Why Binary Search and What Sorted Order Gives You in DSA C# - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the middle index in binary search.

DSA C#

int mid = low + (high - low) / [1];

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

A2

B3

C4

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 or 4 instead of 2 causes wrong middle calculation.

In binary search, we find the middle by dividing the range by 2.

2fill in blank

medium

Complete the code to check if the middle element is the target.

DSA C#

if (arr[mid] == [1]) { return mid; }

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

Ahigh

Btarget

Clow

Dmid

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with mid index instead of target value.

We compare the middle element with the target value we want to find.

3fill in blank

hard

Fix the error in updating the search range when target is less than middle element.

DSA C#

if (target < arr[mid]) { high = [1]; }

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

Amid - 1

Bmid + 1

Clow + 1

Dhigh - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting high to mid + 1 causes infinite loop or wrong search.

If the target is less than the middle element, we move the high pointer just before mid.

4fill in blank

hard

Fill both blanks to update the search range correctly when target is greater than middle element.

DSA C#

if (target > arr[mid]) { low = [1]; high = [2]; }

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

Amid + 1

Bmid - 1

Chigh

Dlow

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping low and high updates causes wrong search range.

When target is greater, we move low just after mid and keep high unchanged.

5fill in blank

hard

Fill all three blanks to complete the binary search method.

DSA C#

public int BinarySearch(int[] arr, int target) {
    int low = 0, high = arr.Length - 1;
    while (low <= high) {
        int mid = low + (high - low) / [1];
        if (arr[mid] == [2]) return mid;
        else if (target < arr[mid]) high = mid [3] 1;
        else low = mid + 1;
    }
    return -1;
}

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

A2

Btarget

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operators or variables causes incorrect search behavior.

This completes the binary search: middle index calculation, comparison with target, and updating high pointer.

Practice

1. Why is it important for a list to be sorted before applying binary search?

easy

A. Because binary search can only find the first element in any list

B. Because binary search works only on lists with unique elements

C. Because binary search divides the list and compares values assuming order

D. Because binary search requires the list to be reversed

Solution

1. Step 1: Understand binary search working principle

   Binary search splits the list into halves and compares the middle element to the target, assuming the list is sorted.

2. Step 2: Importance of sorted order

   If the list is not sorted, the halves cannot be correctly chosen, so binary search fails to find the target efficiently.

3. Final Answer:

   Because binary search divides the list and compares values assuming order -> Option C

4. Quick Check:

   Sorted list needed for binary search = True [OK]

Hint: Binary search needs order to split search space correctly [OK]

Common Mistakes:

• Thinking binary search works on unsorted lists
• Assuming binary search finds duplicates only
• Confusing binary search with linear search

2. Which of the following is the correct syntax to start a binary search on a sorted array named arr in C#?

easy

A. int index = Array.BinarySearch(arr, target);

B. int index = arr.BinarySearch(target);

C. int index = BinarySearch(arr, target);

D. int index = arr.SearchBinary(target);

Solution

1. Step 1: Recall C# binary search method usage

   C# provides a static method Array.BinarySearch that takes the array and the target value.

2. Step 2: Check method call correctness

   int index = Array.BinarySearch(arr, target); correctly calls Array.BinarySearch(arr, target). Other options use invalid syntax or methods.

3. Final Answer:

   int index = Array.BinarySearch(arr, target); -> Option A

4. Quick Check:

   Correct C# binary search syntax = int index = Array.BinarySearch(arr, target); [OK]

Hint: Use Array.BinarySearch for arrays in C# [OK]

Common Mistakes:

• Trying to call BinarySearch as instance method on array
• Using wrong method names like SearchBinary
• Missing the Array class prefix

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

int[] arr = {2, 4, 6, 8, 10};
int index = Array.BinarySearch(arr, 6);
Console.WriteLine(index);

medium

A. -1

B. 2

C. 3

D. 0

Solution

1. Step 1: Understand array and target

   The array is sorted: [2, 4, 6, 8, 10]. The target is 6.

2. Step 2: Find index of target in array

   6 is at index 2 (0-based indexing).

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Index of 6 in array = 2 [OK]

Hint: BinarySearch returns index of found element [OK]

Common Mistakes:

• Confusing index with value
• Assuming 1-based indexing
• Expecting -1 when element is present

4. The following code tries to find the index of 5 in a sorted array but gives an unexpected result. What is the error?

int[] arr = {1, 3, 7, 9, 11};
int index = Array.BinarySearch(arr, 5);
Console.WriteLine(index);

medium

A. 5 is not in the array, so negative index is returned

B. The method call syntax is incorrect

C. BinarySearch cannot be used on integers

D. The array is not sorted

Solution

1. Step 1: Check if array is sorted

   The array [1, 3, 7, 9, 11] is sorted in ascending order.

2. Step 2: Understand BinarySearch return when element not found

   When the target (5) is not found, BinarySearch returns a negative number indicating the bitwise complement of the insertion point.

3. Final Answer:

   5 is not in the array, so negative index is returned -> Option A

4. Quick Check:

   Not found element returns negative index [OK]

Hint: Negative result means element not found [OK]

Common Mistakes:

• Thinking array is unsorted
• Expecting -1 always when not found
• Believing syntax is wrong

5. You have a sorted list of student scores: int[] scores = {55, 67, 78, 89, 92, 95}. You want to find if a score of 80 exists using binary search. If not found, you want to insert 80 in the correct position to keep the list sorted. Which approach correctly achieves this?

hard

A. Sort the array after inserting 80 at the end

B. Use Array.BinarySearch and insert at returned index directly

C. Use linear search to find 80 and insert if missing

D. Use Array.BinarySearch to find 80; if negative, insert at ~index returned

Solution

1. Step 1: Use Array.BinarySearch to find 80

   BinarySearch returns index if found; if not found, returns negative number which is bitwise complement of insertion point.

2. Step 2: Insert 80 at correct position

   If result is negative, invert bits (~index) to get insertion position to keep array sorted.

3. Final Answer:

   Use Array.BinarySearch to find 80; if negative, insert at ~index returned -> Option D

4. Quick Check:

   Negative index means insert at bitwise complement [OK]

Hint: Insert at ~index if BinarySearch returns negative [OK]

Common Mistakes:

• Inserting at negative index directly
• Not using binary search result properly
• Sorting after insertion instead of correct position