Linear Search Algorithm in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Linear Search on Array

What is the output of the following C# code that performs a linear search for the value 7 in the array?

DSA C#

int[] arr = {3, 5, 7, 9, 11};
int target = 7;
int index = -1;
for (int i = 0; i < arr.Length; i++)
{
    if (arr[i] == target)
    {
        index = i;
        break;
    }
}
Console.WriteLine(index);

A 3

B -1

C 1

D 2

💡 Hint

Remember that array indexing in C# starts at 0.

The value 7 is at index 2 in the array. The loop stops when it finds 7 and prints the index 2.

🧠 Conceptual

intermediate

Number of Comparisons in Worst Case

If you perform a linear search for a value that is not present in an array of length 10, how many comparisons will the algorithm make?

A 9

B 11

C 10

D 0

💡 Hint

Think about how many elements the algorithm checks before concluding the value is not found.

In the worst case, linear search checks every element once. For 10 elements, it makes 10 comparisons.

🔧 Debug

advanced

Identify the Bug in Linear Search Implementation

What error will the following C# code produce when searching for the value 4 in the array?

DSA C#

int[] arr = {1, 2, 3, 5, 6};
int target = 4;
int index = 0;
for (int i = 0; i <= arr.Length; i++)
{
    if (arr[i] == target)
    {
        index = i;
        break;
    }
}
Console.WriteLine(index);

A IndexOutOfRangeException

B No output (infinite loop)

C 0

D -1

💡 Hint

Check the loop condition carefully and remember array indexing rules.

The loop uses i <= arr.Length which tries to access arr[arr.Length], causing IndexOutOfRangeException.

🚀 Application

advanced

Linear Search on Linked List

Given a singly linked list with nodes containing values [10, 20, 30, 40], what is the output index when searching for value 30 using linear search?

DSA C#

class Node {
    public int val;
    public Node next;
    public Node(int v) { val = v; next = null; }
}

Node head = new Node(10);
head.next = new Node(20);
head.next.next = new Node(30);
head.next.next.next = new Node(40);

int target = 30;
int index = 0;
Node current = head;
while (current != null)
{
    if (current.val == target)
        break;
    current = current.next;
    index++;
}
Console.WriteLine(current == null ? -1 : index);

A 1

B 2

C -1

D 3

💡 Hint

Count nodes starting from 0 until you find the target value.

The value 30 is in the third node, which is index 2 starting from 0.

🧠 Conceptual

expert

Time Complexity of Linear Search in Different Cases

What is the average time complexity of linear search in an unsorted array of size n?

A O(n)

B O(log n)

C O(1)

D O(n^2)

💡 Hint

Think about how many elements are checked on average before finding the target.

On average, linear search checks half the elements, which is proportional to n, so time complexity is O(n).

Practice

1. What does the linear search algorithm do when searching for a value in a list?

easy

A. It checks each element one by one until it finds the value or reaches the end.

B. It divides the list into halves to find the value quickly.

C. It sorts the list before searching for the value.

D. It uses a hash table to find the value instantly.

Solution

1. Step 1: Understand linear search process

   Linear search checks each element in order from start to end.

2. Step 2: Compare with other search methods

   Other methods like binary search divide the list, but linear search does not.

3. Final Answer:

   It checks each element one by one until it finds the value or reaches the end. -> Option A

4. Quick Check:

   Linear search = check elements one by one [OK]

Hint: Linear search checks elements one by one [OK]

Common Mistakes:

• Confusing linear search with binary search
• Thinking linear search sorts the list first
• Assuming linear search uses extra data structures

2. Which of the following is the correct syntax to start a linear search loop in C# for an array named arr?

easy

A. for (int i = 0; i < arr.Length; i++)

B. for (int i = 1; i <= arr.Length; i++)

C. foreach (int i in arr.Length)

D. while (i < arr.Length) i++"

Solution

1. Step 1: Identify correct for loop syntax for arrays

   In C#, arrays use zero-based indexing, so loop starts at 0 and runs while i < arr.Length.

2. Step 2: Check each option

   for (int i = 0; i < arr.Length; i++) uses correct syntax: for (int i = 0; i < arr.Length; i++). Others have errors like starting at 1 or wrong foreach syntax.

3. Final Answer:

   for (int i = 0; i < arr.Length; i++) -> Option A

4. Quick Check:

   Loop from 0 to Length-1 = for (int i = 0; i < arr.Length; i++) [OK]

Hint: Array loops start at 0 and go to Length-1 [OK]

Common Mistakes:

• Starting loop at 1 instead of 0
• Using <= instead of < in loop condition
• Incorrect foreach syntax for arrays

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

int[] arr = {4, 7, 1, 3, 9};
int target = 3;
int index = -1;
for (int i = 0; i < arr.Length; i++) {
  if (arr[i] == target) {
    index = i;
    break;
  }
}
Console.WriteLine(index);

medium

A. 4

B. 1

C. -1

D. 3

Solution

1. Step 1: Trace the loop to find target 3

   Check elements: arr[0]=4 (no), arr[1]=7 (no), arr[2]=1 (no), arr[3]=3 (yes), so index = 3 and break.

2. Step 2: Print the index found

   After loop, index = 3, so Console.WriteLine prints 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Target 3 found at index 3 [OK]

Hint: Stop loop when target found, print index [OK]

Common Mistakes:

• Forgetting to break loop after finding target
• Printing value instead of index
• Confusing index with element value

4. The following code is intended to perform a linear search but has a bug. What is the bug?

int[] arr = {2, 5, 8, 12};
int target = 8;
int index = -1;
for (int i = 0; i <= arr.Length; i++) {
  if (arr[i] == target) {
    index = i;
    break;
  }
}
Console.WriteLine(index);

medium

A. The index variable should be initialized to 0.

B. The target value is not in the array.

C. The loop condition should be i < arr.Length, not i <= arr.Length.

D. The break statement is missing.

Solution

1. Step 1: Analyze loop condition

   The loop runs while i <= arr.Length, but valid indices are 0 to arr.Length - 1. Using <= causes out-of-range error.

2. Step 2: Correct the loop condition

   Change condition to i < arr.Length to avoid accessing invalid index.

3. Final Answer:

   The loop condition should be i < arr.Length, not i <= arr.Length. -> Option C

4. Quick Check:

   Loop condition must be less than Length [OK]

Hint: Use i < arr.Length to avoid index errors [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Assuming target not found is a bug
• Changing index initialization unnecessarily

5. You have an array int[] arr = {5, 3, 7, 3, 9}. You want to find the index of the second occurrence of the value 3 using linear search. Which approach is correct?

hard

A. Sort the array first, then find the value 3.

B. Use a counter to track occurrences and return index when counter reaches 2.

C. Return the first index where value 3 is found.

D. Use binary search to find the second occurrence.

Solution

1. Step 1: Understand the problem of finding second occurrence

   Linear search must count how many times target appears and return index at second time.

2. Step 2: Evaluate options

   Sort the array first, then find the value 3. sorts the array, changing indices. Return the first index where value 3 is found. returns only the first occurrence. Use binary search to find the second occurrence. uses binary search, unsuitable for unsorted array and specific occurrences. Use a counter to track occurrences and return index when counter reaches 2. correctly uses a counter to track occurrences and return the index of the second match.

3. Final Answer:

   Use a counter to track occurrences and return index when counter reaches 2. -> Option B

4. Quick Check:

   Count occurrences to find second index [OK]

Hint: Count matches, return index at second match [OK]

Common Mistakes:

• Returning first occurrence instead of second
• Sorting array which changes original order
• Using binary search on unsorted array