables commonly traced:
  • Loop condition variables
  • Counters
  • Flags

---

Trace Error Patterns

• Assuming loop runs once automatically
• Missing termination iteration
• Incorrect condition evaluation order

---

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

---

WHILE Loop Vs REPEAT UNTIL (Cambridge Context)

• REPEAT UNTIL:
  • Is a post-test loop
  • Runs at least once
• WHILE:
  • Is a pre-test loop
  • May run zero times
• Cambridge Paper 2:
  • Strongly prefers WHILE
  • Uses REPEAT UNTIL rarely

---

Section B Modifications Involving WHILE Loops

• Common Section B tasks:
  • Add extra condition
  • Restrict number of iterations
  • Prevent infinite looping

---

Safe Modification Strategy

• Keep original WHILE structure
• Modify:
  • Condition only
• Do not:
  • Rewrite as FOR unless asked

---

Illegal WHILE Loop Practices (Guaranteed Mark Loss)

• Missing ENDWHILE
• Condition variable never updated
• INPUT only inside loop without initial INPUT
• Infinite loops
• Ambiguous termination condition

---

Best-Practice WHILE Loop Strategy For Paper 2

• Always initialise condition variables
• Ensure condition can become FALSE
• Update condition variables inside loop
• Use clear, readable conditions
• Avoid over-complicated logic
• Prefer FOR loops if count is fixed

---

Final Quality Checklist

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

---

Final Lock-In Rules

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

---

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