Arrays, Strings And Data Handling: Character-By-Character String Processing (Copy)

Character-By-Character String Processing

What Character-By-Character Processing Means

• Processing a string one character at a time
• Treating a string like a one-dimensional array of characters
• Accessing characters using index positions
• Essential for:
  • Validation
  • Pattern checking
  • Counting specific characters
  • Filtering characters
  • Building new strings

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Core Examiner Rule (Non-Negotiable)

• You cannot directly modify a character inside a string
• Any change requires:
  • Reading characters one by one
  • Building a new string
  • Reassigning it

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String As A Sequence Of Characters

Conceptual Model

text = "HELLO"
Index:   1  2  3  4  5
Chars:   H  E  L  L  O

• text[1] → "H"
• text[5] → "O"

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Standard Character Traversal Pattern (MOST IMPORTANT)

FOR i ← 1 TO LENGTH(text)
  <process text[i]>
NEXT i

• This pattern appears in almost every string-processing question
• Examiner expects:
  • Correct bounds
  • Correct index usage

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Reading Characters One By One

Example 1: Output Each Character On New Line

FOR i ← 1 TO LENGTH(word)
  OUTPUT word[i]
NEXT i

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Example 2: Output Characters With Position

FOR i ← 1 TO LENGTH(word)
  OUTPUT i, ":", word[i]
NEXT i

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Counting Characters Using Conditions

Example 3: Count Vowels

count ← 0
FOR i ← 1 TO LENGTH(text)
  IF text[i] = "A" OR text[i] = "E" OR text[i] = "I" OR
     text[i] = "O" OR text[i] = "U" THEN
    count ← count + 1
  ENDIF
NEXT i

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Example 4: Count Digits

count ← 0
FOR i ← 1 TO LENGTH(text)
  IF text[i] >= "0" AND text[i] <= "9" THEN
    count ← count + 1
  ENDIF
NEXT i

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Example 5: Count Spaces

count ← 0
FOR i ← 1 TO LENGTH(text)
  IF text[i] = " " 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

Character-Based Validation (VERY COMMON)

Example 6: Validate First Character

IF code[1] = "A" THEN
  OUTPUT "Valid"
ELSE
  OUTPUT "Invalid"
ENDIF

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Example 7: Validate Last Character

IF text[LENGTH(text)] = "." THEN
  OUTPUT "Sentence complete"
ENDIF

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Example 8: Check If String Contains At Least One Digit

hasDigit ← FALSE

FOR i ← 1 TO LENGTH(text)
  IF text[i] >= "0" AND text[i] <= "9" THEN
    hasDigit ← TRUE
  ENDIF
NEXT i

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Example 9: Check If String Contains Only Letters

valid ← TRUE

FOR i ← 1 TO LENGTH(text)
  IF NOT (text[i] >= "A" AND text[i] <= "Z") AND
     NOT (text[i] >= "a" AND text[i] <= "z") THEN
    valid ← FALSE
  ENDIF
NEXT i

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Building New Strings Character By Character

Core Rule

• Strings are rebuilt, not edited
• Use concatenation (&)

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Example 10: Copy A String

newText ← ""
FOR i ← 1 TO LENGTH(text)
  newText ← newText & text[i]
NEXT i

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Example 11: Remove Spaces

result ← ""
FOR i ← 1 TO LENGTH(text)
  IF text[i] <> " " THEN
    result ← result & text[i]
  ENDIF
NEXT i

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Example 12: Remove Digits

result ← ""
FOR i ← 1 TO LENGTH(text)
  IF NOT (text[i] >= "0" AND text[i] <= "9") THEN
    result ← result & text[i]
  ENDIF
NEXT i

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Example 13: Extract Only Uppercase Letters

result ← ""
FOR i ← 1 TO LENGTH(text)
  IF text[i] >= "A" AND text[i] <= "Z" THEN
    result ← result & text[i]
  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

Transforming Characters (Logic-Based)

Example 14: Replace Spaces With Underscores

result ← ""
FOR i ← 1 TO LENGTH(text)
  IF text[i] = " " THEN
    result ← result & "_"
  ELSE
    result ← result & text[i]
  ENDIF
NEXT i

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Example 15: Mask Characters (Password Style)

masked ← ""
FOR i ← 1 TO LENGTH(password)
  masked ← masked & "*"
NEXT i

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Example 16: Reverse A String

reverse ← ""
FOR i ← LENGTH(text) TO 1 STEP -1
  reverse ← reverse & text[i]
NEXT i

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Position-Specific Character Logic

Example 17: Check Alternate Characters

FOR i ← 1 TO LENGTH(text) STEP 2
  OUTPUT text[i]
NEXT i

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Example 18: Ignore First And Last Characters

FOR i ← 2 TO LENGTH(text) - 1
  OUTPUT text[i]
NEXT i

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Character Processing With Flags

Example 19: Detect Consecutive Spaces

previousSpace ← FALSE

FOR i ← 1 TO LENGTH(text)
  IF text[i] = " " AND previousSpace = TRUE THEN
    OUTPUT "Double space detected"
  ENDIF

  IF text[i] = " " THEN
    previousSpace ← TRUE
  ELSE
    previousSpace ← FALSE
  ENDIF
NEXT i

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Character Processing Combined With Arrays

Example 20: Store Characters In Array

DECLARE chars : ARRAY[1..LENGTH(text)] OF STRING

FOR i ← 1 TO LENGTH(text)
  chars[i] ← text[i]
NEXT i

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Common Logic Errors In Character Processing

Error 1: Out-Of-Range Index

text[LENGTH(text) + 1]

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Error 2: Trying To Modify Directly

text[i] ← "A"

• Invalid
• Must rebuild string

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Error 3: Forgetting LENGTH In Loop Bound

FOR i ← 1 TO n

• n not defined
• Must use LENGTH(text)

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Error 4: Case Sensitivity Ignored

IF text[i] = "a"

• "A" will not match

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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

Dry-Run Technique For Character Processing

• Write the string with index numbers
• Track loop index
• Track output string as it grows
• Apply condition for each character
• Confirm concatenation order

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Exam-Ready Templates (Copy-Paste Safe)

Template 1: Character Traversal

FOR i ← 1 TO LENGTH(text)
  <process text[i]>
NEXT i

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Template 2: Filter Characters

result ← ""
FOR i ← 1 TO LENGTH(text)
  IF <condition> THEN
    result ← result & text[i]
  ENDIF
NEXT i

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Template 3: Transform Characters

result ← ""
FOR i ← 1 TO LENGTH(text)
  IF <condition> THEN
    result ← result & <new character>
  ELSE
    result ← result & text[i]
  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

Examiner Checklist For Full Marks

• Correct use of LENGTH
• Correct index bounds
• No direct character assignment
• Correct concatenation
• Case sensitivity handled
• No out-of-range access

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One-Line Rules To Memorise

• Strings are processed character by character
• Indexing starts at 1
• LENGTH controls loops
• Modification means rebuilding
• Every character must be checked deliberately