Array Access and Update at Index in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the element at index 2 in the array.

DSA Javascript

const arr = [10, 20, 30, 40];
const value = arr[[1]];
console.log(value);

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

A2

B3

C1

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 3 to access the third element instead of 2.

Using index 4 which is out of range.

The index 2 accesses the third element in the array, which is 30.

2fill in blank

medium

Complete the code to update the element at index 1 to 50.

DSA Javascript

const arr = [10, 20, 30];
arr[[1]] = 50;
console.log(arr);

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

A0

B3

C2

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Updating index 0 instead of 1.

Using index 3 which is out of bounds.

Index 1 refers to the second element, which is updated to 50.

3fill in blank

hard

Fix the error in the code to correctly update the last element of the array.

DSA Javascript

const arr = [5, 10, 15];
arr[[1]] = 20;
console.log(arr);

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

A3

B2

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 3 which is out of range.

Using index 1 which is the middle element.

The last element is at index 2 because array indexes start at 0.

4fill in blank

hard

Fill both blanks to access and update the element at index 0 to 100.

DSA Javascript

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

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

A0

B1

C100

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 1 instead of 0.

Assigning 50 instead of 100.

Index 0 accesses the first element, which is updated to 100.

5fill in blank

hard

Fill all three blanks to access the element at index 2, update it to 75, and then print the updated array.

DSA Javascript

const arr = [10, 20, 30, 40];
arr[[1]] = [2];
console.log([3]);

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

A1

B75

Carr

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong index like 1.

Printing a wrong variable name.

Assigning wrong value.

Index 2 accesses the third element, which is updated to 75, then the array is printed.

Practice

1. What does arr[2] return if arr = [10, 20, 30, 40]?

easy

A. 40

B. 20

C. 10

D. 30

Solution

1. Step 1: Understand array indexing

   Arrays start at index 0, so index 2 is the third element.

2. Step 2: Identify the element at index 2

   The element at index 2 in [10, 20, 30, 40] is 30.

3. Final Answer:

   30 -> Option D

4. Quick Check:

   Index 2 element = 30 [OK]

Hint: Remember arrays start at zero index [OK]

Common Mistakes:

• Thinking index starts at 1
• Confusing element value with index
• Using negative index without explanation

2. Which of the following is the correct way to update the 3rd element of array nums to 50?

easy

A. nums[3] = 50;

B. nums(3) = 50;

C. nums[2] = 50;

D. nums{2} = 50;

Solution

1. Step 1: Recall array indexing syntax

   In JavaScript, arrays use square brackets with zero-based index.

2. Step 2: Identify correct index for 3rd element

   The 3rd element is at index 2, so nums[2] = 50; is correct.

3. Final Answer:

   nums[2] = 50; -> Option C

4. Quick Check:

   Use square brackets and zero-based index [OK]

Hint: Use square brackets and zero-based index [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based index
• Using curly braces for arrays

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

let arr = [5, 10, 15, 20];
arr[1] = arr[3];
console.log(arr);

medium

A. [5, 20, 15, 20]

B. [5, 10, 15, 20]

C. [20, 10, 15, 20]

D. [5, 15, 15, 20]

Solution

1. Step 1: Understand the assignment operation

   The element at index 1 is set to the value at index 3, which is 20.

2. Step 2: Update the array and print

   Original array: [5, 10, 15, 20]. After update: [5, 20, 15, 20].

3. Final Answer:

   [5, 20, 15, 20] -> Option A

4. Quick Check:

   arr[1] = arr[3] updates second element to 20 [OK]

Hint: Remember assignment copies value at index [OK]

Common Mistakes:

• Confusing indexes
• Expecting arr[3] to change
• Thinking assignment swaps values

4. Find the error in the following code snippet:

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

medium

A. No error, array expands and adds 4 at index 3

B. Array becomes empty after assignment

C. Syntax error due to invalid index

D. Index 3 is out of bounds, causes runtime error

Solution

1. Step 1: Understand JavaScript array behavior on out-of-range index

   Assigning to index 3 adds a new element; arrays auto-expand.

2. Step 2: Check the resulting array

   Original: [1, 2, 3]. After assignment: [1, 2, 3, 4]. No error occurs.

3. Final Answer:

   No error, array expands and adds 4 at index 3 -> Option A

4. Quick Check:

   JS arrays auto-expand on assignment [OK]

Hint: JS arrays grow when assigned out-of-range index [OK]

Common Mistakes:

• Expecting runtime error
• Thinking syntax error occurs
• Assuming array clears on assignment

5. Given let arr = [2, 4, 6, 8, 10]; write code to double the value at index 2 and then swap it with the value at index 4. What is the final array?

hard

A. [2, 4, 10, 8, 6]

B. [2, 4, 20, 8, 12]

C. [2, 4, 20, 8, 6]

D. [2, 4, 10, 8, 12]

Solution

1. Step 1: Double the value at index 2

   Value at index 2 is 6. Doubling it: 6 * 2 = 12. Update arr[2] = 12.

2. Step 2: Swap values at index 2 and index 4

   Value at index 4 is 10. Swap arr[2] and arr[4]: arr[2] = 10, arr[4] = 12.

3. Step 3: Write final array

   Array after operations: [2, 4, 10, 8, 12].

4. Final Answer:

   [2, 4, 10, 8, 12] -> Option B

5. Quick Check:

   Double then swap results in [2,4,10,8,12] [OK]

Hint: Double first, then swap values at given indexes [OK]

Common Mistakes:

• Swapping before doubling
• Mixing up indexes
• Not updating array correctly