DSA Bite #4 - Arrays

👋 Hi there, coding explorer!
Welcome back! This week, we’re diving into Arrays - one of the most fundamental yet powerful data structures in programming. Whether you’re prepping for coding interviews or building real-world apps, mastering arrays will give you a huge edge.

We’ll break everything into easy, bite-sized parts - from basic concepts to pro-level tricks.

Part 1: What is an Array?

An array is a collection of elements stored in contiguous memory and accessed using an index.
Think of it as a row of lockers: each locker holds a value, and you can find it by its number (index).

Example

nums = [10, 20, 30, 40]
print(nums[0])  # 10

let nums = [10, 20, 30, 40];
console.log(nums[0]); // 10

Key Array Terms

Term	Description	Example
Index	Position of an element (starts from 0)	nums[2] → 30
Length	Number of elements	len(nums) / nums.length
Element	Individual item in the array	20 in [10,20,30]
Contiguous Memory	All elements stored next to each other	Fast random access (O(1))

Part 2: Array Operations

1️⃣ Traversal

Iterating through every element.

nums = [1, 2, 3]
for n in nums:
    print(n)

let nums = [1, 2, 3];
for (let n of nums) console.log(n);

2️⃣ Insertion

Add elements at a specific position.

nums.insert(1, 99)  # insert 99 at index 1
print(nums)

nums.splice(1, 0, 99);
console.log(nums);

3️⃣ Deletion

Remove elements by index or value.

nums.remove(99)  # by value
del nums[0]      # by index

nums.splice(1, 1);  // remove element at index 1

4️⃣ Searching

Find if an element exists.

if 20 in nums:
    print("Found!")

if (nums.includes(20)) console.log("Found!");

5️⃣ Sorting

Arrange elements in order.

nums = [5, 1, 4]
nums.sort()
print(nums)  # [1,4,5]

nums = [5, 1, 4];
nums.sort((a, b) => a - b);
console.log(nums); // [1,4,5]

Part 3: Common Interview Patterns

1️⃣ Find Maximum/Minimum Element

nums = [4, 7, 1, 9]
max_num = max(nums)
print(max_num)  # 9

let nums = [4, 7, 1, 9];
console.log(Math.max(...nums)); // 9

2️⃣ Reverse an Array

nums = [1, 2, 3, 4]
print(nums[::-1])  # [4,3,2,1]

let nums = [1, 2, 3, 4];
console.log(nums.reverse());

3️⃣ Sum of All Elements

nums = [1, 2, 3]
print(sum(nums))  # 6

let nums = [1, 2, 3];
let total = nums.reduce((a, b) => a + b, 0);
console.log(total); // 6

4️⃣ Find Second Largest Number

nums = [10, 5, 8, 20]
nums.sort()
print(nums[-2])  # 10

let nums = [10, 5, 8, 20];
nums.sort((a,b)=>a-b);
console.log(nums[nums.length-2]); // 10

Part 4: Advanced Patterns

🔹 Two Pointers

Useful when searching pairs in sorted arrays.

Example: Find if two numbers sum to target.

nums = [1,2,4,7,10]
target = 9
left, right = 0, len(nums)-1
while left < right:
    s = nums[left] + nums[right]
    if s == target:
        print(nums[left], nums[right])
        break
    elif s < target:
        left += 1
    else:
        right -= 1

let nums = [1,2,4,7,10];
let target = 9;
let left = 0, right = nums.length-1;
while (left < right) {
  let sum = nums[left] + nums[right];
  if (sum === target) { console.log(nums[left], nums[right]); break; }
  sum < target ? left++ : right--;
}

🔹 Sliding Window

Used for subarray problems like "maximum sum of K elements."

Example (sum of 3 consecutive numbers)

nums = [2,1,5,1,3,2]
k = 3
max_sum = 0
window_sum = sum(nums[:k])
for i in range(k, len(nums)):
    window_sum += nums[i] - nums[i-k]
    max_sum = max(max_sum, window_sum)
print(max_sum)

let nums = [2,1,5,1,3,2];
let k = 3;
let maxSum = 0, windowSum = 0;

for(let i=0;i<k;i++) windowSum+=nums[i];
maxSum=windowSum;

for(let i=k;i<nums.length;i++){
  windowSum+=nums[i]-nums[i-k];
  maxSum=Math.max(maxSum,windowSum);
}
console.log(maxSum);

🔹 Prefix Sum

Used for quick range sum queries.

nums = [1,2,3,4]
prefix = [0]
for n in nums:
    prefix.append(prefix[-1] + n)
print(prefix)  # [0,1,3,6,10]

let nums = [1, 2, 3, 4];
let prefix = [0];

for (let n of nums) {
    prefix.push(prefix[prefix.length - 1] + n);
}

console.log(prefix); // [0, 1, 3, 6, 10]

🔹 2D Arrays (Matrices)

matrix = [[1,2,3],[4,5,6],[7,8,9]]
for row in matrix:
    print(row)

let matrix = [[1,2,3],[4,5,6],[7,8,9]];
matrix.forEach(row => console.log(row));

Part 5: Mini Challenges

1️⃣ Sum of Even Numbers
Write a function to sum only even numbers in an array.

2️⃣ Reverse an Array (without using reverse())
Use a loop and swap elements manually.

3️⃣ Find the Maximum Product of Two Elements.

4️⃣ Matrix Challenge: Print the diagonal elements of a 3×3 matrix.

Expert Challenge

Find the maximum sum of any subarray using Kadane’s Algorithm.

nums = [-2,1,-3,4,-1,2,1,-5,4]
max_sum = cur_sum = nums[0]
for n in nums[1:]:
    cur_sum = max(n, cur_sum + n)
    max_sum = max(max_sum, cur_sum)
print(max_sum)  # 6

let nums = [-2,1,-3,4,-1,2,1,-5,4];
let maxSum = nums[0], curSum = nums[0];
for(let i=1;i<nums.length;i++){
  curSum = Math.max(nums[i], curSum + nums[i]);
  maxSum = Math.max(maxSum, curSum);
}
console.log(maxSum); // 6

AI Tip

Ask AI:

❝

“Explain and generate examples of Two Pointer and Sliding Window techniques on arrays.”

Use it to generate variations and practice multiple array patterns quickly.

✅ Sign-off

That’s your Arrays Bite - Beginner to Expert! 🎉
You’ve now learned how arrays work, how to loop, modify, search, and optimize them using key patterns like two pointers and sliding windows.

Next time, we’ll dive into Strings - one of the most frequently tested interview topics!