Array indexing and access in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the first element of the array.

C Sharp (C#)

int[] numbers = {1, 2, 3, 4};
int first = numbers[[1]];

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

A0

B1

C4

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as the first index instead of 0.

Using a negative index which is invalid.

Arrays in C# start at index 0, so the first element is at index 0.

2fill in blank

medium

Complete the code to access the last element of the array using its length.

C Sharp (C#)

int[] values = {10, 20, 30, 40, 50};
int last = values[values.Length [1] 1];

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

A+

B/

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using +1 which causes an index out of range error.

Using multiplication or division instead of subtraction.

The last element is at index Length - 1 because indexing starts at 0.

3fill in blank

hard

Fix the error in accessing the third element of the array.

C Sharp (C#)

string[] fruits = {"apple", "banana", "cherry"};
string third = fruits[[1]];

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

A3

B2

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 as index which is out of range.

Using 1 or 0 which points to earlier elements.

The third element is at index 2 because indexing starts at 0.

4fill in blank

hard

Fill both blanks to create an array of squares for numbers 1 to 5.

C Sharp (C#)

int[] squares = new int[5];
for (int i = 0; i [1] 4; i++) {
    squares[i] = (i [2] 1) * (i [2] 1);
}

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

A<=

B<

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of <= causes missing the last element.

Subtracting 1 instead of adding 1 changes the numbers incorrectly.

The loop runs while i is less than or equal to 4 (indexes 0 to 4). To get numbers 1 to 5, add 1 to i before squaring.

5fill in blank

hard

Fill all three blanks to create a reversed copy of the array.

C Sharp (C#)

int[] original = {1, 2, 3, 4};
int[] reversed = new int[original.Length];
for (int [1] = 0; [1] < original.Length; [1]++) {
    reversed[[2]] = original[original.Length [3] 1 - [1]];
}

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

Ai

Boriginal.Length - 1 - i

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong loop variable names inconsistently.

Incorrect calculation of reversed index causing errors.

Use 'i' as the loop variable. Assign reversed[i] to original at the reversed index calculated by Length - 1 - i.

Practice

1. What is the index of the first element in a C# array int[] numbers = {10, 20, 30};?

easy

A. 0

B. 1

C. 10

D. Array length - 1

Solution

1. Step 1: Understand zero-based indexing in C# arrays

   Arrays in C# start counting indexes from 0, so the first element is at index 0.

2. Step 2: Identify the first element's index

   The first element 10 is at index 0, not 1 or any other number.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   First element index = 0 [OK]

Hint: Remember: C# arrays start at zero index [OK]

Common Mistakes:

• Thinking the first index is 1
• Confusing element value with index
• Using array length as first index

2. Which of the following is the correct way to access the third element of an array string[] fruits = {"apple", "banana", "cherry"};?

easy

A. fruits[2]

B. fruits[3]

C. fruits(2)

D. fruits{2}

Solution

1. Step 1: Recall C# array indexing syntax

   Arrays use square brackets [] to access elements by zero-based index.

2. Step 2: Identify the correct index for the third element

   The third element is at index 2, so fruits[2] is correct.

3. Final Answer:

   fruits[2] -> Option A

4. Quick Check:

   Third element index = 2 [OK]

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

Common Mistakes:

• Using parentheses instead of brackets
• Using curly braces instead of brackets
• Using index 3 for third element

3. What will be the output of this code?

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

medium

A. 5

B. Runtime error

C. 10

D. 15

Solution

1. Step 1: Identify the element at index 1

   The array values has elements 5 at index 0, 10 at index 1, and 15 at index 2.

2. Step 2: Determine the output of Console.WriteLine(values[1])

   Accessing values[1] prints 10 to the console.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   values[1] = 10 [OK]

Hint: Index 1 means second element in array [OK]

Common Mistakes:

• Printing values[0] instead of values[1]
• Confusing index with element value
• Expecting index to start at 1

4. What is wrong with this code?

int[] arr = {1, 2, 3};
Console.WriteLine(arr[3]);

medium

A. No error, prints 3

B. Index out of range runtime error

C. Syntax error due to missing semicolon

D. Prints 0 by default

Solution

1. Step 1: Check array size and valid indexes

   The array has 3 elements with indexes 0, 1, and 2. Index 3 is outside the array bounds.

2. Step 2: Understand what happens when accessing invalid index

   Accessing arr[3] causes a runtime error: IndexOutOfRangeException.

3. Final Answer:

   Index out of range runtime error -> Option B

4. Quick Check:

   Accessing invalid index causes runtime error [OK]

Hint: Index must be less than array length [OK]

Common Mistakes:

• Assuming index 3 is valid for 3-element array
• Expecting default zero value for invalid index
• Confusing syntax error with runtime error

5. Given int[] data = {2, 4, 6, 8, 10};, which expression correctly changes the last element to 20?

hard

A. data[5] = 20;

B. data[data.Length] = 20;

C. data[-1] = 20;

D. data[4] = 20;

Solution

1. Step 1: Identify the last index of the array

   The array has 5 elements, so indexes go from 0 to 4. The last element is at index 4.

2. Step 2: Choose the correct syntax to assign new value

   Assigning data[4] = 20; changes the last element to 20. Using data.Length or negative indexes is invalid.

3. Final Answer:

   data[4] = 20; -> Option D

4. Quick Check:

   Last index = Length - 1 = 4 [OK]

Hint: Last index = Length - 1 for assignment [OK]

Common Mistakes:

• Using data.Length as index (out of range)
• Using negative index (not supported)
• Using index equal to array size