Array Access and Update at Index in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C#

int[] numbers = {1, 2, 3, 4, 5};
int value = numbers[[1]];
Console.WriteLine(value);

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

A2

B3

C1

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as index instead of 2, which accesses the fourth element.

Confusing array indexes starting at 1 instead of 0.

Index 2 in the array corresponds to the third element, which is 3.

2fill in blank

medium

Complete the code to update the element at index 1 to 10.

DSA C#

int[] numbers = {5, 6, 7, 8};
numbers[[1]] = 10;
Console.WriteLine(string.Join(", ", numbers));

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

A0

B2

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Updating index 0 instead of 1.

Using an index out of range like 4.

Index 1 is the second element, which we want to update to 10.

3fill in blank

hard

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

DSA C#

int[] numbers = {9, 8, 7};
numbers[[1]] = 0;
Console.WriteLine(string.Join(", ", numbers));

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

A3

B0

C-1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 3 which is out of bounds.

Using negative index which is invalid in C# arrays.

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 middle element of the array.

DSA C#

int[] numbers = {4, 5, 6, 7, 8};
numbers[[1]] = [2];
Console.WriteLine(string.Join(", ", numbers));

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

A2

B10

C3

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 3 instead of 2 for the middle element.

Assigning the wrong value to the element.

Index 2 is the middle element in a 5-element array, and we update it to 10.

5fill in blank

hard

Fill all three blanks to create an array, update an element, and print the updated array.

DSA C#

int[] [1] = {1, 2, 3, 4};
[1][[2]] = 100;
Console.WriteLine(string.Join(", ", [1]));

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

Anums

B2

C3

Dvalues

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Updating the wrong index.

We declare the array named 'nums', update index 2, and print 'nums'.

Practice

1. What does the expression arr[2] do in an array arr?

easy

A. Returns the length of the array

B. Accesses the element at index 2 in the array

C. Deletes the element at index 2

D. Adds a new element at index 2

Solution

1. Step 1: Understand array indexing

   Array elements are accessed by their index, starting from 0.

2. Step 2: Access element at index 2

   The expression arr[2] retrieves the element stored at the third position (index 2) in the array.

3. Final Answer:

   Accesses the element at index 2 in the array -> Option B

4. Quick Check:

   arr[2] means get element at index 2 [OK]

Hint: Remember array indexing starts at zero [OK]

Common Mistakes:

• Thinking index starts at 1
• Confusing access with insertion
• Assuming arr[2] returns array length

2. Which of the following is the correct way to update the element at index 1 in an integer array numbers to 10 in C#?

easy

A. numbers(1) = 10;

B. numbers{1} = 10;

C. numbers[1] = 10;

D. numbers<1> = 10;

Solution

1. Step 1: Identify correct syntax for array indexing in C#

   C# uses square brackets [] to access or update array elements by index.

2. Step 2: Apply update syntax

   To update the element at index 1, use numbers[1] = 10;.

3. Final Answer:

   numbers[1] = 10; -> Option C

4. Quick Check:

   Use square brackets for array index update [OK]

Hint: Use square brackets [] to access or update arrays [OK]

Common Mistakes:

• Using parentheses () instead of brackets []
• Using curly braces {} which are invalid for arrays
• Using angle brackets <> which are for generics

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

int[] arr = {5, 10, 15};
arr[1] = 20;
Console.WriteLine(arr[1]);

medium

A. 20

B. 15

C. 10

D. 5

Solution

1. Step 1: Initialize array and update element

   The array arr starts as {5, 10, 15}. Then arr[1] = 20; changes the element at index 1 from 10 to 20.

2. Step 2: Print updated element

   Console.WriteLine(arr[1]); prints the updated value at index 1, which is now 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   arr[1] updated to 20, so output is 20 [OK]

Hint: Remember updates change the element value at given index [OK]

Common Mistakes:

• Printing old value before update
• Confusing index 1 with index 0
• Assuming array is unchanged

4. Identify the error in the following code snippet:

int[] data = new int[3];
data[3] = 100;

medium

A. Index out of range error at data[3]

B. No error, code runs fine

C. Syntax error in array declaration

D. Cannot assign value to array element

Solution

1. Step 1: Check array size and valid indices

   The array data has size 3, so valid indices are 0, 1, and 2.

2. Step 2: Analyze assignment at index 3

   Accessing data[3] is invalid and causes an index out of range error at runtime.

3. Final Answer:

   Index out of range error at data[3] -> Option A

4. Quick Check:

   Array indices go from 0 to length-1 [OK]

Hint: Array index must be less than array length [OK]

Common Mistakes:

• Using index equal to array length
• Assuming array indices start at 1
• Ignoring runtime exceptions

5. Given an array int[] nums = {1, 2, 3, 4, 5};, which code snippet correctly doubles the value of each element in the array?

hard

A. foreach (int i in nums) { i = i * 2; }

B. for (int i = 0; i <= nums.Length; i++) { nums[i] = nums[i] * 2; }

C. for (int i = 1; i < nums.Length; i++) { nums[i] = nums[i] * 2; }

D. for (int i = 0; i < nums.Length; i++) { nums[i] = nums[i] * 2; }

Solution

1. Step 1: Understand array traversal for update

   To update each element, we must loop from index 0 to Length-1 using i < nums.Length.

2. Step 2: Analyze each option

   for (int i = 0; i <= nums.Length; i++) { nums[i] = nums[i] * 2; } uses i <= nums.Length which causes index out of range error.
   for (int i = 0; i < nums.Length; i++) { nums[i] = nums[i] * 2; } correctly loops from 0 to Length-1 and updates each element.
   for (int i = 1; i < nums.Length; i++) { nums[i] = nums[i] * 2; } starts from 1, skipping index 0.
   foreach (int i in nums) { i = i * 2; } uses foreach which does not allow modifying elements directly.

3. Final Answer:

   for (int i = 0; i < nums.Length; i++) { nums[i] = nums[i] * 2; } -> Option D

4. Quick Check:

   Use for loop with i < Length to update array [OK]

Hint: Use for loop with i < Length to safely update all elements [OK]

Common Mistakes:

• Using <= instead of < causing errors
• Starting loop at 1 skipping first element
• Using foreach which cannot modify array elements