Arrays, Records & Data Handling: Traversing Arrays Using Loops (Copy)

Traversing Arrays Using Loops (Cambridge Standard – O Level 2210 + IGCSE 0478)

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Purpose Of Array Traversal In Cambridge Pseudocode

• Traversing an array means:
  • Accessing each element one by one using a loop
• Cambridge tests traversal to assess:
  • Correct loop choice
  • Safe index handling
  • Accurate processing of all elements
• Array traversal is fundamental to:
  • Pre-release material algorithms
  • Section B modifications
  • Trace table and dry-run questions

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What Cambridge Expects By “Traversing An Array”

• Every valid element:
  • Is accessed exactly once (unless stated otherwise)
• Traversal must:
  • Respect declared array bounds
  • Use correct loop control
• No element should be:
  • Skipped
  • Repeated unintentionally
  • Accessed out of range

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Core Traversal Pattern For One-Dimensional Arrays

Standard FOR Loop Traversal (Most Common)

• FOR index ← lowerBound TO upperBound
  • PROCESS arrayName[index]
• ENDFOR

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Example

• FOR i ← 1 TO 30
  • OUTPUT marks[i]
• ENDFOR

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Examiner Expectations

• Loop variable:
  • Is INTEGER
• Loop bounds:
  • Match array declaration exactly
• Index used:
  • Matches loop variable

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Why FOR Loops Are Preferred For Traversal

• Number of elements:
  • Is known from array bounds
• FOR loops:
  • Are inclusive
  • Are predictable
  • Are easier to trace
• Using WHILE unnecessarily:
  • Is allowed
  • But increases error risk

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Traversing Arrays Using WHILE Loops (Allowed But Risky)

Standard WHILE Traversal Pattern

• index ← lowerBound
• WHILE index <= upperBound DO
  • PROCESS arrayName[index]
  • index ← index + 1
• ENDWHILE

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Examiner Focus

• index must be:
  • Initialised before loop
  • Updated inside loop
• Boundary condition must:
  • Match array bounds

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Common WHILE Traversal Errors

• Forgetting to increment index
• Incorrect loop condition
• Infinite loop
• Skipping first or last element

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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 O Level And IGCSE Computer Science Full Scale Course

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Traversing Arrays For Input Operations

Standard Input Traversal

• FOR i ← lowerBound TO upperBound
  • INPUT arrayName[i]
• ENDFOR

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Example

• FOR i ← 1 TO 20
  • INPUT names[i]
• ENDFOR

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Examiner Expectations

• One INPUT per iteration
• No skipped indices
• Loop runs full range

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Traversing Arrays For Output Operations

Standard Output Traversal

• FOR i ← lowerBound TO upperBound
  • OUTPUT arrayName[i]
• ENDFOR

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Example

• FOR i ← 1 TO 10
  • OUTPUT prices[i]
• ENDFOR

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Output Formatting Notes

• Formatting is secondary
• Correct traversal logic is primary
• Do not add unnecessary OUTPUT statements

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Traversing Arrays For Accumulation

Accumulator Pattern (Very High Frequency)

• total ← 0
• FOR i ← lowerBound TO upperBound
  • total ← total + arrayName[i]
• ENDFOR

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Examiner Expectations

• Accumulator initialised before loop
• Update occurs inside loop
• No reset during traversal

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Common Accumulation Errors

• Initialising accumulator inside loop
• Using wrong index
• Forgetting to initialise accumulator

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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 O Level And IGCSE Computer Science Full Scale Course

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Traversing Arrays With Selection (Filtering Elements)

Conditional Traversal Pattern

• FOR i ← lowerBound TO upperBound
  • IF condition(arrayName[i]) THEN
    • PROCESS arrayName[i]
  • ENDIF
• ENDFOR

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Example: Counting Valid Values

• count ← 0
• FOR i ← 1 TO 30
  • IF marks[i] >= 50 THEN
    • count ← count + 1
  • ENDIF
• ENDFOR

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Examiner Focus

• Loop always runs full range
• IF controls processing, not iteration
• Condition applies per element

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Traversing Arrays For Searching

Linear Search Pattern

• found ← FALSE
• index ← lowerBound
• WHILE found = FALSE AND index <= upperBound DO
  • IF arrayName[index] = target THEN
    • found ← TRUE
  • ELSE
    • index ← index + 1
  • ENDIF
• ENDWHILE

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Examiner Expectations

• Boundary condition included
• index increments correctly
• Search terminates safely

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Common Search Traversal Errors

• Missing boundary check
• Infinite loop if target not found
• Incorrect condition order

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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 O Level And IGCSE Computer Science Full Scale Course

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Traversing Arrays In Reverse Order

Reverse Traversal Using FOR

• FOR i ← upperBound TO lowerBound STEP -1
  • PROCESS arrayName[i]
• ENDFOR

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Example

• FOR i ← 10 TO 1 STEP -1
  • OUTPUT values[i]
• ENDFOR

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Examiner Focus

• STEP must be negative
• Bounds must be correct
• No off-by-one errors

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Traversing Subsets Of Arrays

Partial Traversal Pattern

• FOR i ← startIndex TO endIndex
  • PROCESS arrayName[i]
• ENDFOR

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Examiner Expectations

• startIndex and endIndex:
  • Must be within declared bounds
• Must match question requirement

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Traversing Two-Dimensional Arrays (Brief Context)

• Requires:
  • Nested loops
• Pattern:
• FOR row ← rowLower TO rowUpper
  • FOR col ← colLower TO colUpper
    • PROCESS arrayName[row, col]
  • ENDFOR
• ENDFOR
• Inner loop:
  • Traverses columns
• Outer loop:
  • Traverses rows

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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 O Level And IGCSE Computer Science Full Scale Course

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Traversing Arrays And Trace Tables

How Traversal Appears In Trace Questions

• Trace tables often include:
  • Loop index
  • Current array element
  • Updated variables

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Trace Rule

• One table row per iteration
• Index increases sequentially
• Array element changes with index

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Common Trace Errors

• Skipping first element
• Ending traversal early
• Using incorrect index values

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Traversing Arrays In Pre-Release Material

• Pre-release tasks often require:
  • Traversing arrays multiple times
  • Traversing arrays with different conditions
• You must:
  • Preserve array bounds
  • Avoid hardcoded limits
  • Follow task constraints exactly

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Traversing Arrays In Section B Modifications

• Section B frequently asks to:
  • Add validation during traversal
  • Ignore invalid elements
  • Stop traversal early based on condition

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Safe Modification Strategy

• Keep traversal loop unchanged
• Insert IF inside loop
• Do not alter array bounds

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Example Modification

Original:

• FOR i ← 1 TO 30
  • total ← total + marks[i]
• ENDFOR

Modified:

• FOR i ← 1 TO 30
  • IF marks[i] >= 0 AND marks[i] <= 100 THEN
    • total ← total + marks[i]
  • ENDIF
• ENDFOR

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Illegal Array Traversal Practices (Guaranteed Mark Loss)

• Accessing out-of-range indices
• Modifying loop counter manually
• Using undeclared arrays
• Incorrect loop bounds
• Mixing loop variables

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Best-Practice Traversal Strategy For Paper 2

• Prefer FOR loops for full traversal
• Match loop bounds to array declaration
• Initialise accumulators before traversal
• Use IF for filtering, not looping
• Trace one full traversal mentally before finalising

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Final Quality Checklist

• Correct loop type chosen
• Bounds match declaration
• Index variable used consistently
• No skipped or repeated elements
• Logic traceable and safe

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Final Lock-In Rules

• Traversal means accessing every required element
• FOR loops are safest for arrays
• Boundary errors are the most common mistakes
• Selection filters traversal, does not control it
• Clean traversal logic = strong Paper 2 performance

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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 O Level And IGCSE Computer Science Full Scale Course