Selection And Iteration Code Guides: Combining Selection And Iteration Correctly (Copy)

Combining Selection And Iteration Correctly

What “Combining Selection And Iteration” Means

• Iteration controls how many times code runs
• Selection controls what decision is made each time
• Correct combination means:
  • Loop controls repetition
  • IF / ELSE controls logic inside each repetition
• This combination appears in:
  • Almost every medium to large exam question
  • Array processing
  • Validation
  • Searching
  • Counting, summing, filtering

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Core Principle (Exam-Safe Rule)

• Loops repeat
• IF statements decide
• Never confuse their roles

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Correct Structural Hierarchy (Very Important)

Correct Structure

<initialisation>
<loop>
  <selection>
    <processing>
  </selection>
</loop>

Wrong Structure (Common Mistake)

<selection>
  <loop>
    <processing>
  </loop>
</selection>

• Why wrong
  • IF only checks once
  • Loop becomes conditionally unreachable

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The Three Standard Combination Patterns

Pattern	Description	Typical Use
Selection inside Iteration	IF inside loop	Counting, filtering
Iteration inside Selection	Loop inside IF	Rare, risky
Selection controlling loop exit	IF affects loop condition	Searching

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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

Pattern 1: Selection Inside Iteration (MOST IMPORTANT)

When To Use

• Every item must be processed
• Decision is made for each iteration

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Example 1: Counting Values That Meet A Condition

• Scenario
  • Input 10 numbers, count how many are positive

count ← 0
FOR i ← 1 TO 10
  INPUT num
  IF num > 0 THEN
    count ← count + 1
  ENDIF
NEXT i
OUTPUT count

• Examiner focus
  • IF inside loop
  • Correct counter update
• Pitfall
  • Putting IF outside loop (counts only last input)

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Example 2: Summation With Condition

• Scenario
  • Add only even numbers

total ← 0
FOR i ← 1 TO 10
  INPUT num
  IF num MOD 2 = 0 THEN
    total ← total + num
  ENDIF
NEXT i
OUTPUT total

• Why correct
  • Decision applied every iteration
• Pitfall
  • Adding outside IF (adds all values)

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Example 3: Array Filtering

• Scenario
  • Count marks ≥ 50

countPass ← 0
FOR index ← 1 TO n
  IF marks[index] >= 50 THEN
    countPass ← countPass + 1
  ENDIF
NEXT index

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Pattern 2: Selection Controlling Iteration (Early Exit Logic)

When To Use

• Loop should stop early when condition is met
• Searching problems

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Example 4: Linear Search With WHILE + IF

• Scenario
  • Find target value 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

• Why correct
  • Selection updates flag
  • Flag affects loop continuation
• Pitfall
  • Forgetting to increment index when NOT found

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Example 5: Search Using REPEAT UNTIL

index ← 1
found ← FALSE

REPEAT
  IF arr[index] = target THEN
    found ← TRUE
  ELSE
    index ← index + 1
  ENDIF
UNTIL found = TRUE OR index > n

• Important
  • Exit condition must protect array bounds

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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

Pattern 3: Iteration Inside Selection (Use Carefully)

When It Is Acceptable

• Loop runs only if a condition is true
• Rare in exams
• Must be very clear

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Example 6: Conditional Processing

• Scenario
  • Only process list if user agrees

INPUT choice
IF choice = "Y" THEN
  FOR i ← 1 TO n
    OUTPUT data[i]
  NEXT i
ENDIF

• Pitfall
  • Overusing this pattern
• Examiner preference
  • Usually wants loop first, IF inside

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Common Exam Applications (Mapped Clearly)

Task	Loop	Selection
Count passes	FOR	IF
Input validation	REPEAT	IF
Search	WHILE	IF
Menu	WHILE / REPEAT	IF / ELSE
Frequency count	FOR	IF
Stop on condition	WHILE	IF

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Nested Selection Inside Iteration (Very Common)

Example 7: Multiple Conditions Per Iteration

• Scenario
  • Categorise marks

FOR i ← 1 TO n
  INPUT mark
  IF mark >= 80 THEN
    OUTPUT "A"
  ELSEIF mark >= 60 THEN
    OUTPUT "B"
  ELSE
    OUTPUT "Fail"
  ENDIF
NEXT i

• Pitfall
  • Forgetting ELSEIF order
• Rule
  • Highest condition first

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Combining With Counters And Accumulators

Example 8: Count And Sum Together

• Scenario
  • Average of positive numbers

count ← 0
total ← 0

FOR i ← 1 TO n
  INPUT num
  IF num > 0 THEN
    total ← total + num
    count ← count + 1
  ENDIF
NEXT i

IF count > 0 THEN
  average ← total / count
ENDIF

• Why IF after loop
  • Prevents division by zero

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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

Common Examiner Mistakes (High-Frequency)

Mistake 1: IF Outside Loop

FOR i ← 1 TO 10
  INPUT num
NEXT i
IF num > 0 THEN
  count ← count + 1
ENDIF

• Error
  • IF checks only last value

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Mistake 2: Missing ELSE Path

• Leads to:
  • Counter not updated
  • Infinite loops

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Mistake 3: Selection That Does Not Affect Loop

• Example

WHILE i <= 10 DO
  IF i = 5 THEN
    OUTPUT "Five"
  ENDIF
  i ← i + 1
ENDWHILE

• Safe but pointless IF
• Examiner expects logic relevance

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Mistake 4: Incorrect Update Placement

• Example

WHILE i <= 10 DO
  IF condition THEN
    i ← i + 1
  ENDIF
ENDWHILE

• Infinite if condition never true

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Dry-Run Check For Combined Logic

• Does loop variable always update?
• Does IF execute every iteration?
• Can loop exit early if needed?
• Are all paths safe?

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Safe Combination Templates (Copy-Paste Ready)

Template A: Counting With Condition

count ← 0
FOR i ← 1 TO n
  IF <condition> THEN
    count ← count + 1
  ENDIF
NEXT i

Template B: Search With Early Exit

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

Template C: Validation Loop

REPEAT
  INPUT value
UNTIL <valid condition>

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Examiner’s Mental Checklist (Use This Before Writing)

• Loop chosen correctly?
• Selection inside loop where needed?
• All variables initialised?
• All paths update control variable?
• Loop guaranteed to terminate?

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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 Rules To Memorise

• Repeat with loop, decide with IF
• IF inside loop = per-item logic
• IF controlling loop = early exit
• Wrong nesting = lost marks
• If logic looks clever, simplify