Arrays, Strings And Data Handling: Searching Through Arrays Step By Step (Copy)

Searching Through Arrays Step By Step

What “Searching Through Arrays” Means

• Searching means checking array elements to:
  • Find a specific value (target)
  • Find whether the value exists
  • Find the position/index of a value
  • Find all occurrences
• In AS Level style questions, searching is usually:
  • Linear search (sequential)
• Key skills assessed:
  • Correct traversal
  • Correct use of conditions
  • Safe bounds
  • Correct outputs (found/not found, index)

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Before You Start: Search Requirements Checklist

• What must be output?
  • Found / Not found
  • Index position
  • First occurrence only
  • All positions
  • Count of occurrences
• What is the data type?
  • INTEGER, REAL, STRING, CHAR, BOOLEAN
• Does the array contain duplicates?
  • If yes, define whether you need first or all matches

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Key Search Variables (Used In Almost All Answers)

Variable	Type	Purpose
target	same as array type	value to find
index	INTEGER	current position being checked
found	BOOLEAN	tracks whether target has been located
pos	INTEGER	stores found position (if required)
count	INTEGER	counts matches

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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

Step-By-Step Linear Search (Find First Occurrence)

Goal

• Search arr[1..n] for target
• Output:
  • “Found” and the index
  • OR “Not found”

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Step 1: Initialise Control Variables

• Start from first element
• Assume not found

index ← 1
found ← FALSE

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Step 2: Loop With Safe Bounds + Early Exit

• Loop must stop when:
  • index exceeds n
  • OR found becomes TRUE

WHILE index <= n AND found = FALSE DO
  IF arr[index] = target THEN
    found ← TRUE
  ELSE
    index ← index + 1
  ENDIF
ENDWHILE

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Step 3: Output Based On Found

IF found = TRUE THEN
  OUTPUT "Found at ", index
ELSE
  OUTPUT "Not found"
ENDIF

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Full Copy-Paste Version (Exam-Safe)

index ← 1
found ← FALSE

WHILE index <= n AND found = FALSE DO
  IF arr[index] = target THEN
    found ← TRUE
  ELSE
    index ← index + 1
  ENDIF
ENDWHILE

IF found = TRUE THEN
  OUTPUT "Found at ", index
ELSE
  OUTPUT "Not found"
ENDIF

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Dry-Run Example (How To Trace Searching)

Given

• arr[1..6] = [12, 7, 19, 7, 30, 5]
• target = 7

Trace Table

Step	index	arr[index]	Compare to target	found
Start	1	12	12 = 7? No	FALSE
1	2	7	7 = 7? Yes	TRUE

• Loop stops early
• Output: Found at 2

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Common Search Variations (Step-By-Step)

Variation A: Find All Occurrences

• Goal
  • Output all indices where target occurs
• Key difference
  • Do not stop at first match

FOR index ← 1 TO n
  IF arr[index] = target THEN
    OUTPUT "Found at ", index
  ENDIF
NEXT index

• Pitfall
  • Using WHILE + found flag stops too early

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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

Variation B: Count Occurrences

• Goal
  • Count how many times target occurs

count ← 0
FOR index ← 1 TO n
  IF arr[index] = target THEN
    count ← count + 1
  ENDIF
NEXT index
OUTPUT count

• Examiner focus
  • Counter initialised
  • Counter updated only when matched

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Variation C: Return Position Or -1 (Common Pattern)

• Goal
  • If found, output index
  • Else output -1

pos ← -1
FOR index ← 1 TO n
  IF arr[index] = target THEN
    pos ← index
    EXIT FOR
  ENDIF
NEXT index
OUTPUT pos

• If EXIT FOR is not allowed in your expected pseudocode style:
  • Use a found flag instead:

pos ← -1
found ← FALSE
index ← 1

WHILE index <= n AND found = FALSE DO
  IF arr[index] = target THEN
    pos ← index
    found ← TRUE
  ELSE
    index ← index + 1
  ENDIF
ENDWHILE

OUTPUT pos

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Variation D: Search For Minimum / Maximum By Traversal

• Not “searching for target” but still a search-type question
• Maximum:

max ← arr[1]
posMax ← 1

FOR index ← 2 TO n
  IF arr[index] > max THEN
    max ← arr[index]
    posMax ← index
  ENDIF
NEXT index

OUTPUT max
OUTPUT posMax

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Searching In Two-Dimensional Arrays (Table Search)

Goal

• Find target in table[1..r, 1..c]
• Stop when found
• Output row and column

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Step-By-Step Safe Approach (Flag + Nested Loops)

found ← FALSE

FOR row ← 1 TO r
  FOR col ← 1 TO c
    IF table[row, col] = target THEN
      found ← TRUE
      foundRow ← row
      foundCol ← col
    ENDIF
  NEXT col
NEXT row

IF found = TRUE THEN
  OUTPUT "Found at ", foundRow, ",", foundCol
ELSE
  OUTPUT "Not found"
ENDIF

• Note
  • This finds the last occurrence if duplicates exist
• To find the first occurrence, add protection:

found ← FALSE

FOR row ← 1 TO r
  FOR col ← 1 TO c
    IF table[row, col] = target AND found = FALSE THEN
      found ← TRUE
      foundRow ← row
      foundCol ← col
    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

Common Logic Errors In Search Questions (High-Frequency)

Error 1: Out Of Range Access

• Example
  • Loop continues after index = n
• Fix
  • Always use index <= n in condition

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Error 2: Index Not Incremented

WHILE index <= n AND found = FALSE DO
  IF arr[index] = target THEN
    found ← TRUE
  ENDIF
ENDWHILE

• Infinite loop if first element isn’t target
• Fix
  • Increment index when not found

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Error 3: Using Wrong Variable In Comparison

IF arr[i] = target THEN

• But loop variable is index
• Fix
  • Keep loop variable consistent

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Error 4: Forgetting To Initialise found

• found left undefined
• Fix
  • Always set found ← FALSE

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Error 5: Searching Strings Incorrectly

• Comparing parts incorrectly
• Fix
  • Use exact equality unless question asks partial match

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Dry-Run Checklist For Search Questions

• Write array values with indices
• Start at index 1
• For each step:
  • Compare arr[index] with target
  • If match, stop (for first-match search)
  • Else increment index
• Confirm stop condition reached:
  • Found OR index becomes n + 1

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Exam-Ready “Search Toolkit” Table

Required Output	Best Pattern
Found / Not found	WHILE + flag
First position	WHILE + flag (store pos)
All positions	FOR (no early exit)
Count occurrences	FOR + counter
Row/col in 2D	Nested FOR + flag

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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

• Linear search checks each element in order
• Always protect bounds (index <= n)
• Use found flag for early exit
• Use FOR when you need all matches
• Update index every cycle unless found