Selection And Iteration Code Guides: Choosing The Correct Loop For A Problem (Copy)

Choosing The Correct Loop For A Problem

Why Loop Selection Matters

• Choosing the wrong loop:
  • Makes logic harder to understand
  • Increases risk of infinite loops
  • Loses easy method marks even if output is correct
• Examiners expect:
  • FOR when repetitions are known
  • WHILE when repetitions are unknown and condition checked first
  • REPEAT UNTIL when code must run at least once

Core Decision Rule (Exam-Safe Thinking)

• Ask three questions before writing any loop:
  • Do I know how many times this must run?
  • Must the code run at least once?
  • Should the condition be checked before or after execution?

Master Decision Table (Use This First)

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

FOR Loop: When And Why To Choose It

When FOR Is The Correct Choice

• Number of repetitions is known in advance
• Loop runs through:
  • A fixed range (1 to 10)
  • All elements of an array using indices
• Loop control is simple and predictable

Typical FOR Scenarios

• “Repeat 20 times”
• “Process all values in an array of size n”
• “Display numbers from 1 to 50”
• “Add marks of 30 students”

Correct FOR Pattern

FOR i ← 1 TO n
  <process>
NEXT i

FOR Loop Pitfalls

• Using FOR when repetitions are unknown
• Changing the counter inside the loop
• Wrong start/end bounds

WHILE Loop: When And Why To Choose It

When WHILE Is The Correct Choice

• Number of repetitions is unknown
• Loop depends on:
  • User input
  • Data conditions
• Condition must be checked before execution

Typical WHILE Scenarios

• “Keep asking until input is valid”
• “Repeat while user has not chosen exit”
• “Search until item found”
• “Repeat while stock > 0”

Correct WHILE Pattern

WHILE <condition> DO
  <process>
ENDWHILE

WHILE Loop Pitfalls

• Forgetting to update condition variables
• Condition never becoming FALSE
• Using WHILE when loop must run at least once

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

REPEAT UNTIL Loop: When And Why To Choose It

When REPEAT UNTIL Is The Correct Choice

• Loop must execute at least once
• Input must be taken before validation
• Exit condition is clearer than continuation condition

Typical REPEAT UNTIL Scenarios

• “Enter a number until it is valid”
• “Show menu at least once”
• “Repeat until user chooses to quit”
• “Allow limited attempts”

Correct REPEAT UNTIL Pattern

REPEAT
  <process>
UNTIL <condition>

REPEAT UNTIL Pitfalls

• Writing condition like a WHILE loop
• Forgetting that condition stops loop when TRUE
• Infinite loops due to condition never becoming TRUE

Side-By-Side Comparison (Fast Recall Table)

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

Problem-Based Loop Selection (Exam-Style Scenarios)

Scenario 1: “Input 10 numbers and find the total”

• Correct loop: FOR
• Reason
  • Number of inputs is fixed

total ← 0
FOR i ← 1 TO 10
  INPUT num
  total ← total + num
NEXT i

Scenario 2: “Keep asking until the user enters a positive number”

• Correct loop: REPEAT UNTIL
• Reason
  • Input must occur at least once

REPEAT
  INPUT num
UNTIL num > 0

Scenario 3: “Repeat while the user does not choose exit”

• Correct loop: WHILE
• Reason
  • Condition must be checked before repeating

INPUT choice
WHILE choice <> "Q" DO
  <menu actions>
  INPUT choice
ENDWHILE

Scenario 4: “Search array and stop when found”

• Correct loop: WHILE
• Reason
  • Early exit required

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

Scenario 5: “Allow maximum 3 attempts”

• Correct loop: REPEAT UNTIL
• Reason
  • Must allow at least one attempt

attempts ← 0
REPEAT
  INPUT password
  attempts ← attempts + 1
UNTIL password = correct OR attempts = 3

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

Examiner-Focused Mistakes (And How To Avoid Them)

Mistake 1: Using FOR For Input Validation

• Why wrong
  • Number of attempts is unknown
• Correct choice
  • WHILE or REPEAT UNTIL

Mistake 2: Using WHILE When Loop Must Run Once

• Why wrong
  • Loop may never execute
• Correct choice
  • REPEAT UNTIL

Mistake 3: Overcomplicating Simple Repetition

• Example
  • Using WHILE instead of FOR for fixed count
• Fix
  • Use FOR for clarity and safety

Mistake 4: Writing Clever But Unsafe Logic

• Example
  • Multiple conditions without clarity
• Fix
  • Use simplest loop that meets requirements

Mental Checklist Before Writing Any Loop

• Can I write the loop bounds clearly? → FOR
• Do I need to test condition first? → WHILE
• Must the loop run at least once? → REPEAT UNTIL
• Do I need early termination? → WHILE / FOR with flag
• Is readability important? → Choose simplest valid loop

Final Loop-Choice Summary (One-Line Rules)

• Fixed number → FOR
• Unknown number → WHILE
• Must run once → REPEAT UNTIL
• Array traversal → FOR
• Validation after input → REPEAT UNTIL
• Early exit → WHILE

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