Algorithmic Problem-Solving Guides: Identifying And Fixing Logical Errors (Copy)

Identifying And Fixing Logical Errors

What A Logical Error Is (Exam Definition)

• A logical error occurs when:
  • The algorithm runs
  • Syntax is correct
  • But the output is wrong
• These are the most tested and most misunderstood errors in exams

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Core Examiner Rule (Extremely Important)

• Logical errors are identified by:
  • Reasoning
  • Tracing
  • Understanding intent
• Not by guessing output
• Not by rewriting the whole algorithm

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The Difference Between Error Types (Must Be Clear)

Error Type	What Happens
Syntax error	Code does not run
Runtime error	Program crashes
Logical error	Program runs but gives wrong result

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The 5-Step Method To Identify Logical Errors

Step 1: Understand The Intended Outcome

• Read the question
• Write in plain words:
  • What should this algorithm do?
• This is your reference point

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Step 2: Identify Inputs And Expected Outputs

• Ask:
  • What values go in?
  • What should come out?
• Logical errors always appear when:
  • Actual output ≠ expected output

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Step 3: Trace The Algorithm Step By Step

• Use:
  • Trace tables
  • Dry runs
• Track:
  • Variable values
  • Condition results
  • Loop iterations

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Step 4: Locate The First Incorrect Behaviour

• The first wrong step matters most
• Everything after that is a consequence

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Step 5: Apply A Minimal Fix

• Fix only the faulty logic
• Do not redesign the algorithm

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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

Category 1: Initialisation Errors

Faulty Code

total ← total + value

Logical Problem

• total has no starting value
• Accumulator logic broken

Reasoning

• Accumulators must start from a known base value

Fix

total ← 0
total ← total + value

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Category 2: Loop Boundary Errors

Faulty Code

FOR i ← 1 TO 9
  INPUT num
NEXT i

Intended

• Input 10 numbers

Logical Error

• Loop runs only 9 times

Fix

FOR i ← 1 TO 10

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Faulty Code (Off-By-One)

WHILE count < 10 DO
  count ← count + 1
ENDWHILE

Logical Issue

• Loop stops too early

Fix

WHILE count <= 10 DO

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Category 3: Infinite Loop Errors

Faulty Code

WHILE number > 0 DO
  OUTPUT number
ENDWHILE

Logical Error

• Condition variable never changes

Reasoning

• Loop condition stays TRUE forever

Fix

WHILE number > 0 DO
  OUTPUT number
  number ← number - 1
ENDWHILE

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Category 4: Wrong Variable Updated

Faulty Code

FOR i ← 1 TO 5
  sum ← sum + i
  i ← i + 1
NEXT i

Logical Error

• Loop variable manually modified

Effect

• Skipped iterations

Fix

REMOVE i ← i + 1

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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

Category 5: Condition Logic Errors

Condition Reversal

Faulty Code

IF mark < 50 THEN
  OUTPUT "Pass"
ENDIF

Logical Error

• Pass condition reversed

Fix

IF mark >= 50 THEN

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Overlapping Conditions

Faulty Code

IF score >= 80 THEN
  grade ← "A"
ELSEIF score >= 70 THEN
  grade ← "B"
ELSEIF score >= 80 THEN
  grade ← "C"
ENDIF

Logical Error

• Third condition unreachable

Fix

ELSEIF score >= 60 THEN

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Category 6: IF Placement Errors

Faulty Code

FOR i ← 1 TO 10
  INPUT num
NEXT i

IF num > 5 THEN
  count ← count + 1
ENDIF

Logical Error

• Condition applied to only last input

Fix

FOR i ← 1 TO 10
  INPUT num
  IF num > 5 THEN
    count ← count + 1
  ENDIF
NEXT i

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Category 7: Array Logic Errors

Wrong Index Used

Faulty Code

sum ← sum + arr[1]

Logical Error

• Same element reused

Fix

sum ← sum + arr[i]

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Out-Of-Range Access

Faulty Code

FOR i ← 1 TO n + 1
  OUTPUT arr[i]
NEXT i

Logical Error

• Accesses invalid index

Fix

FOR i ← 1 TO n

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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

Category 8: String Logic Errors

Direct Character Modification

Faulty Code

text[2] ← "X"

Logical Error

• Strings cannot be modified directly

Fix

newText ← ""
FOR i ← 1 TO LENGTH(text)
  IF i = 2 THEN
    newText ← newText & "X"
  ELSE
    newText ← newText & text[i]
  ENDIF
NEXT i
text ← newText

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Category 9: Output Placement Errors

Faulty Code

FOR i ← 1 TO 5
  total ← total + i
  OUTPUT total
NEXT i

Logical Error

• Outputs partial results instead of final

Fix

FOR i ← 1 TO 5
  total ← total + i
NEXT i
OUTPUT total

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How Examiners Want Fixes Explained

Examiner-Approved Answer Style

• Identify the error
• Explain why it is incorrect
• Show corrected logic

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Example Explanation

• “The loop never ends because the condition variable is not updated.”
• “This causes an infinite loop.”
• “Updating the variable inside the loop fixes the error.”

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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

Logical Error Identification Checklist

• Are all variables initialised?
• Do all loops terminate?
• Are conditions correct and reachable?
• Are correct variables updated?
• Is output placed after processing?

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Exam-Ready Error-Fixing Template

Error:
<describe the logical error>

Reason:
<explain why it causes incorrect behaviour>

Correction:
<show corrected line(s)>

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One-Line Rules To Memorise

• Code can run and still be wrong
• Trace before fixing
• Fix the first wrong step
• Minimal changes score more
• Explanation matters as much as correction