Linear Search Algorithm in DSA C# - Build from Scratch

Linear Search Algorithm

📖 Scenario: You are helping a store manager find a specific product ID in a list of product IDs. The manager wants to know if the product is available and at which position in the list.

🎯 Goal: Build a simple linear search program that looks for a product ID in a list and tells the manager the position of the product if found, or that it is not available.

📋 What You'll Learn

Create an array of product IDs with exact values

Create a variable for the product ID to search

Use a linear search loop to find the product ID

Print the position if found or a not found message

💡 Why This Matters

🌍 Real World

Stores and inventory systems often need to find if a product is available in their list quickly.

💼 Career

Understanding linear search helps in basic data lookup tasks and is a foundation for learning more efficient search algorithms.

1

Create the product ID list

Create an integer array called productIds with these exact values: 101, 203, 405, 302, 504.

DSA C#

// Create the productIds array with the given values
// Your code here

Hint

Use int[] productIds = {101, 203, 405, 302, 504}; to create the array.

2

Set the product ID to search

Create an integer variable called searchId and set it to 302.

DSA C#

int[] productIds = {101, 203, 405, 302, 504};
// Create the searchId variable and set it to 302
// Your code here

Hint

Use int searchId = 302; to set the product ID to find.

3

Implement the linear search

Use a for loop with variable i to go through productIds. Inside the loop, use an if statement to check if productIds[i] equals searchId. If yes, break the loop.

DSA C#

int[] productIds = {101, 203, 405, 302, 504};
int searchId = 302;

int foundIndex = -1;
// Use a for loop with i to search productIds for searchId
// Your code here

Hint

Use a for loop to check each product ID. When you find the match, save the index and stop the loop.

4

Print the search result

Use an if statement to check if foundIndex is not -1. If yes, print "Product found at position: " followed by foundIndex. Otherwise, print "Product not found".

DSA C#

int[] productIds = {101, 203, 405, 302, 504};
int searchId = 302;

int foundIndex = -1;
for (int i = 0; i < productIds.Length; i++)
{
    if (productIds[i] == searchId)
    {
        foundIndex = i;
        break;
    }
}

// Print the result based on foundIndex
// Your code here

Hint

Check if foundIndex is not -1 to print the position. Otherwise, print the not found message.

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