Insertion Sort Algorithm in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Python

for i in range([1], len(arr)):
    key = arr[i]
    j = i - 1
    while j >= 0 and arr[j] > key:
        arr[j + 1] = arr[j]
        j -= 1
    arr[j + 1] = key

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

A0

B1

C2

Dlen(arr)

Attempts:

3 left

💡 Hint

Common Mistakes

Starting the loop from 0 causes unnecessary comparisons.

Using len(arr) as start index causes the loop to skip all elements.

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

2fill in blank

medium

Complete the code to compare elements correctly inside the while loop.

DSA Python

while j >= 0 and arr[j] [1] key:
    arr[j + 1] = arr[j]
    j -= 1

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

A>

B<=

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will shift smaller elements incorrectly.

Using '==' or '<=' will not correctly sort the list.

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 the code to insert the key at the correct position after shifting.

DSA Python

arr[j [1] 1] = key

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

A/

B-

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' places the key in the wrong position.

Using '*' or '/' causes syntax or runtime errors.

After shifting elements, the key is placed at position j + 1.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps each element to its square if the element is less than 5.

DSA Python

{x: x[1]2 for x in arr if x [2] 5}

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

A**

B>

C<

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '**' will add instead of square.

Using '>' will filter wrong elements.

The comprehension squares elements (x ** 2) and filters those less than 5 (x < 5).

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps uppercase keys to their values only if the value is positive.

DSA Python

result = [1]: [2] for [3], v in data.items() if v > 0}

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

Ak.upper()

Bv

Ck

Dv.upper()

Attempts:

3 left

💡 Hint

Common Mistakes

Using v.upper() causes errors because values may not be strings.

Using wrong variable names breaks the comprehension.

The comprehension uses k.upper() as keys, v as values, and iterates over k, v.

Practice

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

easy

A. Build the sorted list one element at a time by inserting elements in their correct position

B. Divide the list into halves and sort each half separately

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

D. Select the smallest element and swap it with the first unsorted element

Solution

1. Step 1: Understand insertion sort process

   Insertion sort builds a sorted list by taking one element at a time and placing it in the correct position among the already sorted elements.

2. Step 2: Compare with other sorting methods

   Options A, B, and D describe other sorting algorithms: bubble sort, merge sort, and selection sort respectively.

3. Final Answer:

   Build the sorted list one element at a time by inserting elements in their correct position -> Option A

4. Quick Check:

   Insertion sort = Insert elements in order [OK]

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

Common Mistakes:

• Confusing insertion sort with bubble sort
• Thinking insertion sort divides the list
• Mixing insertion sort with selection sort

2. Which of the following is the correct syntax to start the outer loop for insertion sort on a list arr of length n in Python?

easy

A. for i in range(0, n-1):

B. for i in range(n):

C. for i in range(1, n):

D. for i in range(n-1, 0, -1):

Solution

1. Step 1: Identify insertion sort loop start

   The outer loop starts from index 1 because the first element (index 0) is considered sorted.

2. Step 2: Check options

   for i in range(1, n): correctly uses range(1, n). Options B and C start from 0 which is incorrect. for i in range(n-1, 0, -1): loops backwards which is not standard for insertion sort.

3. Final Answer:

   for i in range(1, n): -> Option C

4. Quick Check:

   Start loop at 1 for insertion sort [OK]

Hint: Start outer loop from 1, not 0 [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Using reverse loop incorrectly
• Confusing loop ranges

3. What will be the output of the following code snippet?

def insertion_sort(arr):
    for i in range(1, len(arr)):
        key = arr[i]
        j = i - 1
        while j >= 0 and arr[j] > key:
            arr[j + 1] = arr[j]
            j -= 1
        arr[j + 1] = key

arr = [4, 3, 2, 1]
insertion_sort(arr)
print(arr)

medium

A. [4, 3, 2, 1]

B. [1, 2, 3, 4]

C. [1, 3, 2, 4]

D. [2, 3, 4, 1]

Solution

1. Step 1: Trace the insertion sort on input

   The list [4, 3, 2, 1] is sorted step-by-step by moving smaller elements leftwards until fully sorted.

2. Step 2: Final sorted list

   After sorting, the list becomes [1, 2, 3, 4].

3. Final Answer:

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

4. Quick Check:

   Sorted ascending = [1, 2, 3, 4] [OK]

Hint: Insertion sort outputs sorted ascending list [OK]

Common Mistakes:

• Assuming original list remains unchanged
• Confusing output with partial sorting
• Misreading the while loop condition

4. Find the error in the following insertion sort code:

def insertion_sort(arr):
    for i in range(1, len(arr)):
        key = arr[i]
        j = i
        while j > 0 and arr[j-1] > key:
            arr[j] = arr[j-1]
            j -= 1
        arr[j] = key

arr = [3, 1, 2]
insertion_sort(arr)
print(arr)

medium

A. The for loop should start from 0, not 1

B. The while loop condition should be j >= 0, not j > 0

C. The assignment arr[j] = key should be outside the while loop

D. The variable j should start at i - 1, not i

Solution

1. Step 1: Check initialization of j

   In insertion sort, j should start at i - 1 to compare the key with previous elements using the standard condition arr[j] > key.

2. Step 2: Verify other parts

   The while loop condition and assignment are correct. The for loop correctly starts at 1.

3. Final Answer:

   The variable j should start at i - 1, not i -> Option D

4. Quick Check:

   j = i - 1 to compare previous elements [OK]

Hint: Start j at i-1 to compare with previous elements [OK]

Common Mistakes:

• Using j = i instead of standard j = i-1
• Changing while loop condition incorrectly
• Misplacing key assignment inside loop

5. Given a list arr = [5, 3, 8, 6, 2], how many total shifts (element moves) will insertion sort perform to sort the list in ascending order?

hard

A. 6

B. 8

C. 7

D. 5

Solution

1. Step 1: Trace shifts during sorting

   Insertion sort shifts elements to insert the key in the correct place:
   - i=1 (key=3): shift 5 right (1 shift)
   - i=2 (key=8): no shift (0 shifts)
   - i=3 (key=6): shift 8 right (1 shift)
   - i=4 (key=2): shift 8, 6, 5, 3 right (4 shifts)

2. Step 2: Calculate total shifts

   Total shifts = 1 + 0 + 1 + 4 = 6 shifts. But note that each shift moves one element one position right. Counting carefully:
   At i=1: 1 shift
   At i=3: 1 shift
   At i=4: 4 shifts
   Total = 6 shifts

3. Step 3: Recount carefully

   Actually, at i=4, elements 3,5,6,8 are shifted right one by one (4 shifts). Total shifts = 1 + 0 + 1 + 4 = 6.

4. Final Answer:

   6 -> Option A

5. Quick Check:

   Count shifts per insertion = 6 [OK]

Hint: Count shifts by tracing each insertion step [OK]

Common Mistakes:

• Counting comparisons instead of shifts
• Missing multiple shifts in one insertion
• Confusing shifts with swaps