Module 1

1. Arrays & Strings

Foundation data structures - contiguous memory and character sequences

Theory

Arrays & Strings

Data Structure Module - Learn how data is stored and organized

Arrays and Strings are the building blocks of programming. Every other data structure builds on these concepts!

What You'll Learn

By the end of this module, you will be able to:

Understand how arrays store data in memory
Perform common array operations (access, insert, delete)
Work with 2D arrays (matrices)
Manipulate strings efficiently

1. Arrays - The Foundation

What is an Array? (Simple Explanation)

Think of an array like a row of lockers in a school:

Each locker has a number (index) - 0, 1, 2, 3...
Each locker can hold one item (value)
All lockers are side by side (contiguous memory)
You can go directly to locker #5 without checking lockers 0-4

Real-world examples:

Your contacts list on phone
A row of seats in a movie theater
Shopping cart items

Creating and Using Arrays

python

# Creating an array (list in Python)
fruits = ["apple", "banana", "cherry", "date"]
#         index 0   index 1   index 2   index 3

# ACCESS: Get item at index 2 - Super fast! O(1)
print(fruits[2])  # Output: "cherry"

# MODIFY: Change item at index 1
fruits[1] = "blueberry"
print(fruits)  # ["apple", "blueberry", "cherry", "date"]

# ADD: Append to end - Fast! O(1)
fruits.append("elderberry")

# INSERT: Add at specific position - Slow! O(n)
# Why slow? All elements after must shift right
fruits.insert(0, "avocado")  # Add at beginning

# DELETE: Remove by value - Slow! O(n)
fruits.remove("cherry")

# LENGTH: How many items? - Fast! O(1)
print(len(fruits))  # 5

Why Some Operations Are Slow?

Inserting at the beginning (O(n)): Imagine inserting a person at the front of a queue - everyone must step back!

Before: [10, 20, 30, 40]
Insert 5 at position 0:
Step 1: [10, 20, 30, 40, _]  ← Make space
Step 2: [_, 10, 20, 30, 40]  ← Shift all right
Step 3: [5, 10, 20, 30, 40]  ← Insert 5

Time Complexity Summary

Operation	Time	Why?
Access by index	O(1)	Direct jump to location
Search for value	O(n)	Must check each element
Add at end	O(1)	Just add after last item
Add at beginning	O(n)	Shift all elements right
Delete	O(n)	Shift all elements left

2. 2D Arrays (Matrices)

A 2D array is like a table with rows and columns - think of Excel spreadsheet!

python

# Creating a 3x3 matrix
matrix = [
    [1, 2, 3],    # Row 0
    [4, 5, 6],    # Row 1
    [7, 8, 9]     # Row 2
]
#   Col0 Col1 Col2

# Access element at row 1, column 2
print(matrix[1][2])  # Output: 6 (row 1, col 2)

# Traverse all elements (row by row)
for row in range(3):
    for col in range(3):
        print(matrix[row][col], end=" ")
    print()  # New line after each row

# Output:
# 1 2 3
# 4 5 6
# 7 8 9

Common uses:

Game boards (chess, tic-tac-toe)
Image pixels
Maps and grids

3. Strings - Array of Characters

A string is essentially an array of characters!

python

# Strings in Python
name = "Hello World"

# Access character (just like array!)
print(name[0])    # 'H'
print(name[6])    # 'W'

# Get length
print(len(name))  # 11 (includes space)

# Slicing - Get part of string
print(name[0:5])   # "Hello"
print(name[6:])    # "World"

# IMPORTANT: Strings are IMMUTABLE in Python
# This means you can't modify them directly
# name[0] = 'h'  ← This would cause ERROR!

# Instead, create a new string:
name_lower = name.lower()  # "hello world"

Common String Operations

python

text = "hello world"

# Find position of substring
position = text.find("world")  # Returns 6

# Replace text
new_text = text.replace("world", "python")  # "hello python"

# Split into list
words = text.split(" ")  # ["hello", "world"]

# Join list into string
joined = "-".join(words)  # "hello-world"

# Check start/end
text.startswith("hello")  # True
text.endswith("world")    # True

Key Takeaways

Concept	Array	String
Access	O(1) - Instant	O(1) - Instant
Modify	O(1) - Can change	O(n) - Create new
Insert	O(n) - Shift needed	O(n) - Create new
Search	O(n) - Check each	O(n) - Check each

Remember:

Arrays = Mutable (can change)
Strings = Immutable (cannot change in Python)

Common Interview Patterns

Two Pointers - Use left and right pointers
Sliding Window - Move a window through array
Frequency Count - Count occurrences using dict
Reverse - Swap from both ends

Practice

Test your understanding

📝

Practice Quiz

12 questions · 90s per question

Each question has a 90-second time limit. Unanswered questions will be auto-submitted when time runs out.