# Python program for insertion sort

In this post, you will learn about the insertion sort and how to write an insertion sort program using Python.

The **Insertion Sort** is a very simple technique, it inserts each item in the proper place in the final list. Insertion sort is less effective as compared to the other sorting technique. In this, the array elements are divided into two parts.

## Insertion sort example

These are the insertion sorting techniques.

Search for the element less than the first element. **A[1] < A[0]**, so interchange them.

Next search for the next element less than the first element. **A[2] < A[0]**, so interchange them.

Now there is no element in the right which is less than the first element. Similarly, we repeat the same process for **A[1]**, **A[2]** and **A[3]**.

While sorting of **A[3]**, we found **A[3] > A[4]**, so we interchange them.

Finally, we get the sorted array element.

## Complexity of Insertion sort

Suppose **n** is the number of elements in an array.

`Best Time Complexity - `**O(n)**
Worst Time Complexity - **O(n**^{2})
Average Time Complexity - **O(n**^{2})

## Insertion sort program in Python

In the given example, we create a function **insertion_sort()** that takes an array as an argument. Inside the function, we create a for loop with a loop variable **i** that counts from 1 to the length of the array. Next, we set a variable **key** equal to the element at index **i** and set **j** equal to **i – 1**. We create a while loop that runs as long as **j** is non-negative and key is smaller than the element at index **j**. Within this loop, we set the element at index **j + 1** equal to the element at index **j** and decrement **j**. When the while loop finishes, we set the element at index **j + 1** equal to **key**.

```
# Insertion sort in Python
def insertion_sort(arr):
for i in range(1, len(arr)):
key = arr[i]
# Move elements of arr[0..i-1], that are greater than key,
# to one position ahead of their current position
j = i-1
while j >=0 and key < arr[j] :
arr[j+1] = arr[j]
j -= 1
# Place key at after the element just smaller than it.
arr[j+1] = key
# main
# Initializing array
arr = [42, 55, 23, 64, 12, 66, 32, 14, 16, 63]
insertion_sort(arr)
print ("The sorted array is:")
for i in range(len(arr)):
print (arr[i])
```

**Output of the above code:**

```
The sorted array is:
12
14
16
23
32
42
55
63
64
66
```

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