String and Array Manipulation

This lesson covers string operations and array manipulation for the OCR A-Level Computer Science (H446) specification. These are essential programming skills that appear frequently in exam questions and practical programming tasks.

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String Operations

A string is a sequence of characters. Strings are immutable in Python -- once created, individual characters cannot be changed.

Common String Operations

Operation	Python	Pseudocode	Example
Length	len(s)	LENGTH(s)	len("Hello") returns 5
Concatenation	s1 + s2	s1 + s2	"Hi" + " there" returns "Hi there"
Substring	s[start:end]	SUBSTRING(s, start, length)	"Hello"[1:4] returns "ell"
Character at index	s[i]	s[i]	"Hello"[0] returns "H"
Find/search	s.find(sub)	FIND(s, sub)	"Hello".find("ll") returns 2
Upper case	s.upper()	UPPER(s)	"hello".upper() returns "HELLO"
Lower case	s.lower()	LOWER(s)	"HELLO".lower() returns "hello"
Strip whitespace	s.strip()	TRIM(s)	" hi ".strip() returns "hi"
Replace	s.replace(old, new)	--	"cat".replace("c", "b") returns "bat"
Split	s.split(delim)	--	"a,b,c".split(",") returns ["a", "b", "c"]

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String Traversal

# Character by character
text = "Hello"
for char in text:
    print(char)

# With index
for i in range(len(text)):
    print(f"Index {i}: {text[i]}")

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ASCII and Character Conversion

Every character has a numeric ASCII code. Python provides functions to convert between characters and their codes:

Function	Description	Example
ord(c)	Character to ASCII code	ord("A") returns 65
chr(n)	ASCII code to character	chr(65) returns "A"

Useful ASCII Ranges

Characters	Range
Digits 0-9	48-57
Uppercase A-Z	65-90
Lowercase a-z	97-122

Example: Caesar Cipher

def caesar_encrypt(text: str, shift: int) -> str:
    result = ""
    for char in text:
        if char.isalpha():
            base = ord("A") if char.isupper() else ord("a")
            shifted = (ord(char) - base + shift) % 26
            result += chr(base + shifted)
        else:
            result += char
    return result

print(caesar_encrypt("Hello World", 3))  # Khoor Zruog

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String Searching

Linear Search in a String

def count_vowels(text: str) -> int:
    vowels = "aeiouAEIOU"
    count = 0
    for char in text:
        if char in vowels:
            count += 1
    return count

print(count_vowels("Hello World"))  # 3

Checking String Properties

text = "Hello123"

print(text.isalpha())    # False (contains digits)
print(text.isdigit())    # False (contains letters)
print(text.isalnum())    # True (all alphanumeric)
print(text.startswith("He"))  # True
print(text.endswith("23"))    # True

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String Conversion

# Number to string
num = 42
text = str(num)       # "42"

# String to number
text = "3.14"
num = float(text)     # 3.14
integer = int("42")   # 42

# String to list of characters
chars = list("Hello")  # ['H', 'e', 'l', 'l', 'o']

# List to string
joined = "".join(chars)  # "Hello"
csv_line = ",".join(["Alice", "17", "A"])  # "Alice,17,A"

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Arrays (Lists in Python)

An array is an ordered collection of elements, typically of the same type. In Python, arrays are implemented as lists.

Creating and Accessing Arrays

# Creating arrays
numbers = [10, 20, 30, 40, 50]
names = ["Alice", "Bob", "Charlie"]
empty = []

# Accessing elements (0-indexed)
print(numbers[0])    # 10 (first element)
print(numbers[-1])   # 50 (last element)
print(numbers[1:3])  # [20, 30] (slice)

# Modifying elements
numbers[2] = 35      # [10, 20, 35, 40, 50]

Common Array Operations