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Python Insertion Sort

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Insertion Sort builds the final sorted list one element at a time. It is simple, intuitive, and works well with small or nearly sorted lists.

It’s the same way we often sort playing cards in our hands!

🔹 How Insertion Sort Works

  1. Start from the second element

  2. Compare it with elements before

  3. Shift larger elements one position to the right

  4. Insert the current element into its correct position

  5. Repeat for all elements

🔹 Insertion Sort in Python

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
Example
numbers = [12, 11, 13, 5, 6]

insertion_sort(numbers)
print("Sorted array:", numbers)

✅ Output:

Sorted array: [5, 6, 11, 12, 13]

🔹 Visual Example (First 2 Steps)

Start: [12, 11, 13, 5, 6]
→ Compare 11 with 12 → [11, 12, 13, 5, 6]
→ Compare 13 with 12 → No change

🔹 Time Complexity

Case Time
Best Case O(n)
Average O(n²)
Worst Case O(n²)
Space O(1)

It performs better than bubble/selection sort for nearly sorted lists.

🔹 Use Cases of Insertion Sort

  • Best for small datasets

  • When input is already partially sorted

  • Stable sort (preserves order of equal elements)

Practice Questions

Q1. Write a Python program to sort the list [7, 3, 5, 2] using insertion sort.

Q2. Write a Python program to modify insertion sort to sort a list in descending order.

Q3. Write a Python program to accept a list of numbers from the user and sort them using insertion sort.

Q4. Write a Python program to count and print how many times the array is modified during insertion sort.

Q5. Write a Python program to sort a list of strings alphabetically using insertion sort.

Q6. Write a Python program to print the list after each insertion step during insertion sort.

Q7. Write a Python program to sort only the first half of a list using insertion sort.

Q8. Write a Python program to implement insertion sort without using a separate function (write code directly in the main block).

Q9. Write a Python program to sort a list using reverse logic in insertion sort (from end to start).

Q10. Write a Python program to compare the time taken by insertion sort and selection sort to sort a list of 1,000 random numbers.

Python Insertion Sort Quiz

Q1: What is the key idea of insertion sort?

A. Swap adjacent elements
B. Pick max and move to end
C. Insert element in sorted part
D. Divide and merge

Q2: In which case is insertion sort most efficient?

A. Random list
B. Sorted list
C. Reversed list
D. Large list

Q3: Which sort works like sorting cards in hand?

A. Bubble
B. Insertion
C. Selection
D. Merge

Q4: What is the worst-case time complexity?

A. O(n)
B. O(log n)
C. O(n²)
D. O(n log n)

Q5: What happens when current element is smaller than all before?

A. Inserted at front
B. Ignored
C. Swapped
D. Reversed

Q6: Insertion sort is a __________ sort.

A. Divide-and-conquer
B. Greedy
C. Stable
D. Unstable

Q7: Which loop is commonly used to shift elements in Insertion Sort?

A. For
B. While
C. Do-while
D. Nested for

Q8: Can Insertion Sort be used to sort strings?

A. Yes
B. No
C. Only short strings
D. Only if strings are numeric

Q9: Space complexity of insertion sort?

A. O(1)
B. O(n)
C. O(n²)
D. O(log n)

Q10: What happens after each outer loop iteration in Insertion Sort?

A. Whole list is sorted
B. One more item is sorted
C. All elements are reversed
D. Largest element moves to the end
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