Strings — Basics & Operations

Why read this? Text processing is one of the most common programming tasks — reading names, parsing documents, validating input, generating reports. Strings are how Python handles text, and mastering them is essential for almost everything you'll write.

🎯 Learning Objectives

1. Create strings with single, double, and triple quotes
2. Access individual characters using indexing (positive and negative)
3. Extract substrings using slicing with start, stop, and step
4. Use common string methods: upper(), lower(), strip(), replace(), find()
5. Understand string immutability and its implications

📋 Prerequisites

• Variables & Data Types — Understanding string type basics.
• Operators & Expressions — Useful but not essential.

📖 Core Content

5.1 What Problem Do Strings Solve?

5.2 Creating Strings

python
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# runnable
# Single quotes
s1 = 'Hello'
print(s1)
# Double quotes (most common)
s2 = "World"
print(s2)
# Triple quotes (multi-line strings)
s3 = """This is a
multi-line
string!"""
print(s3)
# Triple quotes with single quotes too
s4 = '''This also
works'''
print(s4)

pseudo
Hello
World
This is a
multi-line
string!
This also
works

python
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print("It's a sunny day")     # Double-quoted string containing apostrophe
print('She said "Hello"')     # Single-quoted string containing double quotes

5.3 String Indexing — Getting Individual Characters

python
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# runnable
word = "Python"
#       012345   (positive indices)
#      -6-5-4-3-2-1   (negative indices)
print(word[0])    # P (first character)
print(word[1])    # y (second character)
print(word[5])    # n (sixth character)
print(word[-1])   # n (last character)
print(word[-2])   # o (second-to-last)
print(word[-6])   # P (first character, using negative)

pseudo
P
y
n
n
o
P

5.4 String Slicing — Extracting Substrings

• start: Where to begin (inclusive). Default: beginning.
• stop: Where to end (exclusive — the character at stop is NOT included). Default: end.
• step: How many characters to skip. Default: 1.

python
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# runnable
word = "Python Programming"
# Basic slicing
print(word[0:6])       # "Python"  (characters 0 to 5)
print(word[7:18])      # "Programming" (characters 7 to 17)
print(word[:6])        # "Python"  (from start to index 6)
print(word[7:])        # "Programming" (from index 7 to end)
print(word[:])         # "Python Programming" (entire string)
print(word[-11:])      # "Programming" (using negative index)
# With step
print(word[0:6:2])     # "Pto"  (index 0, 2, 4)
print(word[::-1])      # "gnimmargorP nohtyP" (reversed!)
print(word[::2])       # "Pto rgamn" (every other character)

pseudo
Python
Programming
Python
Programming
Python Programming
Programming
Pto
gnimmargorP nohtyP
Pto rgamn

text
Index:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17
        P  y  t  h  o  n     P  r  o  g  r  a  m  m  i  n  g
       [0:6] → Python
       [7: ] → Programming
       [   :6] → Python
       [   : :2] → P t o   r g a m n

5.5 Slicing Rules and Edge Cases

python
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# runnable
text = "Python"
# start beyond end returns empty string
print(text[10:15])     # "" (empty)
# stop before start returns empty
print(text[4:1])       # "" (empty)
# Negative step naturally reverses
print(text[4:1:-1])    # "oht" (indices 4,3,2)
# Omitting both with step -1 reverses
print(text[::-1])      # "nohtyP"

pseudo
                     (empty line)
                     (empty line)
oht
nohtyP

5.6 Strings Are Immutable

python
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# runnable
word = "Python"
# word[0] = "J"   # This would cause an error!
# Instead, create a NEW string:
new_word = "J" + word[1:]
print(new_word)      # "Jython"

5.7 Common String Methods

python
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# runnable
text = "  Hello, Python World!  "
print(text.upper())           # "  HELLO, PYTHON WORLD!  "
print(text.lower())           # "  hello, python world!  "
print(text.strip())           # "Hello, Python World!" (removes leading/trailing spaces)
print(text.replace("World", "Universe"))  # "  Hello, Python Universe!  "
print(text.find("Python"))    # 9 (index where "Python" starts)
print(text.find("Java"))      # -1 (not found)
print(text.count("o"))        # 3 (how many times 'o' appears)
print(text.startswith("  He")) # True
print(text.endswith("!  "))   # True

pseudo
  HELLO, PYTHON WORLD!
  hello, python world!
Hello, Python World!
  Hello, Python Universe!
9
-1
3
True
True

5.8 The len() Function

python
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# runnable
word = "Python"
print(len(word))          # 6
sentence = "Hello World"
print(len(sentence))      # 11 (includes the space)
empty = ""
print(len(empty))         # 0

pseudo
6
11
0

5.9 String Concatenation and Repetition

python
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# runnable
# Concatenation (joining)
first = "Hello"
second = "World"
message = first + " " + second
print(message)            # "Hello World"
# Repetition
laugh = "Ha" * 3
print(laugh)              # "HaHaHa"
# Useful for separators
print("=" * 30)           # "=============================="

pseudo
Hello World
HaHaHa
==============================

5.10 Worked Example 1: Name Formatter

python
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# runnable
name = input("Enter your full name: ")
# Clean up extra spaces
name = name.strip()
# Find the space
space_index = name.find(" ")
first_name = name[:space_index]
last_name = name[space_index + 1:]
print(f"First name: {first_name}")
print(f"Last name: {last_name}")
print(f"Initials: {first_name[0]}.{last_name[0]}.")
print(f"Name capitalized: {name.title()}")

pseudo
Enter your full name:   alice smith
First name: alice
Last name: smith
Initials: a.s.
Name capitalized: Alice Smith

5.11 Worked Example 2: Palindrome Checker

python
Run Inline
# runnable
word = input("Enter a word: ").strip().lower()
reversed_word = word[::-1]
if word == reversed_word:
    print(f"'{word}' is a palindrome!")
else:
    print(f"'{word}' is NOT a palindrome.")
    print(f"Reversed: {reversed_word}")

pseudo
Enter a word: Racecar
'racecar' is a palindrome!

5.12 Worked Example 3: Counting Vowels

python
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# runnable
text = input("Enter text: ").lower()
vowels = "aeiou"
count = 0
for ch in text:
    if ch in vowels:
        count += 1
print(f"Number of vowels: {count}")

pseudo
Enter text: Beautiful Python
Number of vowels: 7

5.13 Worked Example 4: Email Extractor

python
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# runnable
email = "  [email protected]  "
email = email.strip()
# Find @ position
at_pos = email.find("@")
username = email[:at_pos]
domain = email[at_pos + 1:]
print(f"Email: {email}")
print(f"Username: {username}")
print(f"Domain: {domain}")
# Extract top-level domain
dot_pos = domain.rfind(".")
tld = domain[dot_pos + 1:]
print(f"TLD: {tld}")

pseudo
Email: user@example.com
Username: user
Domain: example.com
TLD: com

5.14 Worked Example 5: Caesar Cipher (Simple Encryption)

python
Run Inline
# runnable
text = input("Enter text: ").upper()
shift = int(input("Shift amount: "))
result = ""
for ch in text:
    if ch.isalpha():
        # Shift character, wrapping around alphabet
        new_pos = (ord(ch) - ord('A') + shift) % 26
        result += chr(ord('A') + new_pos)
    else:
        result += ch
print(f"Encrypted: {result}")

pseudo
Enter text: Hello World
Shift amount: 3
Encrypted: KHOOR ZRUOG

📐 Key Concepts Reference

⚠️ Common Pitfalls

Pitfall 1: Index Out of Range

Pitfall 2: Forgetting Strings Are Immutable

Pitfall 3: Confusing find() Return Value with Boolean

Pitfall 4: Slicing Stop Value Is Exclusive

📝 Practice Questions

Q1: What is the output?

python
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text = "Programming"
print(text[0:6])
print(text[-7:])

Answer:

pseudo
Progra
amming

• text[0:6] → indices 0,1,2,3,4,5 → "Progra"
• text[-7:] → last 7 characters → "amming" Q2: What does this return?

python
Run Inline
"Python"[::-1]

Answer:

pseudo
"nohtyP"

The [::-1] slice means: start at the end, go backwards by 1 step, so the entire string is reversed. Q3: Why does this code fail? Fix it.

python
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name = "Alice"
name[0] = "B"

Answer: Error: TypeError: 'str' object does not support item assignment Why: Strings are immutable — you can't change a character in place. Fix: name = "B" + name[1:] → "Blice" Q4: What does this output?

python
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s = "Hello World"
print(s.find("o"))
print(s.find("o", 5))
print(s.rfind("o"))

Answer:

pseudo
4
7
7

• s.find("o") — finds first 'o' at index 4
• s.find("o", 5) — starts searching from index 5, finds 'o' at index 7
• s.rfind("o") — reverse find, finds last 'o' at index 7 Q5: Write code to extract the file extension from a filename like "document.pdf".

Answer:

python
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# runnable
filename = "document.pdf"
dot_pos = filename.find(".")
extension = filename[dot_pos + 1:]
print(f"Extension: {extension}")

# For multiple dots (e.g., "my.file.name.txt"):
ext = filename.split(".")[-1]  # we'll learn split() later
print(f"Extension: {ext}")

Q6: What is the output of this code?

python
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text = "  spaces  "
print(text.strip())
print(text.lstrip())
print(text.rstrip())

Answer:

pseudo
spaces
spaces
  spaces

• strip() removes leading AND trailing spaces
• lstrip() removes only leading (left) spaces
• rstrip() removes only trailing (right) spaces Q7: What does len("") return?

Answer: 0

An empty string has no characters, so its length is 0. Q8: Write code to check if a string starts with a vowel.

Answer:

python
Run Inline
# runnable
word = input("Enter a word: ").strip().lower()
if word[0] in "aeiou":
    print(f"'{word}' starts with a vowel.")
else:
    print(f"'{word}' starts with a consonant.")

Q9: What is the output?

python
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s = "abracadabra"
print(s.count("a"))
print(s.replace("a", "o"))

Answer:

pseudo
5
obrocodobro

• count("a") counts how many times 'a' appears → 5
• replace("a", "o") replaces ALL occurrences of 'a' with 'o' Q10: Write code to reverse each word in a two-word string.

Answer:

python
Run Inline
# runnable
text = "Hello World"
space_pos = text.find(" ")
first = text[:space_pos]
second = text[space_pos + 1:]
reversed_text = first[::-1] + " " + second[::-1]
print(reversed_text)  # "olleH dlroW"

🔗 Cross-References

• Next Topic: Conditionals — if/elif/else
• Previous Topic: Operators & Expressions
• BSCS1001 Computational Thinking: String operations connect to text processing in computational problems.
• Reference: Python for Everybody, Chapter 6 — "Strings"
• Video: L8: Introduction to strings & string slicing, L11: More on strings Join Discord Previous4. Operators & ExpressionsNext6. Conditionals