Array Reversal Techniques in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

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

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

Areverse

Bsort

Csplice

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sort' instead of 'reverse' changes order but not necessarily reverses.

Using 'splice' or 'push' does not reverse the array.

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

2fill in blank

medium

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

DSA Javascript

const arr = [10, 20, 30, 40, 50];
for(let i = 0; i < arr.length / 2; i++) {
  let 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'

Aarr.length

B0

Ci

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed number instead of 'i' causes incorrect swapping.

Using 'temp' or '0' in the index is incorrect.

To swap elements from start and end, use the current index 'i' to find the matching element from the end.

3fill in blank

hard

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

DSA Javascript

function reverseArray(arr) {
  if(arr.length === 0) return [];
  return [arr.pop()][1] reverseArray(arr);
}

const result = reverseArray([1, 2, 3]);
console.log(result);

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

A+

B-

C,

Dconcat

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' causes arrays to convert to strings.

Using ',' or '-' is invalid syntax here.

To combine arrays in JavaScript, use 'concat' method instead of '+' operator.

4fill in blank

hard

Fill both blanks to create a new reversed array using map and length.

DSA Javascript

const arr = [5, 10, 15, 20];
const reversed = arr.map((_, [1]) => arr[[2] - 1 - [1]]);
console.log(reversed);

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

Aindex

Blength

Ci

Darr.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using '_' as index causes errors.

Using 'length' alone is undefined.

Using 'index' without declaration.

In map, the second parameter is the index, here named 'i'. The array length is accessed by 'arr.length'.

5fill in blank

hard

Fill all three blanks to reverse an array using reduce.

DSA Javascript

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

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

Aacc

Bcurr

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping accumulator and current causes wrong results.

Not using spread operator causes nested arrays.

In reduce, 'acc' is accumulator and 'curr' is current element. We prepend 'curr' to 'acc' to reverse.

Practice

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

easy

A. It removes duplicate elements from the array.

B. It sorts the array in ascending order.

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

D. It creates a new array with elements doubled.

Solution

1. Step 1: Understand the reverse() method

   The reverse() method changes the original array by reversing the order of its elements.

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

   It does not create a new array or sort; it simply reverses the existing array in place.

3. Final Answer:

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

4. Quick Check:

   reverse() = reverses array order [OK]

Hint: reverse() flips array order 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 JavaScript?

easy

A. arr.reverse();

B. reverse(arr);

C. arr = reverse();

D. arr.reverse(arr);

Solution

1. Step 1: Identify the method call syntax

   The reverse() method is called on the array object itself, so the syntax is arr.reverse();.

2. Step 2: Check other options for errors

   Options A, B, and C are incorrect because reverse is not a standalone function and does not take arguments.

3. Final Answer:

   arr.reverse(); -> Option A

4. Quick Check:

   Method called on array object = arr.reverse() [OK]

Hint: Use arrayName.reverse() to reverse in place [OK]

Common Mistakes:

• Calling reverse as a standalone function
• Passing the array as an argument to reverse
• Assigning reverse() without calling on array

3. What will be the output of the following code?

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

medium

A. undefined

B. [1, 2, 3, 4]

C. [1, 4, 3, 2]

D. [4, 3, 2, 1]

Solution

1. Step 1: Understand reverse() return value

   The reverse() method reverses the original array and returns the same reversed array.

2. Step 2: Trace the code output

   After calling arr.reverse(), arr becomes [4, 3, 2, 1], and reversed holds this reversed array. Logging reversed prints [4, 3, 2, 1].

3. Final Answer:

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

4. Quick Check:

   reverse() returns reversed array [OK]

Hint: reverse() returns the reversed array itself [OK]

Common Mistakes:

• Expecting original array unchanged
• Thinking reverse() returns undefined
• Confusing reverse() with sorting

4. Find the error in the following code that tries to reverse an array nums:

function reverseArray(nums) {
  for(let i = 0; i < nums.length / 2; i++) {
    let temp = nums[i];
    nums[i] = nums[nums.length - i];
    nums[nums.length - i] = temp;
  }
  return nums;
}

medium

A. The index nums.length - i is out of bounds; should be nums.length - i - 1.

B. The temp variable is not needed for swapping.

C. The loop should run while i < nums.length / 2, not <=.

D. The function should return a new array, not modify nums.

Solution

1. Step 1: Check index calculations for swapping

   The code uses nums.length - i as the index for the element to swap, but array indices go from 0 to nums.length - 1, so it should be nums.length - i - 1.

2. Step 2: Confirm loop condition and swapping logic

   The loop condition i < nums.length / 2 is acceptable, and temp is needed for swapping. Modifying the original array is allowed here.

3. Final Answer:

   The index nums.length - i is out of bounds; should be nums.length - i - 1. -> Option A

4. Quick Check:

   Array indices start at 0, so subtract 1 [OK]

Hint: Array last index = length - 1, not length [OK]

Common Mistakes:

• Using nums.length - i instead of nums.length - i - 1
• Removing temp variable causing swap errors
• Incorrect loop boundary causing extra swaps

5. Given an array arr, which code snippet correctly reverses the array without modifying the original array?

hard

A. const reversed = arr.reverse();

B. const reversed = arr.slice().reverse();

C. const reversed = [...arr].sort().reverse();

D. const reversed = arr.reverse().map(x => x);

Solution

1. Step 1: Understand how to avoid modifying original array

   Using arr.slice() creates a shallow copy of the array. Calling reverse() on this copy reverses it without changing the original.

2. Step 2: Analyze other options

   const reversed = arr.reverse(); reverses the original array. const reversed = [...arr].sort().reverse(); sorts before reversing, changing order. const reversed = arr.reverse().map(x => x); reverses the original array because reverse() is called on arr before mapping.

3. Final Answer:

   const reversed = arr.slice().reverse(); -> Option B

4. Quick Check:

   Copy then reverse to keep original safe [OK]

Hint: Use slice() to copy before reverse() [OK]

Common Mistakes:

• Using reverse() directly modifies original array
• Sorting before reversing changes order
• Thinking reverse().map(x => x) preserves original array