Array Reversal Techniques in DSA Typescript - Build from Scratch

Array Reversal Techniques

📖 Scenario: You are working on a simple app that processes a list of daily temperatures recorded in Celsius. Sometimes, you need to reverse the order of these temperatures to analyze trends backwards.

🎯 Goal: Build a TypeScript program that creates an array of temperatures, sets up a helper variable for the array length, reverses the array using a loop, and finally prints the reversed array.

📋 What You'll Learn

Create an array called temperatures with these exact values: 15, 22, 18, 30, 25

Create a variable called n that stores the length of the temperatures array

Use a for loop with a variable i to reverse the temperatures array in place

Print the reversed temperatures array using console.log

💡 Why This Matters

🌍 Real World

Reversing arrays is useful in data analysis, such as looking at time series data backwards or undoing operations.

💼 Career

Understanding array manipulation is fundamental for software developers, data analysts, and anyone working with collections of data.

1

Create the temperatures array

Create an array called temperatures with these exact values: 15, 22, 18, 30, 25

DSA Typescript

# Your code here

Hint

Use const temperatures: number[] = [15, 22, 18, 30, 25]; to create the array.

2

Create a variable for the array length

Create a variable called n that stores the length of the temperatures array

DSA Typescript

const temperatures: number[] = [15, 22, 18, 30, 25];
# Your code here

Hint

Use const n: number = temperatures.length; to get the array length.

3

Reverse the array using a for loop

Use a for loop with a variable i to reverse the temperatures array in place by swapping elements

DSA Typescript

const temperatures: number[] = [15, 22, 18, 30, 25];
const n: number = temperatures.length;
# Your code here

Hint

Swap elements from start and end moving towards the center using a loop.

4

Print the reversed array

Print the reversed temperatures array using console.log(temperatures)

DSA Typescript

const temperatures: number[] = [15, 22, 18, 30, 25];
const n: number = temperatures.length;
for (let i = 0; i < n / 2; i++) {
    const temp = temperatures[i];
    temperatures[i] = temperatures[n - 1 - i];
    temperatures[n - 1 - i] = temp;
}
# Your code here

Hint

Use console.log(temperatures); to print the reversed array.

Practice

1. What does the reverse() method do to an array in TypeScript?

easy

A. It reverses the order of elements in the original array.

B. It sorts the array in ascending order.

C. It removes duplicate elements from the array.

D. It creates a new array with elements doubled.

Solution

1. Step 1: Understand the reverse() method

   The reverse() method changes the order of elements in the array so the last element becomes first and the first becomes last.

2. Step 2: Confirm the effect on the original array

   This method modifies the original array itself, not creating a new one.

3. Final Answer:

   It reverses the order of elements in the original array. -> Option A

4. Quick Check:

   reverse() reverses array order [OK]

Hint: Remember: reverse() flips the array in place [OK]

Common Mistakes:

• Thinking reverse() sorts the array
• Assuming reverse() creates a new array
• Confusing reverse() with removing duplicates

2. Which of the following is the correct syntax to reverse an array named arr in TypeScript?

easy

A. arr->reverse();

B. reverse(arr);

C. arr = reverse();

D. arr.reverse();

Solution

1. Step 1: Identify the method call on the array

   In TypeScript, reverse() is a method called directly on the array object using dot notation.

2. Step 2: Verify syntax

   arr.reverse(); is correct. reverse(arr); incorrectly treats reverse as a standalone function. arr = reverse(); misuses the method. arr->reverse(); uses invalid syntax.

3. Final Answer:

   arr.reverse(); -> Option D

4. Quick Check:

   Use dot notation: arr.reverse() [OK]

Hint: Use dot notation: array.reverse() [OK]

Common Mistakes:

• Calling reverse() as a standalone function
• Assigning reverse() without calling on array
• Using incorrect symbols like '->'

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

const arr = [1, 2, 3, 4];
arr.reverse();
console.log(arr);

medium

A. [4, 3, 2, 1]

B. [1, 2, 3, 4]

C. [1, 4, 3, 2]

D. Error: reverse is not a function

Solution

1. Step 1: Understand the initial array

   The array starts as [1, 2, 3, 4].

2. Step 2: Apply the reverse() method

   Calling arr.reverse() reverses the array in place, changing it to [4, 3, 2, 1].

3. Final Answer:

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

4. Quick Check:

   reverse() flips array order [OK]

Hint: reverse() changes array order from end to start [OK]

Common Mistakes:

• Expecting original order after reverse()
• Confusing reverse() with sorting
• Thinking reverse() returns a new array

4. Identify the error in this TypeScript code that tries to reverse an array:

const numbers = [5, 6, 7];
const reversed = numbers.reverse;
console.log(reversed);

medium

A. reverse is not a valid method on arrays.

B. Missing parentheses to call reverse method.

C. reverse method should be called with an argument.

D. Array cannot be reversed using reverse method.

Solution

1. Step 1: Check how reverse is used

   The code assigns numbers.reverse without parentheses, so it assigns the function itself, not the reversed array.

2. Step 2: Correct usage of reverse()

   To reverse the array, the method must be called with parentheses: numbers.reverse().

3. Final Answer:

   Missing parentheses to call reverse method. -> Option B

4. Quick Check:

   Call reverse() with () to execute [OK]

Hint: Always use () to call methods like reverse() [OK]

Common Mistakes:

• Forgetting parentheses when calling methods
• Thinking reverse needs arguments
• Believing reverse is not a valid method

5. Given the array const data = [10, 20, 30, 40, 50];, which code snippet correctly reverses the array without modifying the original array?

hard

A. const reversed = data.reverse();

B. const reversed = data.sort().reverse();

C. const reversed = [...data].reverse();

D. const reversed = data.map().reverse();

Solution

1. Step 1: Understand the problem requirement

   We want to reverse the array but keep the original array unchanged.

2. Step 2: Analyze each option

   const reversed = data.reverse(); reverses the original array directly, modifying it.
   const reversed = [...data].reverse(); creates a shallow copy using spread [...data] and then reverses the copy, preserving the original.
   const reversed = data.sort().reverse(); sorts the original array first, changing order before reversing, and modifies original.
   const reversed = data.map().reverse(); uses map() incorrectly without a callback, causing error.

3. Final Answer:

   const reversed = [...data].reverse(); -> Option C

4. Quick Check:

   Copy then reverse to keep original safe [OK]

Hint: Copy array first with [...array] before reverse() [OK]

Common Mistakes:

• Using reverse() directly modifies original array
• Sorting before reversing changes order incorrectly
• Using map() without a callback causes error