Python – Check If List Is Sorted Or Not

In this tutorial, we will look at how to check if a list is sorted or not in Python with the help of some examples.

How to check if a list is sorted in Python?

To check if a list is sorted, you can use the following methods –

1. Compare the list with its corresponding sorted list.
2. Iterate through the list and see if the elements are in order.

Let’s now take a look at the above methods in detail with the help of some examples.

Compare the list with corresponding sorted list

To check if a list is sorted in Python, compare the list to the list resulting from a sort operation on the original list. If the list resulting after a sort operation is the same as the original list we can say that the original list is sorted.

You can use the python built-in sorted() function to get the sorted list. Let’s take a look at an example.

# create a list
ls = [1, 2, 3, 4, 5]
# check if list is sorted
print(ls == sorted(ls))

Output:

True

Here we get True as the output because the original list ls is already sorted.

Let’s look at another example.

# create a list
ls = [1, 2, 4, 3, 5]
# check if list is sorted
print(ls == sorted(ls))

Output:

False

Here we get False as the output because the list ls does not contain elements in the sorted order.

The worst-case time complexity of this method is O(N Log N) where N is the size of the list.

By Iterating through the list

You can also check if the list is sorted or not by iterating through the list elements. If any element is out of order (for example, it’s smaller than the previous element) we can say that the list is not sorted in ascending order.

Let’s look at an example.

# function to check if list is sorted in ascending order
def is_list_sorted(ls):
    for i in range(1, len(ls)):
        # return False if the element is smaller than the previous element
        if ls[i] < ls[i-1]:
            return False
    return True

# create a list
ls = [1, 2, 3, 4, 5]
# check if list is sorted
print(is_list_sorted(ls))

Output:

True

We get the same result as above. The list ls is in sorted order and thus we get True as the output.

Let’s see what happens if we use this method on a list that is not sorted.

# create a list
ls = [1, 2, 4, 3, 5]
# check if list is sorted
print(is_list_sorted(ls))

Output:

False

We get False as the output.

The worst-case time complexity of this method is O(N) where N is the size of the list.

You can also use the above methods to check if a list is sorted in descending order or not.

# function to check if list is sorted in descending order
def is_list_sorted_desc(ls):
    for i in range(1, len(ls)):
        # return False if the element is larger than the previous element
        if ls[i] > ls[i-1]:
            return False
    return True

# create a list
ls = [5, 4, 3, 2, 1]
# check if list is sorted
print(ls == sorted(ls, reverse=True))
print(is_list_sorted_desc(ls))

Output:

True
True

Here we use both the methods to check if the list ls is sorted in descending order or not.

You might also be interested in –

• Python List Sort – With Examples
• Python – Check If All Elements in a List are Unique