Array indexing and access in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the starting index of arrays in C#?

Arrays in C# start indexing at 0. This means the first element is at index 0, the second at index 1, and so on.

beginner

How do you access the third element of an array named numbers?

You access it using numbers[2] because indexing starts at 0.

intermediate

What happens if you try to access an index outside the array bounds in C#?

You get a System.IndexOutOfRangeException error because the index is invalid.

beginner

How can you find the length of an array named items?

Use items.Length to get the number of elements in the array.

beginner

Explain how to change the value of the first element in an array colors to "red".

Assign the new value using colors[0] = "red"; to update the first element.

What is the index of the last element in an array with 10 elements?

A 9

B 10

C 0

D 11

Since indexing starts at 0, the last element is at index 9 (10 - 1).

Which of the following accesses the second element of an array named arr?

A arr[2]

B arr[1]

C arr[0]

D arr[3]

The second element is at index 1 because indexing starts at 0.

What property gives the number of elements in a C# array?

A Length

B Count

C Size

D Capacity

The Length property returns the number of elements in the array.

What error occurs if you try to access array[10] when the array has 10 elements?

A NullReferenceException

B InvalidOperationException

C ArgumentException

D IndexOutOfRangeException

Accessing index 10 is out of bounds since valid indices are 0 to 9, causing an IndexOutOfRangeException.

How do you change the value of the first element in an array named data?

A data.change(0, newValue);

B data[1] = newValue;

C data[0] = newValue;

D data.set(0, newValue);

You assign the new value directly using the index: data[0] = newValue;.

Describe how array indexing works in C# and how to access elements safely.

Think about how you count positions starting from zero.

Explain how to update an element in a C# array and what happens if you try to access an invalid index.

Consider what happens if you try to open a door that doesn't exist.

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