Insertion Sort Algorithm in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the outer loop from the second element in the array.

DSA C#

for (int i = [1]; i < arr.Length; i++)
{
    int key = arr[i];
    int j = i - 1;
    while (j >= 0 && arr[j] > key)
    {
        arr[j + 1] = arr[j];
        j--;
    }
    arr[j + 1] = key;
}

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

A1

B0

C2

Darr.Length - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting the loop at 0 causes unnecessary comparisons.

Starting at the last index skips sorting the array.

The insertion sort starts from the second element (index 1) because the first element is considered sorted.

2fill in blank

medium

Complete the condition to move elements greater than the key to the right.

DSA C#

while (j >= 0 && arr[j] [1] key)
{
    arr[j + 1] = arr[j];
    j--;
}

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

A<

B<=

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' causes incorrect shifting.

Using '==' does not move elements properly.

We move elements that are greater than the key to the right to make space for the key.

3fill in blank

hard

Fix the error in assigning the key to its correct position after shifting.

DSA C#

arr[[1]] = key;

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

Ai - 1

Bj + 1

Cj

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using j places the key in the wrong position.

Using i or i - 1 does not reflect the shifted position.

After shifting elements, the key should be placed at index j + 1.

4fill in blank

hard

Fill both blanks to complete the insertion sort outer and inner loops correctly.

DSA C#

for (int [1] = 1; [1] < arr.Length; [1]++)
{
    int key = arr[[2]];
    int j = [2] - 1;
    while (j >= 0 && arr[j] > key)
    {
        arr[j + 1] = arr[j];
        j--;
    }
    arr[j + 1] = key;
}

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

Ai

Bj

Ck

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names causes errors.

Using 'j' for the outer loop conflicts with inner loop variable.

The outer loop variable is commonly 'i' and must be consistent in the loop and key assignment.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each element to its sorted position if greater than 2.

DSA C#

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

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

Aarr[{{BLANK_3}}]

B{{BLANK_3}}

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names causes syntax errors.

Mixing keys and values incorrectly in the dictionary.

The dictionary maps index ({{BLANK_3}}) to the element value (arr[{{BLANK_3}}]) for elements greater than 2.

Practice

1. What is the main idea behind the Insertion Sort algorithm?

easy

A. It builds the sorted array one element at a time by inserting elements in their correct position.

B. It divides the array into halves and sorts each half recursively.

C. It repeatedly swaps adjacent elements if they are in the wrong order.

D. It selects the smallest element and swaps it with the first unsorted element.

Solution

1. Step 1: Understand insertion sort process

   Insertion sort takes one element at a time and inserts it into the correct position in the already sorted part of the array.

2. Step 2: Compare with other sorting methods

   Other options describe merge sort (B), bubble sort (C), and selection sort (D), which are different algorithms.

3. Final Answer:

   It builds the sorted array one element at a time by inserting elements in their correct position. -> Option A

4. Quick Check:

   Insertion sort = Insert elements step-by-step [OK]

Hint: Insertion sort inserts elements one by one in order [OK]

Common Mistakes:

• Confusing insertion sort with bubble sort
• Thinking insertion sort divides array recursively
• Mixing up selection sort with insertion sort

2. Which of the following is the correct syntax to start the outer loop for insertion sort in C# for an array arr of length n?

easy

A. for (int i = 0; i < n; i++)

B. for (int i = 1; i <= n; i++)

C. for (int i = n - 1; i >= 0; i--)

D. for (int i = 1; i < n; i++)

Solution

1. Step 1: Identify starting index for insertion sort

   Insertion sort starts from the second element (index 1) because the first element is considered sorted.

2. Step 2: Check loop conditions

   The loop runs while i < n to cover all elements. for (int i = 1; i < n; i++) matches this.

3. Final Answer:

   for (int i = 1; i < n; i++) -> Option D

4. Quick Check:

   Start from 1 to n-1 for insertion sort loop [OK]

Hint: Start outer loop at 1, not 0, for insertion sort [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Using <= n instead of < n causing out-of-bounds
• Counting backwards incorrectly

3. What will be the output array after applying insertion sort on int[] arr = {4, 3, 2, 1}?

medium

A. [1, 3, 2, 4]

B. [4, 3, 2, 1]

C. [1, 2, 3, 4]

D. [2, 3, 4, 1]

Solution

1. Step 1: Trace insertion sort on given array

   Start with 4 (sorted). Insert 3 before 4 -> [3,4,2,1]. Insert 2 before 3 -> [2,3,4,1]. Insert 1 before 2 -> [1,2,3,4].

2. Step 2: Confirm final sorted array

   The array is sorted in ascending order: [1, 2, 3, 4].

3. Final Answer:

   [1, 2, 3, 4] -> Option C

4. Quick Check:

   Insertion sort sorts ascending -> [1, 2, 3, 4] [OK]

Hint: Insertion sort sorts array ascending step-by-step [OK]

Common Mistakes:

• Assuming array stays unchanged
• Mixing up positions during insertion
• Stopping before fully sorting

4. Identify the error in this insertion sort code snippet:

for (int i = 1; i < arr.Length; i++) {
    int key = arr[i];
    int j = i + 1;
    while (j >= 0 && arr[j] > key) {
        arr[j + 1] = arr[j];
        j--;
    }
    arr[j + 1] = key;
}

medium

A. The variable j should start at i - 1 instead of i + 1.

B. The while loop condition should use j < arr.Length instead of j >= 0.

C. The key should be assigned after the while loop, not before.

D. The array index arr[j + 1] causes out-of-bounds error.

Solution

1. Step 1: Check initialization of j

   In insertion sort, j starts at the element before i, so it should be i - 1, not i + 1.

2. Step 2: Verify while loop condition and shifting

   The condition j >= 0 is correct to move backwards. The shifting logic is correct if j starts properly.

3. Final Answer:

   The variable j should start at i - 1 instead of i + 1. -> Option A

4. Quick Check:

   j starts at i-1 to compare previous elements [OK]

Hint: j must start left of i to compare previous elements [OK]

Common Mistakes:

• Starting j at i+1 causing wrong comparisons
• Incorrect while loop boundary causing errors
• Misplacing key assignment inside loop

5. Given a nearly sorted array int[] arr = {1, 2, 4, 3, 5, 6}, how many element shifts will insertion sort perform to fully sort it?

hard

A. 2

B. 1

C. 3

D. 4

Solution

1. Step 1: Identify where shifts occur

   Only the element 3 is out of order. It needs to be inserted before 4.

2. Step 2: Count shifts to insert 3

   To insert 3, j=2 (arr[2]=4 > 3), shift arr[3]=arr[2] (1 shift), j=1 (arr[1]=2 < 3), stop. Place arr[2]=3 (not a shift).

3. Step 3: Check if other shifts occur

   No other elements need shifting. Total shifts = 1.

4. Final Answer:

   1 -> Option B

5. Quick Check:

   One shift when inserting 3 [OK]

Hint: Count only elements moved to insert out-of-place items [OK]

Common Mistakes:

• Counting swaps instead of shifts
• Ignoring that placing key is not a shift
• Assuming all elements shift