Linear Search Algorithm in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the linear search loop correctly.

DSA C#

for (int [1] = 0; [1] < arr.Length; [1]++) {
    if (arr[[1]] == target) {
        return [1];
    }
}

Drag options to blanks, or click blank then click option'

Aindex

Bi

Cpos

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name not declared in the loop header.

Using a variable name that does not match the one used inside the loop.

The variable i is commonly used as the loop counter in C# for loops. It must be declared in the for loop to iterate over the array indices.

2fill in blank

medium

Complete the code to return -1 when the target is not found.

DSA C#

public int LinearSearch(int[] arr, int target) {
    for (int i = 0; i < arr.Length; i++) {
        if (arr[i] == target) {
            return i;
        }
    }
    return [1];
}

Drag options to blanks, or click blank then click option'

A-1

Bnull

C0

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 0 which could be a valid index.

Returning null which is invalid for int return type.

Returning -1 is a common way to indicate that the target was not found in the array.

3fill in blank

hard

Fix the error in the loop condition to avoid runtime errors.

DSA C#

for (int i = 0; i [1] arr.Length; i++) {
    if (arr[i] == target) {
        return i;
    }
}

Drag options to blanks, or click blank then click option'

A<=

B>

C<

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= causes the loop to access an invalid index.

Using > or >= will not enter the loop correctly.

The loop should run while i is less than arr.Length because array indices go from 0 to Length-1.

4fill in blank

hard

Fill both blanks to complete the linear search method signature and return type.

DSA C#

public [1] LinearSearch([2][] arr, int target) {
    for (int i = 0; i < arr.Length; i++) {
        if (arr[i] == target) {
            return i;
        }
    }
    return -1;
}

Drag options to blanks, or click blank then click option'

Aint

Bstring

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using string or bool as return type.

Using wrong array type for the parameter.

The method returns an int index and takes an array of int as input.

5fill in blank

hard

Fill all three blanks to complete the code that populates a dictionary mapping elements to their indices if they match the target.

DSA C#

var result = new Dictionary<int, int>();
for (int [3] = 0; [3] < arr.Length; [3]++) {
    if (arr[[3]] == target) {
        result[[1]] = [2];
    }
}

Drag options to blanks, or click blank then click option'

Aarr[i]

Bi

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names.

Swapping keys and values.

This creates a dictionary where keys are the matching elements and values are their indices. The loop variable is i.

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