Arrays

Part of Data Structures & Algorithms

What is an Array?

An array is a fundamental data structure that stores elements in contiguous memory locations. Think of it like a row of boxes, where each box can hold one item and has a number (index) so you can find it quickly.

Real-world Analogy

Imagine a parking lot where each spot has a number. You can go directly to spot #5 without checking spots 1-4. That is how arrays work - you can jump to any position instantly!

Key Characteristics

Fixed vs Dynamic Size

Traditional arrays have fixed size (C, Java), while dynamic arrays (Python lists, JavaScript arrays) can grow automatically.

Same Data Type

In typed languages, arrays typically hold elements of the same type. Dynamic languages like Python allow mixed types.

Zero-Indexed

Most languages start counting from 0, so the first element is at index 0, the second at index 1, and so on.

Contiguous Memory

Elements are stored next to each other in memory, enabling fast access and efficient iteration.

Interactive Array Visualization

Experiment with array operations below. Insert, delete, and sort elements to see how the array changes.

64
[0]
34
[1]
25
[2]
12
[3]
22
[4]
11
[5]
90
[6]
Click on operations to see the array in action
Length
7
First
64
Last
90
Sum
258

Time & Space Complexity

OperationTime ComplexityExplanation
Access by IndexO(1)Direct calculation of memory address
Search (unsorted)O(n)May need to check every element
Insert at EndO(1)*Amortized for dynamic arrays
Insert at BeginningO(n)All elements must shift right
Delete from EndO(1)Just decrease the size
Delete from BeginningO(n)All elements must shift left

Space Complexity: O(n) where n is the number of elements

Code Examples

# Creating an array (list in Python)
numbers = [10, 20, 30, 40, 50]

# Accessing elements by index
first = numbers[0]    # 10
last = numbers[-1]    # 50

# Inserting an element
numbers.append(60)    # Add to end
numbers.insert(2, 25) # Insert at index 2

# Removing elements
numbers.pop()         # Remove last element
numbers.remove(30)    # Remove first occurrence of 30

# Iterating through array
for num in numbers:
    print(num)

Common Mistakes to Avoid

Off-by-One Errors

Remember that arrays are 0-indexed. An array of length 5 has indices 0 to 4, not 1 to 5.

Array Index Out of Bounds

Always check if an index is valid before accessing. Accessing index -1 or length will cause errors in most languages.

Modifying While Iterating

Avoid adding or removing elements while looping through an array. This can skip elements or cause infinite loops.

Interview Patterns

Two Pointer Technique

Use two pointers (start/end or slow/fast) to solve problems like finding pairs, reversing arrays, or detecting cycles.

Sliding Window

Maintain a window of elements for problems involving subarrays - maximum sum subarray, longest substring, etc.

Prefix Sum

Pre-calculate cumulative sums for quick range sum queries. Turns O(n) queries into O(1).

Test Your Knowledge

Arrays Quiz

Question 1 of 4

What is the time complexity of accessing an element by index in an array?

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