6.3.3 String manipulation

String Manipulation Operations

String manipulation is a core programming skill — programs frequently need to process text input, format output, validate data and extract sub-parts of strings. The four main operations required at GCSE are: length, position (indexing), substrings and case conversion.

Strings are zero-indexed: the first character is at index 0, the second at index 1, and so on.

Operation Reference

text = "Hello, World!"

# Length — number of characters including spaces and punctuation
print(len(text))              # 13

# Position (indexing) — zero-indexed
print(text[0])                # H  (first character)
print(text[7])                # W
print(text[-1])               # !  (last character, negative indexing)

# Substring (slicing) — text[start:stop]  stop is EXCLUSIVE
print(text[0:5])              # Hello  (indices 0,1,2,3,4)
print(text[7:12])             # World
print(text[7:])               # World!  (from index 7 to end)

# Case conversion
print(text.upper())           # HELLO, WORLD!
print(text.lower())           # hello, world!

# Find position of a substring (returns -1 if not found)
print(text.find("World"))     # 7  (index where "World" starts)

# Check start/end
print(text.startswith("He"))  # True
print(text.endswith("!"))     # True

string text = "Hello, World!";

// Length
Console.WriteLine(text.Length);                   // 13

// Position (indexing) — zero-indexed
Console.WriteLine(text[0]);                       // H
Console.WriteLine(text[7]);                       // W

// Substring — Substring(startIndex, length)
Console.WriteLine(text.Substring(0, 5));          // Hello
Console.WriteLine(text.Substring(7, 5));          // World

// Case conversion
Console.WriteLine(text.ToUpper());               // HELLO, WORLD!
Console.WriteLine(text.ToLower());               // hello, world!

// Find
Console.WriteLine(text.IndexOf("World"));        // 7

Dim text As String = "Hello, World!"

' Length
Console.WriteLine(text.Length)                    ' 13

' Position (indexing) — zero-indexed
Console.WriteLine(text(0))                        ' H
Console.WriteLine(text(7))                        ' W

' Substring — Substring(startIndex, length)
Console.WriteLine(text.Substring(0, 5))           ' Hello
Console.WriteLine(text.Substring(7, 5))           ' World

' Case conversion
Console.WriteLine(text.ToUpper())                ' HELLO, WORLD!
Console.WriteLine(text.ToLower())                ' hello, world!

' Find
Console.WriteLine(text.IndexOf("World"))          ' 7

Practical Examples

# Generate a username: first 3 letters of first name + last name in lower case
first = input("First name: ")
last  = input("Last name: ")

username = first[0:3].lower() + last.lower()
print("Username:", username)

# Example: "Alice" + "Smith" -> "alismith"

# Extract initials from a full name
full_name = input("Enter full name (first last): ")
parts = full_name.split()     # split on spaces: ["Alice", "Smith"]
initials = parts[0][0].upper() + parts[1][0].upper()
print("Initials:", initials)  # AS

Key Takeaways

Strings are zero-indexed: first character at index 0.
len(s) — number of characters. s[i] — character at index i.
Python slicing: s[start:stop] — stop index is exclusive.
C#/VB.NET: Substring(startIndex, length) — second argument is length, not stop index.
.upper() / .lower() return a new string — they do not modify the original.