Array Insertion at End in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after inserting an element at the end of the array?

Consider the following C# code that inserts an element at the end of an array by creating a new array with increased size. What will be printed after insertion?

DSA C#

int[] InsertAtEnd(int[] arr, int value) {
    int[] newArr = new int[arr.Length + 1];
    for (int i = 0; i < arr.Length; i++) {
        newArr[i] = arr[i];
    }
    newArr[newArr.Length - 1] = value;
    return newArr;
}

int[] original = {1, 2, 3};
int[] result = InsertAtEnd(original, 4);
for (int i = 0; i < result.Length; i++) {
    Console.Write(result[i] + " -> ");
}
Console.WriteLine("null");

A 4 -> 1 -> 2 -> 3 -> null

B 1 -> 2 -> 3 -> null

C 1 -> 2 -> 3 -> 0 -> null

D 1 -> 2 -> 3 -> 4 -> null

💡 Hint

Think about how the new array copies old elements and adds the new value at the last position.

The function creates a new array one element larger, copies all old elements, then places the new value at the last index. So the output shows the original elements followed by the new value 4.

❓ Predict Output

intermediate

What error occurs when inserting at end without resizing the array?

What happens if you try to insert an element at the end of a fixed-size array in C# without creating a new array?

DSA C#

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

A No error, output: 1, 2, 3, 4

B IndexOutOfRangeException

C NullReferenceException

D Compilation error

💡 Hint

Arrays have fixed size; accessing index equal to length is invalid.

Arrays in C# have fixed size. Trying to access arr[arr.Length] is out of bounds and throws IndexOutOfRangeException.

🔧 Debug

advanced

Find the bug in this array insertion code

This code tries to insert a value at the end of an array but produces wrong output. What is the bug?

DSA C#

int[] InsertAtEnd(int[] arr, int value) {
    int[] newArr = new int[arr.Length];
    for (int i = 0; i < arr.Length; i++) {
        newArr[i] = arr[i];
    }
    newArr[newArr.Length - 1] = value;
    return newArr;
}

int[] original = {1, 2, 3};
int[] result = InsertAtEnd(original, 4);
for (int i = 0; i < result.Length; i++) {
    Console.Write(result[i] + " -> ");
}
Console.WriteLine("null");

A The value is inserted at the start instead of the end

B The loop copies elements incorrectly causing IndexOutOfRangeException

C The new array size is not increased, so last element overwrites existing data

D The function returns the original array instead of new array

💡 Hint

Check the size of the new array compared to the original.

The new array has the same length as original, so assigning value at last index overwrites the last original element instead of adding a new one.

🧠 Conceptual

advanced

Why is inserting at the end of an array costly?

Inserting an element at the end of a fixed-size array requires creating a new array and copying elements. Why is this operation costly compared to linked lists?

A Because arrays require copying all elements to a new larger array, which takes time proportional to array size

B Because arrays do not allow storing integers

C Because linked lists use more memory than arrays

D Because arrays automatically resize without copying

💡 Hint

Think about what happens internally when you add an element beyond the array's fixed size.

Arrays have fixed size. To add an element, a new larger array must be created and all elements copied, which takes time proportional to the number of elements.

❓ Predict Output

expert

What is the output after multiple insertions at the end?

Given the code below that inserts multiple elements at the end of an array, what is the final printed output?

DSA C#

int[] InsertAtEnd(int[] arr, int value) {
    int[] newArr = new int[arr.Length + 1];
    for (int i = 0; i < arr.Length; i++) {
        newArr[i] = arr[i];
    }
    newArr[newArr.Length - 1] = value;
    return newArr;
}

int[] arr = {1};
arr = InsertAtEnd(arr, 2);
arr = InsertAtEnd(arr, 3);
arr = InsertAtEnd(arr, 4);
for (int i = 0; i < arr.Length; i++) {
    Console.Write(arr[i] + " -> ");
}
Console.WriteLine("null");

A 1 -> 2 -> 3 -> 4 -> null

B 1 -> 2 -> 3 -> 4 -> 5 -> null

C 1 -> 2 -> 3 -> null

D 4 -> 3 -> 2 -> 1 -> null

💡 Hint

Each insertion creates a new array with one more element, preserving previous elements in order.

Each call adds one element at the end, so after three insertions starting from {1}, the array becomes {1, 2, 3, 4}.

Practice

1. What happens when you insert an element at the end of a fixed-size array in C# without resizing?

easy

A. An error occurs because the array size is fixed

B. The array automatically resizes to fit the new element

C. The element replaces the last element if the array is full

D. The element is added at the first position

Solution

1. Step 1: Understand fixed-size array behavior

   In C#, arrays have a fixed size once created and cannot grow automatically.

2. Step 2: Inserting at the end without resizing

   Inserting at the end requires accessing index arr.Length, which is out of bounds and throws IndexOutOfRangeException.

3. Final Answer:

   An error occurs because the array size is fixed -> Option A

4. Quick Check:

   Fixed-size array insert at end causes error [OK]

Hint: Fixed arrays can't grow; insert at end causes error [OK]

Common Mistakes:

• Assuming array resizes automatically
• Thinking insertion shifts elements
• Believing insertion adds new slot

2. Which of the following is the correct syntax to insert an element at the end of a List<int> named numbers in C#?

easy

A. numbers.Push(5);

B. numbers.InsertEnd(5);

C. numbers.Add(5);

D. numbers.Append(5);

Solution

1. Step 1: Identify List insertion method

   In C#, List<T> uses the Add() method to insert elements at the end.

2. Step 2: Check other options

   InsertEnd, Push, and Append are not valid List<T> methods in C#.

3. Final Answer:

   numbers.Add(5); -> Option C

4. Quick Check:

   List Add method inserts at end [OK]

Hint: Use Add() to insert at end of List [OK]

Common Mistakes:

• Using non-existent methods like InsertEnd or Push
• Confusing List with Stack methods
• Trying Append which is for strings

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

int[] arr = new int[3] {1, 2, 3};
arr[2] = 10;
Console.WriteLine(string.Join(", ", arr));

medium

A. 1, 2, 3

B. 1, 2, 10

C. 10, 2, 3

D. Runtime error

Solution

1. Step 1: Understand array initialization

   The array arr has 3 elements: 1, 2, 3 at indexes 0, 1, 2.

2. Step 2: Assign new value at last index

   arr[2] = 10 replaces the last element (3) with 10.

3. Final Answer:

   1, 2, 10 -> Option B

4. Quick Check:

   Last element replaced with 10 [OK]

Hint: Index 2 is last element; assignment replaces it [OK]

Common Mistakes:

• Thinking array resizes or shifts elements
• Expecting original array unchanged
• Assuming runtime error on assignment

4. Identify the error in the following code that tries to insert 7 at the end of an array:

int[] arr = new int[3] {1, 2, 3};
arr[arr.Length] = 7;

medium

A. Syntax error due to missing semicolon

B. Null reference exception

C. No error, code runs fine

D. Index out of range error

Solution

1. Step 1: Check array index used

   arr.Length is 3, but valid indexes are 0,1,2 only.

2. Step 2: Understand index out of range

   Accessing arr[3] causes an IndexOutOfRangeException at runtime.

3. Final Answer:

   Index out of range error -> Option D

4. Quick Check:

   Array index must be less than length [OK]

Hint: Array indexes go 0 to Length-1 only [OK]

Common Mistakes:

• Using arr.Length as index directly
• Ignoring runtime exceptions
• Confusing syntax errors with runtime errors

5. You have an array int[] arr = new int[3] {1, 2, 3};. You want to insert 4 at the end without losing existing elements. Which approach is correct?

hard

A. Create a new array of size 4, copy old elements, then add 4 at last index

B. Assign arr[3] = 4 directly

C. Use arr.Add(4) method

D. Resize arr using arr.Resize(4) method

Solution

1. Step 1: Understand fixed array size

   Arrays in C# have fixed size; cannot add elements beyond initial size.

2. Step 2: Correct way to add element

   Create a new larger array, copy existing elements, then insert new element at the end.

3. Step 3: Check other options

   Assigning arr[3] causes error; arr.Add() doesn't exist for arrays; arr.Resize() is invalid.

4. Final Answer:

   Create a new array of size 4, copy old elements, then add 4 at last index -> Option A

5. Quick Check:

   Fixed arrays need new array to grow [OK]

Hint: Arrays fixed size; create new bigger array to add [OK]

Common Mistakes:

• Trying to assign beyond array length
• Using List methods on arrays
• Assuming Resize method exists