Arrays, Strings And Data Handling: Common Array And String Mistakes In Exams (Copy)

Common Array And String Mistakes In Exams

Why Examiners Love This Topic

• Array and string questions:
  • Look simple
  • Hide precision traps
• Most students lose marks without realising why
• Errors usually:
  • Do not crash the logic
  • Produce technically valid but wrong answers
• Examiner marking is unforgiving here

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Category 1: Indexing Errors (NUMBER ONE MARK KILLER)

Mistake 1.1: Using Index 0 Instead Of Starting At 1

arr[0]

• Why wrong
  • Indexing starts at 1 unless explicitly stated
• Result
  • Out-of-range access
• Correct

arr[1]

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Mistake 1.2: Accessing Index Beyond Declared Size

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

• Why wrong
  • arr[n + 1] does not exist
• Correct

FOR i ← 1 TO n

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Mistake 1.3: Using Wrong Variable As Index

FOR i ← 1 TO n
  OUTPUT arr[j]
NEXT i

• Why wrong
  • j never changes
• Result
  • Same element accessed repeatedly

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Category 2: Loop Bound Errors With Arrays

Mistake 2.1: Hardcoding Loop Limits

FOR i ← 1 TO 10

• But array declared as:

DECLARE arr : ARRAY[1..n] OF INTEGER

• Why wrong
  • Loop no longer matches array size
• Correct

FOR i ← 1 TO n

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Mistake 2.2: Using LENGTH With Arrays

FOR i ← 1 TO LENGTH(arr)

• Why wrong
  • LENGTH applies to strings, not arrays
• Correct
  • Use known bounds (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 3: Uninitialised Arrays And Strings

Mistake 3.1: Using Array Before Input Or Assignment

OUTPUT arr[i]

• No prior assignment
• Examiner view
  • Undefined behaviour
• Fix
  • Always populate first

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Mistake 3.2: Using String Without Assignment

DECLARE name : STRING
OUTPUT name

• Value undefined
• Automatic logic error

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Category 4: String Indexing Errors

Mistake 4.1: Accessing Character Outside String Length

text[LENGTH(text) + 1]

• Always invalid

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Mistake 4.2: Forgetting Strings Are Indexed

IF text = "A" THEN

• Intended
  • Check first character
• Correct

IF text[1] = "A" THEN

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Mistake 4.3: Treating Characters As Numbers

IF text[i] > 5 THEN

• Invalid comparison
• Must compare characters to characters

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Category 5: Direct Modification Of Strings (VERY COMMON)

Mistake 5.1: Trying To Modify A Character Directly

text[i] ← "X"

• Why wrong
  • Strings are immutable at character level
• Correct approach
  • Rebuild the string

newText ← ""
FOR i ← 1 TO LENGTH(text)
  IF i = 3 THEN
    newText ← newText & "X"
  ELSE
    newText ← newText & text[i]
  ENDIF
NEXT i
text ← newText

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Category 6: Confusing LENGTH Usage

Mistake 6.1: LENGTH Used On Arrays

LENGTH(arr)

• Wrong
• LENGTH is string-only

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Mistake 6.2: Not Using LENGTH In String Traversal

FOR i ← 1 TO n
  OUTPUT text[i]
NEXT i

• n undefined
• Correct

FOR i ← 1 TO LENGTH(text)

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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 7: Logic Placement Errors

Mistake 7.1: IF Placed Outside Loop Instead Of Inside

FOR i ← 1 TO n
  INPUT arr[i]
NEXT i

IF arr[i] > 0 THEN
  count ← count + 1
ENDIF

• Why wrong
  • Only last element checked
• Correct

FOR i ← 1 TO n
  INPUT arr[i]
  IF arr[i] > 0 THEN
    count ← count + 1
  ENDIF
NEXT i

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Mistake 7.2: Updating Loop Variable Instead Of Array Element

FOR i ← 1 TO n
  i ← i + 1
NEXT i

• Causes:
  • Skipped elements
  • Infinite loops

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Category 8: Infinite Loops In Array/String Logic

Mistake 8.1: Missing Index Increment

WHILE index <= n DO
  OUTPUT arr[index]
ENDWHILE

• Infinite loop
• Fix

index ← index + 1

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Mistake 8.2: Condition Variable Never Changes

WHILE text[i] <> " " DO
  OUTPUT text[i]
ENDWHILE

• i never updated
• Infinite 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

Category 9: Mixing Array And String Logic Incorrectly

Mistake 9.1: Assigning Wrong Data Type

DECLARE ages : ARRAY[1..5] OF INTEGER
ages[1] ← "Ten"

• Invalid
• Data type mismatch

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Mistake 9.2: Treating Character As Full String Carelessly

DECLARE ch : STRING
ch ← text[i]

• Acceptable logically
• But must be consistent in later comparisons

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Category 10: Two-Dimensional Array Errors

Mistake 10.1: Swapping Row And Column Indices

table[col, row]

• Instead of

table[row, col]

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Mistake 10.2: Incorrect Loop Nesting For Intended Logic

• Writing column-first traversal
• But describing row-based processing

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Category 11: Output Mistakes

Mistake 11.1: Outputting Index Instead Of Value

OUTPUT i

• Intended

OUTPUT arr[i]

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Mistake 11.2: Forgetting Final OUTPUT

• Computation done
• No output statement

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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

Examiner Red Flags (Instant Mark Loss)

• Out-of-range index
• LENGTH used on arrays
• Direct string modification
• IF placed incorrectly
• Infinite loop risk
• Uninitialised variables

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Final Exam Safety Checklist

• Arrays populated before use
• Loop bounds match array size
• LENGTH used only with strings
• Strings rebuilt, not edited
• All WHILE loops update condition variables
• Every index stays in range

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One-Line Rules To Memorise

• Indexing starts at 1
• LENGTH is string-only
• Strings are rebuilt
• IF placement matters
• Bounds discipline wins marks