Linear Search Algorithm in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind the Linear Search Algorithm?

Linear Search checks each item in a list one by one until it finds the target or reaches the end.

beginner

What is the time complexity of Linear Search in the worst case?

The worst-case time complexity is O(n), where n is the number of items in the list.

beginner

Show the basic steps of Linear Search in simple terms.

1. Start from the first item.
2. Compare it with the target.
3. If it matches, stop and return the position.
4. If not, move to the next item.
5. Repeat until found or list ends.

intermediate

In which cases is Linear Search preferred over other search algorithms?

When the list is small or unsorted, or when simplicity is more important than speed.

beginner

What is the output of Linear Search if the target is not found in the list?

It returns -1 or a special value indicating the target is not present.

What does Linear Search do when it finds the target element?

A Sorts the list

B Continues searching the entire list

C Deletes the element

D Stops and returns the position

Linear Search stops immediately once it finds the target and returns its position.

What is the best case time complexity of Linear Search?

A O(1)

B O(n)

C O(log n)

D O(n^2)

If the target is the first element, Linear Search finds it immediately, so best case is O(1).

Which data structure does Linear Search work on?

A Any list or array

B Only sorted arrays

C Only linked lists

D Only trees

Linear Search can be used on any list or array regardless of sorting.

If the list has 10 elements and the target is not present, how many comparisons does Linear Search make?

A 1

B 10

C 0

D 5

Linear Search checks every element, so it makes 10 comparisons if target is not found.

Which of these is NOT true about Linear Search?

A It checks elements one by one

B It works on unsorted data

C It is very fast for large sorted lists

D It returns -1 if target is missing

Linear Search is slow for large lists; other algorithms like binary search are faster for sorted data.

Explain how Linear Search works step-by-step on a list of numbers.

Think about checking each item one by one.

Describe when you would choose Linear Search over other search methods.

Consider the size and order of the list.

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