🔤 String Manipulation

Learn to work with text data by treating strings as arrays of characters.

What you'll learn:

Why strings are called "strings" (they're arrays of characters!)
Accessing individual characters using index notation
String slicing to extract substrings
Concatenation - joining strings together
Useful string methods: len(), upper(), lower(), find()

Why Are They Called "Strings"?

In programming, we call text data a "string" rather than "text" because it's actually a sequence of characters strung together - like beads on a string!

The word "PYTHON" as a string:

Each character has an index (position number), just like in an array!

Strings ARE Arrays!

Because strings are arrays of characters, you can use the same techniques you learned with arrays:

Arrays

numbers = [10, 20, 30]
numbers[0] → 10
len(numbers) → 3

Strings

word = "CAT"
word[0] → "C"
len(word) → 3

String Operations

Accessing Characters

Use square brackets with an index to get a single character.

word = "HELLO"
print(word[0])   # H (first character)
print(word[4])   # O (fifth character)
print(word[-1])  # O (last character)
print(word[-2])  # L (second to last)

String Slicing

Extract a substring using [start:end]. End is NOT included!

text = "PYTHON"
print(text[0:3])  # PYT (positions 0,1,2)
print(text[:3])   # PYT (from start to 2)
print(text[3:])   # HON (from 3 to end)
print(text[-3:])  # HON (last 3 chars)

Concatenation

Join strings together using the + operator.

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

name = "Alice"
greeting = "Hi, " + name + "!"
print(greeting) # Hi, Alice!

String Length

Use len() to find how many characters are in a string.

word = "Python"
length = len(word)
print(length)     # 6

# Useful for loops!
for i in range(len(word)):
    print(word[i])

Useful String Methods

Method	What it does	Example	Result
`len(s)`	Returns the length (number of characters)	`len("Hello")`	5
`s.upper()`	Converts to UPPERCASE	`"hello".upper()`	"HELLO"
`s.lower()`	Converts to lowercase	`"HELLO".lower()`	"hello"
`s.find(x)`	Finds position of substring (-1 if not found)	`"hello".find("l")`	2
`s.replace(a, b)`	Replaces all occurrences of a with b	`"hello".replace("l", "x")`	"hexxo"
`s.strip()`	Removes whitespace from start and end	`" hi ".strip()`	"hi"
`x in s`	Checks if substring exists (True/False)	`"el" in "hello"`	True

String Slicing Guide

text = "COMPUTING"

-9-8-7-6-5-4-3-2-1

text[0] → "C" First character

text[-1] → "G" Last character

text[:3] → "COM" First 3 characters

text[-3:] → "ING" Last 3 characters

text[2:6] → "MPUT" From index 2 to 5

Learning Path

Follow these activities in order.

🔮

1. Predict: String Output

Predict what string operations will produce.

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📖

2. Study: Worked Example

See how to manipulate strings step by step.

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📊

3. Investigate: Trace String Operations

Track how strings change through operations.

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🧩

4. Modify: Arrange the Code

Put code blocks in order for string manipulation.

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5. Test: Knowledge Quiz

Check your understanding of string manipulation.

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Key Vocabulary

string

A sequence of characters - like an array of letters. Called "string" because characters are strung together.

index

The position of a character in a string. Starts at 0, not 1!

slicing

Extracting a portion of a string using [start:end] notation.

concatenation

Joining strings together using the + operator.

substring

A smaller string extracted from a larger string.

immutable

Cannot be changed. Strings are immutable - you create new strings instead of modifying existing ones.

Quick Reference

string_manipulation.py

# Strings are arrays of characters
text = "Python"

# Access single characters
first = text[0]       # "P"
last = text[-1]       # "n"

# Slicing (extracting substrings)
start = text[:3]      # "Pyt" (first 3)
end = text[-3:]       # "hon" (last 3)
middle = text[1:4]    # "yth" (positions 1,2,3)

# Concatenation
greeting = "Hello" + " " + "World"  # "Hello World"

# Length
length = len(text)    # 6

# Case conversion
upper = text.upper()  # "PYTHON"
lower = text.lower()  # "python"

# Finding substrings
pos = text.find("th") # 2 (position where "th" starts)
exists = "th" in text # True

# Loop through characters (just like an array!)
for char in text:
    print(char)