Arrays, Strings And Data Handling: Updating And Modifying Array Elements (Copy)

Updating And Modifying Array Elements

What “Updating And Modifying Array Elements” Means

• Updating = changing the value stored at an existing index
• Modifying = recalculating values based on rules or conditions
• Very common in:
  • Data cleaning
  • Adjusting marks
  • Applying rules (discounts, bonuses, penalties)
  • Normalising values
• Almost always done using loops

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Core Examiner Rule (Non-Negotiable)

• An array element can only be updated if:
  • It is accessed using a valid index
  • The new value matches the array’s data type
• Updates must occur inside traversal loops

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Basic Update Syntax (Single Element)

Direct Update

arr[3] ← 50

• Changes only one element
• Rare in exams unless index is explicitly given

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Updating All Elements Using A Loop (MOST COMMON)

Example 1: Multiply All Values By 2

FOR i ← 1 TO n
  arr[i] ← arr[i] * 2
NEXT i

• Used when:
  • Every value must be changed
• Examiner focus:
  • Correct loop bounds
  • Correct index usage

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Example 2: Add Bonus Marks To All Students

FOR i ← 1 TO n
  arr[i] ← arr[i] + 5
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

Conditional Updates (MOST TESTED)

When Conditional Updates Are Used

• Only some elements should change
• Decision based on:
  • Value range
  • Status
  • Category

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Example 3: Replace Negative Values With Zero

FOR i ← 1 TO n
  IF arr[i] < 0 THEN
    arr[i] ← 0
  ENDIF
NEXT i

• Data cleaning pattern
• Very examiner-friendly

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Example 4: Increase Only Passing Marks

FOR i ← 1 TO n
  IF arr[i] >= 50 THEN
    arr[i] ← arr[i] + 3
  ENDIF
NEXT i

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Example 5: Cap Maximum Value

• Scenario
  • Marks must not exceed 100

FOR i ← 1 TO n
  IF arr[i] > 100 THEN
    arr[i] ← 100
  ENDIF
NEXT i

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Updating Based On Index (Position-Based Logic)

Example 6: Update Alternate Elements

FOR i ← 1 TO n STEP 2
  arr[i] ← arr[i] * 2
NEXT i

• Updates elements at:
  • Index 1, 3, 5, …

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Example 7: Update Elements At Even Indices

FOR i ← 2 TO n STEP 2
  arr[i] ← arr[i] - 1
NEXT i

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Updating Using WHILE Loops

When WHILE Is Appropriate

• Update should stop early
• Update depends on condition becoming false

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Example 8: Reduce Values Until Zero

index ← 1
WHILE index <= n AND arr[index] > 0 DO
  arr[index] ← arr[index] - 1
  index ← index + 1
ENDWHILE

• Pitfall
  • Forgetting to increment index

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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

Updating Arrays Using REPEAT UNTIL

When REPEAT UNTIL Is Used

• Update must happen at least once
• Validation-style logic

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Example 9: Force Minimum Value

index ← 1
REPEAT
  IF arr[index] < 10 THEN
    arr[index] ← 10
  ENDIF
  index ← index + 1
UNTIL index > n

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Updating One Array Using Another Array

Example 10: Copy And Modify

• Scenario
  • Create adjusted marks

FOR i ← 1 TO n
  adjusted[i] ← marks[i] + 2
NEXT i

• Original array remains unchanged

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Example 11: Conditional Copy

FOR i ← 1 TO n
  IF marks[i] >= 50 THEN
    passMarks[i] ← marks[i]
  ELSE
    passMarks[i] ← 0
  ENDIF
NEXT i

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Updating Two-Dimensional Arrays (Tables)

Core Rule

• Nested loops are mandatory
• Update happens at [row, col]

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Example 12: Increase Every Value In A Table

FOR row ← 1 TO r
  FOR col ← 1 TO c
    table[row, col] ← table[row, col] + 1
  NEXT col
NEXT row

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Example 13: Conditional Table Update

• Replace negative values with zero

FOR row ← 1 TO r
  FOR col ← 1 TO c
    IF table[row, col] < 0 THEN
      table[row, col] ← 0
    ENDIF
  NEXT col
NEXT row

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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

High-Frequency Exam Patterns For Updates

Task	Update Type
Bonus marks	Add constant
Penalty	Subtract constant
Data cleaning	Replace invalid values
Normalisation	Cap min/max
Index-based update	STEP loop
Table update	Nested loops

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Common Logic Errors When Updating Arrays

Error 1: Updating Outside Loop

arr[i] ← arr[i] + 1

• But i is not inside a loop
• Causes undefined behaviour

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Error 2: Wrong Index Variable

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

• j not changing
• Only one element updated repeatedly

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Error 3: Missing ELSE Path

IF arr[i] > 0 THEN
  arr[i] ← arr[i] - 1
ENDIF

• May be fine or logically incomplete
• Depends on question requirement

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Error 4: Modifying Loop Control Variable Instead Of Array

FOR i ← 1 TO n
  i ← i + 1
NEXT i

• Dangerous
• Can skip elements or cause infinite loops

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Dry-Run Technique For Update Questions

Step-By-Step Method

• Write indices horizontally
• Write original values
• Apply update rule per iteration
• Write updated values after each loop pass
• Confirm no index skipped or repeated

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Safe Update Templates (Copy-Paste Ready)

Template 1: Update All Elements

FOR i ← 1 TO n
  arr[i] ← <new value using arr[i]>
NEXT i

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Template 2: Conditional Update

FOR i ← 1 TO n
  IF <condition using arr[i]> THEN
    arr[i] ← <updated value>
  ENDIF
NEXT i

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Template 3: Table Update

FOR row ← 1 TO r
  FOR col ← 1 TO c
    table[row, col] ← <new value>
  NEXT col
NEXT row

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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 Checklist For Full Marks

• Correct loop chosen
• Loop bounds correct
• Correct index used
• Correct update placement
• No out-of-range access
• Data type preserved

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One-Line Rules To Memorise

• Updates happen inside loops
• Every index must be valid
• Conditional updates need IF
• Tables need nested loops
• If you change i, you’re wrong