Selection And Iteration Code Guides: Common Logic Errors In Conditional Statements (Copy)

Common Logic Errors In Conditional Statements

Why Logic Errors Are So Costly In Exams

• Logic errors:
  • Do not cause syntax errors
  • Produce wrong output silently
• Examiners penalise:
  • Incorrect conditions
  • Impossible conditions
  • Conditions that are always TRUE or always FALSE
• Most logic errors occur in:
  • IF statements
  • Loop conditions
  • Combined selection + iteration

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Core Examiner Rule

• A condition must be:
  • Logically correct
  • Reachable
  • Meaningful
  • Dependent on changing variables

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Error Category 1: Using = Instead Of Comparison Logic

The Mistake

• Confusing assignment logic with comparison intent

IF x = 5 THEN

• In pseudocode, this is acceptable as comparison
• But students often:
  • Misinterpret what the condition is checking
  • Assume it “sets” x to 5 (wrong mental model)

Real Logic Error

x ← 3
IF x = 5 THEN
  OUTPUT "Yes"
ENDIF

• Condition is FALSE
• IF body never runs

Examiner Expectation

• You must mentally evaluate the condition
• Do not assume assignment happens in IF

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Error Category 2: Impossible Conditions (Always FALSE)

The Mistake

IF num < 0 AND num > 100 THEN

• Why impossible
  • A number cannot be less than 0 and greater than 100 at the same time

Correct Logic (Depending On Intent)

• To detect invalid input:

IF num < 0 OR num > 100 THEN

Examiner View

• Impossible conditions = dead code
• Treated as logical misunderstanding

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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

Error Category 3: Conditions That Are Always TRUE

The Mistake

IF num >= 0 OR num <= 100 THEN

• Why always TRUE
  • Every number satisfies at least one side

Correct Version

IF num >= 0 AND num <= 100 THEN

Exam Tip

• Read condition in English:
  • “Number is between 0 and 100 inclusive”
  • Requires AND, not OR

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Error Category 4: Reversed Logic (Opposite Meaning)

The Mistake

IF age > 18 THEN
  OUTPUT "Child"
ENDIF

• Logical meaning reversed

Why It Happens

• Rushing
• Not translating condition into English

Safe Fix Method

• Always say condition aloud:
  • “If age greater than 18, then… adult”

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Error Category 5: Incorrect ELSE / ELSEIF Ordering

The Mistake

IF mark >= 50 THEN
  OUTPUT "Pass"
ELSEIF mark >= 80 THEN
  OUTPUT "A"
ENDIF

• Why wrong
  • mark >= 80 is never reached
  • First condition already catches it

Correct Ordering

IF mark >= 80 THEN
  OUTPUT "A"
ELSEIF mark >= 50 THEN
  OUTPUT "Pass"
ENDIF

Examiner Expectation

• Conditions ordered most specific → least specific

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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

Error Category 6: Missing ELSE Path (Incomplete Logic)

The Mistake

IF num > 0 THEN
  total ← total + num
ENDIF

• Problem
  • What happens if num <= 0?
  • Sometimes logic requires explicit handling

When Missing ELSE Is Acceptable

• When doing optional processing only

When It Loses Marks

• When:
  • Every case must be handled
  • Question implies two outcomes

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Error Category 7: Updating The Wrong Variable

The Mistake

IF found = FALSE THEN
  count ← count + 1
ENDIF

• Intended:
  • Change found
• Actual:
  • found never changes
  • Loop may become infinite

Correct Logic

IF arr[index] = target THEN
  found ← TRUE
ENDIF

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Error Category 8: Condition Variable Never Changes

The Mistake

IF choice = "Y" THEN
  OUTPUT "Yes"
ENDIF

• Inside a loop, but:
  • choice is never updated

Result

• IF behaves the same every iteration
• Loop logic becomes meaningless

Fix

• Update condition variable inside loop

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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

Error Category 9: Confusing AND / OR In Multi-Part Conditions

Common Student Error

IF score >= 50 OR score <= 100 THEN

Correct Logic For “Between”

IF score >= 50 AND score <= 100 THEN

OR Is Correct When

• Any one condition is enough

IF choice = "Y" OR choice = "y" THEN

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Error Category 10: Using Equality Instead Of Range Logic

The Mistake

IF age = 18 THEN
  OUTPUT "Adult"
ENDIF

• What about 19, 20, 25?

Correct Logic

IF age >= 18 THEN
  OUTPUT "Adult"
ENDIF

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Error Category 11: Selection Placed Outside Iteration

The Mistake

FOR i ← 1 TO 10
  INPUT num
NEXT i

IF num > 0 THEN
  count ← count + 1
ENDIF

• Only last input checked

Correct Placement

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

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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

Error Category 12: Logic That Causes Infinite Loops

Example

WHILE num > 0 DO
  IF num > 10 THEN
    OUTPUT num
  ENDIF
ENDWHILE

• Why infinite
  • num never changes

Fix

num ← num - 1

• Every condition variable must be updated

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High-Frequency Exam Traps (Quick Table)

Error	Result
Impossible condition	IF never runs
Always TRUE condition	IF always runs
Wrong AND / OR	Validation fails
Bad ELSEIF order	Wrong category
Missing update	Infinite loop
IF outside loop	Partial logic

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Dry-Run Method To Catch Logic Errors

• Write values of all condition variables
• Evaluate condition manually
• Ask:
  • Can this ever be TRUE?
  • Can this ever be FALSE?
• If answer is “no” → logic error

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Safe Condition-Writing Checklist

• Translate condition into English
• Check AND / OR meaning
• Ensure ranges are correct
• Order ELSEIF correctly
• Ensure condition variable changes
• Ensure all required cases handled

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Mini Exam Practice: Identify The Error

Practice 1

IF temp < 0 AND temp > 100 THEN

• Error: impossible condition

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Practice 2

IF mark >= 40 THEN
  OUTPUT "Pass"
ELSEIF mark >= 70 THEN
  OUTPUT "A"
ENDIF

• Error: ELSEIF unreachable

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Practice 3

IF num >= 0 OR num <= 10 THEN

• Error: always TRUE

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Practice 4

WHILE choice = "Y" DO
  OUTPUT "Continue"
ENDWHILE

• Error: choice never updated

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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

• Read conditions in English
• AND narrows, OR widens
• Order ELSEIF from strict to loose
• Every condition variable must change
• If it can’t ever flip, it’s wrong