Array Rotation Techniques in DSA Typescript - Build from Scratch

Array Rotation Techniques

📖 Scenario: You are working on a simple app that manages a list of daily tasks. Sometimes, you want to rotate the tasks so that the first task moves to the end, and the others shift forward. This helps you change the order of tasks easily.

🎯 Goal: Build a small program that rotates an array of tasks to the left by a given number of positions using array rotation techniques.

📋 What You'll Learn

Create an array called tasks with exact string values.

Create a variable called rotateBy to specify how many positions to rotate.

Write a function called rotateArrayLeft that rotates the array to the left by rotateBy positions.

Print the rotated array.

💡 Why This Matters

🌍 Real World

Rotating arrays is useful in task scheduling, game development, and data processing where order changes dynamically.

💼 Career

Understanding array rotation helps in coding interviews and real-world programming tasks involving data manipulation.

1

Create the tasks array

Create an array called tasks with these exact string values in order: 'email', 'call', 'meeting', 'lunch', 'report'.

DSA Typescript

// Create the tasks array with the given values
// Your code here

Hint

Use square brackets [] to create an array and separate values with commas.

2

Set the rotation count

Create a variable called rotateBy and set it to the number 2 to specify how many positions to rotate the array to the left.

DSA Typescript

const tasks = ['email', 'call', 'meeting', 'lunch', 'report'];
// Create rotateBy variable and set it to 2
// Your code here

Hint

Use const rotateBy = 2; to create the variable.

3

Write the rotation function

Write a function called rotateArrayLeft that takes two parameters: arr (an array of strings) and positions (a number). The function should return a new array rotated to the left by positions places. Use array slicing and concatenation inside the function.

DSA Typescript

const tasks = ['email', 'call', 'meeting', 'lunch', 'report'];
const rotateBy = 2;

// Write function rotateArrayLeft(arr: string[], positions: number): string[] here
// Your code here

Hint

Use arr.slice() to split the array and .concat() to join parts.

4

Print the rotated array

Call the function rotateArrayLeft with tasks and rotateBy as arguments. Print the returned rotated array using console.log.

DSA Typescript

const tasks = ['email', 'call', 'meeting', 'lunch', 'report'];
const rotateBy = 2;

function rotateArrayLeft(arr: string[], positions: number): string[] {
    const n = arr.length;
    const pos = positions % n;
    return arr.slice(pos).concat(arr.slice(0, pos));
}

// Call rotateArrayLeft with tasks and rotateBy, then print the result
// Your code here

Hint

Use console.log(rotatedTasks); to print the rotated array.

Practice

1. What does rotating an array to the right by 1 position mean?
Example: [1, 2, 3, 4] rotated right by 1 becomes?

easy

A. [4, 1, 2, 3]

B. [2, 3, 4, 1]

C. [1, 2, 3, 4]

D. [3, 4, 1, 2]

Solution

1. Step 1: Understand right rotation

   Right rotation by 1 means the last element moves to the front, shifting others right.

2. Step 2: Apply to example array

   Last element 4 moves to front, rest shift right: [4, 1, 2, 3].

3. Final Answer:

   [4, 1, 2, 3] -> Option A

4. Quick Check:

   Right rotate by 1 = last element front [OK]

Hint: Right rotate by 1 means last element moves front [OK]

Common Mistakes:

• Confusing right rotation with left rotation
• Not moving the last element to front
• Shifting elements incorrectly

2. Which of the following TypeScript code snippets correctly rotates an array arr to the left by 1 position?

easy

A. arr.push(arr.shift()!)

B. arr.unshift(arr.pop()!)

C. arr.pop(); arr.unshift(arr.pop()!)

D. arr.shift(); arr.push(arr.shift()!)

Solution

1. Step 1: Understand left rotation by 1

   Left rotation by 1 means first element moves to the end.

2. Step 2: Check code snippets

   arr.push(arr.shift()!) uses arr.shift() to remove first element and arr.push() to add it at end, correct for left rotation.

3. Final Answer:

   arr.push(arr.shift()!) -> Option A

4. Quick Check:

   Left rotate = shift then push [OK]

Hint: Left rotate by 1 = remove first, add to end [OK]

Common Mistakes:

• Using pop instead of shift for left rotation
• Using unshift with pop for left rotation
• Calling shift or pop twice incorrectly

3. What is the output of the following TypeScript code?

const arr = [10, 20, 30, 40, 50];
const k = 2;
const rotated = arr.slice(-k).concat(arr.slice(0, arr.length - k));
console.log(rotated);

medium

A. [50, 10, 20, 30, 40]

B. [40, 50, 10, 20, 30]

C. [20, 30, 40, 50, 10]

D. [30, 40, 50, 10, 20]

Solution

1. Step 1: Understand slicing for rotation

   arr.slice(-k) takes last 2 elements: [40, 50]. arr.slice(0, arr.length - k) takes first 3 elements: [10, 20, 30].

2. Step 2: Concatenate slices

   Concatenating gives [40, 50, 10, 20, 30], which is right rotation by 2.

3. Final Answer:

   [40, 50, 10, 20, 30] -> Option B

4. Quick Check:

   Right rotate by 2 = last 2 front [OK]

Hint: Right rotate by k = last k elements + rest [OK]

Common Mistakes:

• Using slice with wrong indices
• Confusing left and right rotation
• Not concatenating slices in correct order

4. Find the error in this TypeScript function that rotates an array to the left by k positions:

function rotateLeft(arr: number[], k: number): number[] {
  for (let i = 0; i < k; i++) {
    arr.push(arr.shift());
  }
  return arr;
}

medium

A. arr.push() should be arr.unshift()

B. Loop should run from i = 1 to k

C. arr.shift() can return undefined, causing push(undefined)

D. Function should return void, not number[]

Solution

1. Step 1: Check arr.shift() return type

   arr.shift() returns element or undefined if array empty. Here, no check for undefined.

2. Step 2: Effect on arr.push()

   If arr.shift() returns undefined, arr.push(undefined) adds undefined to array, causing incorrect result or runtime issues.

3. Final Answer:

   arr.shift() can return undefined, causing push(undefined) -> Option C

4. Quick Check:

   shift may be undefined, must check [OK]

Hint: Check shift() return before push to avoid undefined [OK]

Common Mistakes:

• Ignoring undefined from shift()
• Wrong loop range assumptions
• Confusing push and unshift methods

5. Given an array arr of length n, how can you rotate it to the left by k positions efficiently without using extra space? Choose the correct approach.

hard

A. Use a temporary array to store first k elements, then shift rest left, then copy temp at end

B. Sort the array and then rotate by k positions

C. Rotate one by one k times using shift and push

D. Reverse first k elements, reverse last n-k elements, then reverse whole array

Solution

1. Step 1: Understand in-place rotation using reversal

   Reversing parts of array and then whole array rotates left by k without extra space.

2. Step 2: Apply reversal steps

   Reverse first k elements, reverse last n-k elements, then reverse entire array to get rotated array.

3. Final Answer:

   Reverse first k, reverse last n-k, reverse whole array -> Option D

4. Quick Check:

   Reversal method = in-place left rotate [OK]

Hint: Use triple reversal for in-place left rotation [OK]

Common Mistakes:

• Using extra array when not needed
• Rotating one by one causing O(n*k) time
• Sorting array unrelated to rotation