String Manipulation

String manipulation means inspecting and transforming text data. Strings are one of the most commonly used data types, and being able to work with them is essential for GCSE Computer Science (AQA 3.2 / OCR J277 2.2).

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What Is a String?

A string is a sequence of characters. Each character has a position (index), starting from 0:

Index	0	1	2	3	4	5	6	7	8	9	10	11
Char	H	e	l	l	o		W	o	r	l	d	!

greeting = "Hello World!"
print(greeting[0])   # H
print(greeting[6])   # W
print(greeting[11])  # !

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String Length

To find the number of characters in a string:

• Pseudocode: LEN("Hello") returns 5
• Python: len("Hello") returns 5

Note: Spaces, digits, and punctuation all count as characters.

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Substrings (Slicing)

A substring is a portion of a string. You extract substrings using slicing.

Pseudocode (AQA)

word ← "Computer"
OUTPUT SUBSTRING(3, 4, word)
# Extracts 4 characters starting from position 3 → "pute"

AQA format: SUBSTRING(startPosition, numberOfCharacters, stringName)

Python

word = "Computer"
print(word[3:7])    # "pute" (index 3, 4, 5, 6)
print(word[:4])     # "Comp" (first 4 characters)
print(word[4:])     # "uter" (from index 4 to the end)
print(word[-3:])    # "ter" (last 3 characters)

Exam Tip: Remember that Python slicing uses [start:stop] where stop is exclusive — the character at the stop index is not included.

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Concatenation

Concatenation means joining strings together using the + operator:

first = "Hello"
second = "World"
combined = first + " " + second
print(combined)  # "Hello World"

In pseudocode, concatenation uses + as well.

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Case Conversion

Operation	Pseudocode	Python	Example
Uppercase	UPPER("hello")	"hello".upper()	"HELLO"
Lowercase	LOWER("HELLO")	"HELLO".lower()	"hello"

This is useful for case-insensitive comparisons:

user_input = input("Enter yes or no: ")
if user_input.lower() == "yes":
    print("You chose yes")

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ASCII Values

Every character has a numeric ASCII (American Standard Code for Information Interchange) value:

Character	ASCII Value
'A'	65
'Z'	90
'a'	97
'z'	122
'0'	48
'9'	57

Converting Between Characters and ASCII

• Pseudocode: ASC("A") returns 65; CHR(65) returns "A"
• Python: ord("A") returns 65; chr(65) returns "A"

print(ord("A"))  # 65
print(chr(97))   # "a"

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String Operations Map

flowchart LR
    A[String value] --> B[LEN length]
    A --> C[SUBSTRING slice]
    A --> D[UPPER / LOWER case]
    A --> E[Concatenate with +]
    A --> F[ASC / CHR ASCII]
    A --> G[split into list]
    A --> H[replace text]
    A --> I[find position]

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Other Useful String Operations

Checking Characters

"hello".isalpha()    # True (all letters)
"12345".isdigit()    # True (all digits)
"hello".isalnum()    # True (letters or digits)

Splitting a String

sentence = "the cat sat on the mat"
words = sentence.split(" ")
print(words)  # ["the", "cat", "sat", "on", "the", "mat"]
print(len(words))  # 6

Stripping Whitespace

text = "  hello  "
print(text.strip())   # "hello"
print(text.lstrip())  # "hello  "
print(text.rstrip())  # "  hello"

Finding a Substring

email = "user@example.com"
position = email.find("@")
print(position)  # 4

Replacing Parts of a String

message = "Hello World"
new_message = message.replace("World", "Python")
print(new_message)  # "Hello Python"

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Traversing a String Character by Character

word = "GCSE"
for char in word:
    print(char)

Or using indices:

for i in range(len(word)):
    print(f"Index {i}: {word[i]}")

Practical Example: Counting Vowels

text = "Computer Science"
vowels = "aeiouAEIOU"
count = 0
for char in text:
    if char in vowels:
        count += 1
print(f"Number of vowels: {count}")

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Common Exam Mistakes

1. Forgetting zero-based indexing — the first character is at index 0.
2. Confusing length with last index — a string of length 5 has indices 0 to 4.
3. Strings are immutable in Python — you cannot do word[0] = "X". Instead, create a new string.
4. Not converting case before comparing — "Yes" is not equal to "yes".

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Summary

• Strings are sequences of characters accessed by index (starting at 0).
• LEN returns the number of characters.
• Slicing extracts substrings.
• Concatenation (+) joins strings.
• UPPER and LOWER convert case.
• ASC / CHR convert between characters and ASCII codes.
• You can traverse a string character by character using a loop.

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Extended Worked Examples

Example 1: Palindrome detector

A palindrome reads the same forwards and backwards.

def is_palindrome(text):
    text = text.lower()
    reversed_text = ""
    for i in range(len(text) - 1, -1, -1):
        reversed_text = reversed_text + text[i]
    return text == reversed_text

print(is_palindrome("Racecar"))  # True
print(is_palindrome("Hello"))    # False

Trace for "Racecar":

Step	text	reversed_text after loop
lowered	"racecar"	"racecar"

Result: True.

Example 2: Caesar cipher shift using ASCII

def shift_letter(letter, shift):
    base = ord('A') if letter.isupper() else ord('a')
    offset = (ord(letter) - base + shift) % 26
    return chr(base + offset)

def caesar(text, shift):
    result = ""
    for ch in text:
        if ch.isalpha():
            result = result + shift_letter(ch, shift)
        else:
            result = result + ch
    return result

print(caesar("Hello, World!", 3))   # Khoor, Zruog!
print(caesar("Khoor, Zruog!", -3))  # Hello, World!

Example 3: Counting each vowel

text = input("Enter a sentence: ").lower()
vowels = ["a", "e", "i", "o", "u"]
counts = [0, 0, 0, 0, 0]

for ch in text:
    for i in range(len(vowels)):
        if ch == vowels[i]:
            counts[i] = counts[i] + 1