Cost And Management Accounting Formats (AS Level): Cost–Volume–Profit Decision Table Format (Copy)

REPEAT UNTIL Loops: Exit-Controlled Iteration

What A REPEAT UNTIL Loop Is (Exit-Controlled Loop)

• A REPEAT UNTIL loop runs the loop body at least once
• The condition is checked after the loop body executes
• The loop stops when the condition becomes TRUE
• Best used when:
  • You must run the code at least once
  • Input must be taken before validation
  • Menu systems must display at least one time

Standard REPEAT UNTIL Structure (Pseudocode)

REPEAT
  <statements>
UNTIL <condition>

• Execution order
  • Execute loop body
  • Check condition
  • If condition is FALSE → repeat
  • If condition is TRUE → exit loop
• Key difference from WHILE
  • REPEAT UNTIL guarantees minimum one execution

REPEAT UNTIL Vs WHILE (Quick Comparison)

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 (Exam-Focused)

Loop Body

• Executes before condition is checked
• Usually contains:
  • INPUT
  • Processing
  • Update logic

UNTIL Condition

• Must be a Boolean expression
• Loop exits when condition becomes TRUE
• Common patterns
  • choice = "X"
  • num >= 0
  • attempts = 3

Critical Logic Rule

• Condition must be written as:
  • “Stop when this becomes TRUE”
• Common mistake
  • Writing the condition as if it were a WHILE loop

Core REPEAT UNTIL Patterns (High-Frequency Exam Use)

Pattern 1: Input Validation (Guaranteed One Input)

• Scenario
  • User must enter a positive number

REPEAT
  INPUT num
UNTIL num > 0

• Why REPEAT UNTIL is perfect here
  • Input must occur at least once
• Pitfall
  • Writing UNTIL num < 0 (logic reversed)

Pattern 2: Menu System (Exit Option)

• Scenario
  • Display menu until user chooses “Q”

REPEAT
  OUTPUT "A. Add"
  OUTPUT "B. Delete"
  OUTPUT "Q. Quit"
  INPUT choice
UNTIL choice = "Q"

• Examiner focus
  • Correct exit condition
  • Menu displayed at least once

Pattern 3: Sentinel-Controlled Input

• Scenario
  • Input numbers until user enters -1

REPEAT
  INPUT num
  IF num <> -1 THEN
    total ← total + num
  ENDIF
UNTIL num = -1
OUTPUT total

• Why this structure works
  • Sentinel not added to total
• Pitfall
  • Adding sentinel before checking

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

Controlled Repetition Using Counters

Pattern 4: Limited Attempts (Password Example)

• Scenario
  • Allow maximum 3 attempts

attempts ← 0
REPEAT
  INPUT password
  attempts ← attempts + 1
UNTIL password = "ABC123" OR attempts = 3

• Examiner marking points
  • Counter initialised
  • Counter updated inside loop
  • Compound condition used correctly

Pattern 5: Fixed Repetition With REPEAT (Less Common)

• Scenario
  • Output numbers 1 to 5

i ← 1
REPEAT
  OUTPUT i
  i ← i + 1
UNTIL i > 5

• Why FOR is better here
  • Fixed repetitions are clearer with FOR
• Why REPEAT still works
  • Condition stops loop correctly

Using REPEAT UNTIL With Arrays

Pattern 6: Search Until Found

• Scenario
  • Search for value in array until found

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

• Strength of REPEAT UNTIL
  • Guarantees at least one check
• Pitfall
  • Accessing arr[index] after index > n

Pattern 7: Input Validation With Range Check

• Scenario
  • Input number between 1 and 10 inclusive

REPEAT
  INPUT num
UNTIL num >= 1 AND num <= 10

• Common mistake
  • Using OR instead of AND
• Correct logic
  • Both bounds must be satisfied

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 REPEAT UNTIL Loop (Dry Run Method)

Tracing Checklist

• Assume loop runs once minimum
• Track:
  • Variable values
  • Condition result after each iteration
• Stop when condition becomes TRUE

Worked Trace Example

i ← 1
REPEAT
  OUTPUT i
  i ← i + 1
UNTIL i = 4

• Output: 1 2 3

Common Examiner Pitfalls (And Fixes)

Pitfall 1: Reversed Condition Logic

• Mistake
  • Writing the condition as “keep going while”
• Example (wrong)

UNTIL num < 0

• Correct thinking
  • “Stop when input is valid”

UNTIL num >= 0

Pitfall 2: Infinite Loop

• Cause
  • Condition never becomes TRUE
• Example

REPEAT
  OUTPUT "Hello"
UNTIL 1 = 2

• Fix
  • Ensure condition depends on changing variable

Pitfall 3: Misplaced Processing Logic

• Example

REPEAT
  total ← total + num
  INPUT num
UNTIL num = -1

• Error
  • Adds previous invalid value
• Fix
  • Input first, then process conditionally

Pitfall 4: Wrong Use Of OR / AND

• Example (wrong)

UNTIL num >= 1 OR num <= 10

• Why wrong
  • Condition always TRUE
• Correct

UNTIL num >= 1 AND num <= 10

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

Entry-Controlled Vs Exit-Controlled Loops (Decision Table)

Copy-Paste REPEAT UNTIL Templates (Exam-Safe)

Template A: Input Validation

REPEAT
  INPUT value
UNTIL value > 0

Template B: Sentinel Loop

REPEAT
  INPUT num
UNTIL num = -1

Template C: Limited Attempts

attempts ← 0
REPEAT
  INPUT data
  attempts ← attempts + 1
UNTIL attempts = limit

Template D: Menu Exit

REPEAT
  <display menu>
  INPUT choice
UNTIL choice = "X"

“Spot The Bug” Mini Practice

Bug 1

REPEAT
  INPUT num
UNTIL num < 0

• Error
  • Stops only on invalid input
• Fix
  • Reverse condition to valid exit

Bug 2

REPEAT
  OUTPUT "Menu"
UNTIL choice = "Q"

• Error
  • choice never changes
• Fix
  • Add input inside loop

Bug 3

REPEAT
  INPUT num
  total ← total + num
UNTIL num = -1

• Error
  • Adds sentinel
• Fix
  • Add conditional check before summation

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