Unit 11.2A · Term 2

String Indexing & Slicing

Strings in Python are sequences of characters. You can access individual characters by index and extract parts of a string using slicing. These skills are essential for text processing and manipulation.

Learning Objectives

11.2.2.1 Use indexing in strings
11.2.2.2 Apply slicing operations on strings

Lesson Presentation

11.2A-lesson-11-string-indexing.pdf · Slides for classroom use

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Conceptual Anchor

The Train Car Analogy

A string is like a train where each car holds one character. Each car has a number (index), starting from 0. You can access any single car by its number (indexing) or request a sequence of cars (slicing). Negative indices count from the back of the train: car -1 is the last car.

Rules & Theory

String Indexing

# String:    P   y   t   h   o   n
# Positive:  0   1   2   3   4   5
# Negative: -6  -5  -4  -3  -2  -1

s = "Python"
print(s[0])     # P — first character
print(s[5])     # n — last character
print(s[-1])    # n — last character (negative index)
print(s[-6])    # P — first character (negative index)

print(len(s))   # 6 — length of string

# IndexError if out of range
# print(s[6])   # IndexError: string index out of range

String Slicing

# Syntax: string[start:stop:step]
# start — inclusive (default 0)
# stop  — exclusive (default len)
# step  — default 1

s = "Programming"

print(s[0:4])    # "Prog"     — from index 0 to 3
print(s[4:7])    # "ram"      — from index 4 to 6
print(s[:3])     # "Pro"      — from start to index 2
print(s[7:])     # "ming"     — from index 7 to end
print(s[:])      # "Programming"  — entire string (copy)

# With step
print(s[::2])    # "Pormig"   — every 2nd character
print(s[1::2])   # "rgamn"    — every 2nd, starting at 1

# Reverse a string
print(s[::-1])   # "gnimmargorP"

# Negative indices in slicing
print(s[-4:])    # "ming"     — last 4 characters
print(s[:-4])    # "Program"  — all except last 4

Strings are Immutable

You cannot change individual characters: s[0] = "p" raises TypeError. Instead, create a new string: s = "p" + s[1:].

Worked Examples

1 Extracting Parts of a Date

date = "2025-02-19"
year = date[:4]       # "2025"
month = date[5:7]     # "02"
day = date[8:]        # "19"
print(f"Day: {day}, Month: {month}, Year: {year}")
# Day: 19, Month: 02, Year: 2025

2 Palindrome Check

word = input("Enter a word: ").lower()
reversed_word = word[::-1]

if word == reversed_word:
    print(f"'{word}' is a palindrome!")
else:
    print(f"'{word}' is NOT a palindrome")

# "racecar" → "racecar" → palindrome ✓
# "hello"   → "olleh"   → NOT palindrome ✗

3 Processing Each Character

text = "Hello World"

# Method 1: iterate over characters
for char in text:
    print(char, end="-")
# H-e-l-l-o- -W-o-r-l-d-

# Method 2: iterate by index
for i in range(len(text)):
    print(f"Index {i}: '{text[i]}'")

# Count uppercase letters
upper_count = 0
for char in text:
    if char.isupper():
        upper_count += 1
print("Uppercase letters:", upper_count)  # 2

4 Masking Part of a String

email = "student@school.kz"
at_pos = email.index("@")

# Show first 2 chars, mask the rest before @
masked = email[:2] + "*" * (at_pos - 2) + email[at_pos:]
print(masked)   # "st*****@school.kz"

Pitfalls & Common Errors

Off-by-One in Slicing

s[1:4] gives characters at indices 1, 2, 3 — NOT 4. The stop index is always excluded.

Modifying Strings

s[0] = "X" raises TypeError. Strings are immutable. Create a new string instead.

Indexing with len()

A string of length 5 has indices 0-4. s[len(s)] is always an IndexError! Use s[len(s)-1] or s[-1] for the last character.

Pro-Tips for Exams

Quick Reference

s[-1] = last character (safer than s[len(s)-1])
s[::-1] = reverse a string (very common exam question!)
s[:n] = first n characters
s[n:] = everything from index n onwards
Slicing never causes IndexError (it just returns less)

Graded Tasks

Remember

For s = "Computer", what is s[0], s[-1], s[3], s[-3]?

Understand

Explain why s[1:5] returns 4 characters and not 5.

Apply

Write a program that extracts and prints the first name and last name from "LastName, FirstName".

Apply

Write a program to check if a word is a palindrome (case-insensitive) using slicing.

Analyze

What is the output of s = "abcdefgh"; print(s[1:7:2])? Trace step by step.

Create

Create a "string cipher" that takes every other character starting from index 0 and combines with every other character from index 1 reversed.

Self-Check Quiz

1. What does "Hello"[1] return?

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2. What does "Python"[::-1] return?

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3. What does "Hello"[1:4] return?

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4. Can you do "Hello"[0] = "J"?

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5. What does "Hello"[-2:] return?

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