Array methods (Sort, Reverse, IndexOf) in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C# code using Array.Sort and Array.IndexOf?

Look at the code below. What will be printed to the console?

C Sharp (C#)

using System;

class Program {
    static void Main() {
        int[] numbers = {5, 3, 8, 1, 2};
        Array.Sort(numbers);
        int index = Array.IndexOf(numbers, 3);
        Console.WriteLine(index);
    }
}

A 1

B 3

C 2

D 0

💡 Hint

Remember that Array.Sort arranges the array in ascending order before searching.

After sorting, the array becomes [1, 2, 3, 5, 8]. The number 3 is at index 2 (0-based). So the output is 2.

❓ Predict Output

intermediate

What does this code print after reversing the array?

What will be the output of the following C# program?

C Sharp (C#)

using System;

class Program {
    static void Main() {
        string[] fruits = {"apple", "banana", "cherry"};
        Array.Reverse(fruits);
        Console.WriteLine(fruits[1]);
    }
}

A banana

B apple

C cherry

D IndexOutOfRangeException

💡 Hint

Think about how reversing changes the order of elements.

Original array: ["apple", "banana", "cherry"]. After reversing: ["cherry", "banana", "apple"]. The element at index 1 is "banana".

🔧 Debug

advanced

Why does this code throw an exception?

Examine the code below. It throws an exception when run. What is the cause?

C Sharp (C#)

using System;

class Program {
    static void Main() {
        int[] values = {10, 20, 30};
        Array.Reverse(values, 1, 5);
        Console.WriteLine(string.Join(",", values));
    }
}

A ArgumentOutOfRangeException because the length parameter exceeds array bounds

B NullReferenceException because the array is null

C InvalidOperationException because Reverse cannot be used on int arrays

D No exception, output is '10,30,20'

💡 Hint

Check the parameters passed to Array.Reverse carefully.

The method Array.Reverse(array, index, length) requires that index + length does not exceed the array length. Here, 1 + 5 = 6, which is greater than the array length 3, causing ArgumentOutOfRangeException.

❓ Predict Output

advanced

What is the output of this code using Array.IndexOf with a missing element?

What will this program print?

C Sharp (C#)

using System;

class Program {
    static void Main() {
        char[] letters = {'a', 'b', 'c', 'd'};
        int pos = Array.IndexOf(letters, 'z');
        Console.WriteLine(pos);
    }
}

A IndexOutOfRangeException

B -1

C 0

D 4

💡 Hint

What does Array.IndexOf return if the item is not found?

If the element is not found, Array.IndexOf returns -1.

🧠 Conceptual

expert

How many elements are in the array after these operations?

Consider this C# code snippet:

int[] arr = {4, 7, 1, 9};
Array.Sort(arr);
Array.Reverse(arr);
int index = Array.IndexOf(arr, 7);

After running this code, how many elements does arr contain?

A 0

B 3

C 1

D 4

💡 Hint

Sorting and reversing do not change the number of elements in the array.

The array arr starts with 4 elements. Sorting and reversing only change the order, not the size. So it still has 4 elements.

Practice

1. What does the Array.Sort method do to an array in C#?

easy

A. It reverses the order of the items.

B. It arranges the items in ascending order.

C. It finds the index of a specific item.

D. It removes duplicate items from the array.

Solution

1. Step 1: Understand Array.Sort purpose

   The Array.Sort method organizes the elements of an array in ascending order by default.

2. Step 2: Compare with other methods

   Array.Reverse flips the array, and Array.IndexOf finds an element's position, so they do different things.

3. Final Answer:

   It arranges the items in ascending order. -> Option B

4. Quick Check:

   Array.Sort = ascending order [OK]

Hint: Sort means arrange from smallest to largest [OK]

Common Mistakes:

• Confusing Sort with Reverse
• Thinking Sort finds an element's position
• Assuming Sort removes duplicates

2. Which of the following is the correct way to reverse an array named numbers in C#?

easy

A. numbers.Reverse();

B. numbers.ReverseArray();

C. Array.Sort(numbers);

D. Array.Reverse(numbers);

Solution

1. Step 1: Identify correct method call syntax

   In C#, Array.Reverse is a static method that takes the array as an argument, so Array.Reverse(numbers); is correct.

2. Step 2: Check other options

   numbers.Reverse() is a LINQ extension returning IEnumerable, not reversing in place. Array.Sort sorts, not reverses. ReverseArray() does not exist.

3. Final Answer:

   Array.Reverse(numbers); -> Option D

4. Quick Check:

   Reverse array = Array.Reverse(array) [OK]

Hint: Use Array.Reverse(array) to flip order [OK]

Common Mistakes:

• Using instance method Reverse() which doesn't reverse in place
• Confusing Sort with Reverse
• Calling non-existent methods

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

string[] fruits = {"apple", "banana", "cherry"};
Array.Sort(fruits);
Array.Reverse(fruits);
Console.WriteLine(fruits[0]);

medium

A. cherry

B. banana

C. apple

D. Runtime error

Solution

1. Step 1: Sort the array

   After Array.Sort(fruits);, the array becomes ["apple", "banana", "cherry"] (already sorted).

2. Step 2: Reverse the array

   After Array.Reverse(fruits);, the array becomes ["cherry", "banana", "apple"].

3. Step 3: Print first element

   fruits[0] is now "cherry".

4. Final Answer:

   cherry -> Option A

5. Quick Check:

   Sort then Reverse = descending order [OK]

Hint: Sort then Reverse = descending order, first item is largest [OK]

Common Mistakes:

• Assuming array stays sorted ascending
• Confusing Reverse with Sort
• Expecting runtime error due to method calls

4. The following code throws an error. What is the mistake?

int[] nums = {3, 1, 4};
nums.Sort();
Console.WriteLine(nums[0]);

medium

A. Arrays don't have a Sort() instance method.

B. The array must be reversed before sorting.

C. You must use Console.WriteLine(nums.Sort()); instead.

D. The array must be converted to a list before sorting.

Solution

1. Step 1: Identify method call error

   Arrays in C# do not have an instance method called Sort(). Sorting is done via the static method Array.Sort(array).

2. Step 2: Understand correct usage

   The correct way is Array.Sort(nums); to sort the array in place.

3. Final Answer:

   Arrays don't have a Sort() instance method. -> Option A

4. Quick Check:

   Use Array.Sort(array), not array.Sort() [OK]

Hint: Array.Sort is static; no instance Sort() method [OK]

Common Mistakes:

• Calling Sort() directly on array instance
• Confusing List.Sort() with Array.Sort()
• Trying to print Sort() return value

5. Given the array int[] values = {5, 3, 8, 3, 9};, which code snippet finds the index of the first occurrence of the number 3 and then reverses the array?

hard

A. int index = Array.FindIndex(values, 3);
Array.Sort(values);

B. int index = values.IndexOf(3);
Array.Reverse(values);

C. int index = Array.IndexOf(values, 3);
Array.Reverse(values);

D. int index = Array.IndexOf(values, 3);
Array.Sort(values);

Solution

1. Step 1: Find index of first 3

   Array.IndexOf(values, 3) correctly returns the first index where 3 appears, which is 1.

2. Step 2: Reverse the array

   Array.Reverse(values); reverses the array in place as required.

3. Final Answer:

   int index = Array.IndexOf(values, 3); Array.Reverse(values); -> Option C

4. Quick Check:

   IndexOf + Reverse = int index = Array.IndexOf(values, 3); Array.Reverse(values); [OK]

Hint: Use Array.IndexOf(array, value) to find index [OK]

Common Mistakes:

• Using instance method IndexOf on array (not valid)
• Confusing FindIndex with IndexOf
• Sorting instead of reversing