Array Reversal Techniques in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse the array using a built-in method.

DSA C#

Array.[1](arr);

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

AReverse

BClear

CSort

DCopy

Attempts:

3 left

💡 Hint

Common Mistakes

Using Array.Sort which sorts instead of reversing.

Using Array.Clear which empties the array.

The Array.Reverse method reverses the elements of the array in place.

2fill in blank

medium

Complete the code to swap elements at positions i and j in the array.

DSA C#

int temp = arr[[1]];

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

Aj

Bi

C0

Darr.Length

Attempts:

3 left

💡 Hint

Common Mistakes

Using j instead of i for the temporary variable.

Using 0 or arr.Length which are not the correct indices.

To swap elements, first store the element at index i in a temporary variable.

3fill in blank

hard

Fix the error in the loop condition to reverse the array by swapping.

DSA C#

for (int i = 0; i < arr.[1] / 2; i++)

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

ASize

BCount

CLength

DCapacity

Attempts:

3 left

💡 Hint

Common Mistakes

Using Count which is for collections, not arrays.

Using Size or Capacity which do not exist for arrays.

The property Length gives the number of elements in the array.

4fill in blank

hard

Fill both blanks to complete the swap inside the loop for array reversal.

DSA C#

arr[[1]] = arr[[2]];

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

Ai

Barr.Length - 1 - i

Cj

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using j instead of the calculated opposite index.

Using 0 which always points to the first element.

Swap element at index i with element at index arr.Length - 1 - i to reverse the array.

5fill in blank

hard

Fill all three blanks to create a dictionary that maps each element to its reversed index.

DSA C#

var reversedIndices = new Dictionary<int, int> { { [1], [2] } };
int reversedIndex = arr.Length [3] 1 - i;

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

Aarr[i]

Bi

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using plus instead of minus for reversed index calculation.

Swapping key and value in the dictionary.

Map the element arr[i] to its reversed index i. Calculate reversedIndex as arr.Length - 1 - i.

Practice

1. What does reversing an array mean in simple terms?

easy

A. Removing duplicate elements from the array

B. Sorting the array in ascending order

C. Changing the order of elements so the last becomes first and first becomes last

D. Adding new elements at the end of the array

Solution

1. Step 1: Understand array order

   An array has elements arranged from first to last in a sequence.

2. Step 2: Define reversal

   Reversing means swapping elements so the last element moves to the front and the first moves to the end, flipping the order.

3. Final Answer:

   Changing the order of elements so the last becomes first and first becomes last -> Option C

4. Quick Check:

   Reversal = Flip order [OK]

Hint: Reversal flips the array order front to back [OK]

Common Mistakes:

• Confusing reversal with sorting
• Thinking reversal removes elements
• Assuming reversal adds elements

2. Which of the following is the correct syntax to reverse an array named arr in C# using built-in methods?

easy

A. Array.Reverse(arr);

B. arr.Reverse();

C. reverse(arr);

D. arr.reverse();

Solution

1. Step 1: Identify C# array reversal method

   C# provides a static method Array.Reverse() to reverse arrays in place.

2. Step 2: Check syntax correctness

   The correct syntax calls Array.Reverse(arr); with capital 'A' and 'R' and no return value.

3. Final Answer:

   Array.Reverse(arr); -> Option A

4. Quick Check:

   Use Array.Reverse(arr) in C# [OK]

Hint: Use Array.Reverse(arr) to reverse arrays in C# [OK]

Common Mistakes:

• Using lowercase method names
• Calling reverse as instance method on array
• Using non-existent reverse functions

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

int[] arr = {1, 2, 3, 4, 5};
Array.Reverse(arr);
Console.WriteLine(string.Join(", ", arr));

medium

A. 1, 2, 3, 4, 5

B. 5, 2, 3, 4, 1

C. Syntax Error

D. 5, 4, 3, 2, 1

Solution

1. Step 1: Understand Array.Reverse effect

   The method reverses the array elements in place, so the order flips.

2. Step 2: Trace the output

   Original array: 1, 2, 3, 4, 5 becomes 5, 4, 3, 2, 1 after reversal.

3. Final Answer:

   5, 4, 3, 2, 1 -> Option D

4. Quick Check:

   Reversed array prints 5 to 1 [OK]

Hint: Reversal flips array order exactly [OK]

Common Mistakes:

• Assuming reversal sorts the array
• Expecting original order after reversal
• Confusing output format

4. Identify the error in the following C# code that attempts to reverse an array:

int[] arr = {10, 20, 30};
arr.Reverse();
Console.WriteLine(string.Join(", ", arr));

medium

A. arr.Reverse() does not reverse the array in place

B. Array must be converted to list before reversing

C. Console.WriteLine syntax is incorrect

D. No error, code works fine

Solution

1. Step 1: Check arr.Reverse() method

   In C#, arr.Reverse() returns a reversed sequence but does not change the original array.

2. Step 2: Understand effect on original array

   The original array remains unchanged, so printing it shows original order, not reversed.

3. Final Answer:

   arr.Reverse() does not reverse the array in place -> Option A

4. Quick Check:

   arr.Reverse() returns reversed IEnumerable, no in-place change [OK]

Hint: Array.Reverse(arr) reverses in place; arr.Reverse() does not [OK]

Common Mistakes:

• Assuming arr.Reverse() changes original array
• Confusing LINQ Reverse with Array.Reverse
• Ignoring return value of arr.Reverse()

5. Given an integer array arr with an odd number of elements, which approach correctly reverses the array in place without using built-in methods?

hard

A. Create a new array and copy elements from end to start, then assign back to original array

B. Use two pointers, one at start and one at end, swap elements and move pointers towards center until they meet

C. Sort the array in descending order to reverse it

D. Remove the middle element and reverse the rest of the array

Solution

1. Step 1: Understand in-place reversal with two pointers

   Start with one pointer at the first element and another at the last element, swap their values.

2. Step 2: Move pointers towards center and swap

   Increment the start pointer and decrement the end pointer, continue swapping until pointers meet or cross.

3. Final Answer:

   Use two pointers, one at start and one at end, swap elements and move pointers towards center until they meet -> Option B

4. Quick Check:

   Two-pointer swap reverses array in place [OK]

Hint: Swap start and end elements moving inward [OK]

Common Mistakes:

• Using sorting instead of reversal
• Creating extra arrays unnecessarily
• Removing elements instead of swapping