Array Reversal Techniques in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse the array using the built-in method.

DSA Typescript

const arr = [1, 2, 3, 4, 5];
arr.[1]();
console.log(arr);

Drag options to blanks, or click blank then click option'

Amap

Bsort

Creverse

Dfilter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sort()' which sorts the array instead of reversing it.

Using 'map()' or 'filter()' which do not reverse the array.

The built-in method to reverse an array in TypeScript is 'reverse()'.

2fill in blank

medium

Complete the code to reverse the array by swapping elements using a for loop.

DSA Typescript

const arr = [10, 20, 30, 40, 50];
for(let i = 0; i < arr.length / 2; i++) {
  const temp = arr[i];
  arr[i] = arr[arr.length - 1 - [1]];
  arr[arr.length - 1 - i] = temp;
}
console.log(arr);

Drag options to blanks, or click blank then click option'

Ai

B1

C0

Darr.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed number instead of the loop index.

Using 'arr.length' which is out of array bounds.

To swap elements from start and end, use the same index 'i' for both positions.

3fill in blank

hard

Fix the error in the recursive function to reverse an array.

DSA Typescript

function reverseArray(arr: number[], start: number, end: number): void {
  if (start >= end) return;
  const temp = arr[start];
  arr[start] = arr[end];
  arr[end] = temp;
  reverseArray(arr, start + 1, [1]);
}

const array = [1, 2, 3, 4, 5];
reverseArray(array, 0, array.length - 1);
console.log(array);

Drag options to blanks, or click blank then click option'

Aend - 1

Bend + 1

Cend

Dstart - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Keeping the end index the same, which results in incorrect reversal.

Increasing the end index, which goes out of bounds.

The recursive call should increment the start pointer and decrement the end pointer: start + 1, end - 1.

4fill in blank

hard

Fill both blanks to create a new reversed array using a for loop.

DSA Typescript

const original = [5, 10, 15, 20];
const reversed = [];
for(let i = original.length - 1; i [1] 0; i--) {
  reversed.[2](original[i]);
}
console.log(reversed);

Drag options to blanks, or click blank then click option'

A>

Bpush

C>=

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' which skips the first element at index 0.

Using 'pop()' which removes elements instead of adding.

The loop runs while i is greater than or equal to 0, and elements are added to reversed using push().

5fill in blank

hard

Fill all three blanks to reverse an array using the reduce method.

DSA Typescript

const arr = [2, 4, 6, 8];
const reversed = arr.[1](([2], [3]) => [[3]].concat([2]), []);
console.log(reversed);

Drag options to blanks, or click blank then click option'

Areduce

Bacc

Ccurrent

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'map' instead of 'reduce' which does not accumulate.

Swapping 'acc' and 'current' causing incorrect order.

The reduce method is used to accumulate elements in reverse order by concatenating current element before accumulator.

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