Check if a List is Sorted or not - Python

We are given a list of numbers and our task is to check whether the list is sorted in increasing or decreasing order. For example, if the input is [1, 2, 3, 4], the output should be True, but for [3, 1, 2], it should be False.

Using all()

all() function checks if every pair of consecutive elements in the list is in non-decreasing order.

a = [1, 2, 3, 4, 5]

# For checking ascending order
print(all(a[i] <= a[i + 1] for i in range(len(a) - 1)))

b = [5, 4, 3, 2, 1]

# For checking descending order
print(all(b[i] >= b[i + 1] for i in range(len(b) - 1)))

True
True

Explanation:

• For the list a = [1, 2, 3, 4, 5], we are checking for ascending order and here all() method is used verify if each element is less than or equals to its next element in the list. It only returns True if the list is sorted in ascending order.
• For the list b = [5, 4, 3, 2, 1], we are checking descending order all() method verify if each element is greater than or equals to its next element in the list. It returns True if the list is sorted in descending order.

Note: To check for strictly increasing or decreasing sequences remove the equals sign (=) from the comparison.

Using sorted()

sorted() function returns a sorted version of the list. If the original list is equal to its sorted version it means the list is already sorted.

a = [1, 2, 3, 4, 5]
b = [5, 4, 3, 2, 1]

# Here, sorted(), sort list in ascending order
print(a == sorted(a))  
print(b == sorted(b)) 

True
False

Explanation:

• sorted(a) returns a new sorted list.
• a == sorted(a) compares the original list with its sorted version. If they match, the list is sorted.

Note: We can also use list.sort() method as well. To do this, we'll create a copy of the original list, sort the copy and then compare the two lists.

Using for Loop

We can iterate through the list and check if each element is less than or equal to the next one, if the loop executes completely that means the list is sorted but if we break out of the loop that means the list is not sorted.

a = [1, 2, 3, 4]
res = True

for i in range(len(a) - 1):
    if a[i] > a[i + 1]:
        res = False
        break

print(res)

True

Explanation:

• Iterates through the list and checks if each element is greater than the next one.
• If any element is greater than the next, res is set to False and we break the loop.

Related Articles:

• Check for Descending Sorted List
• Sort List of Lists Ascending and then Descending
• Difference between sorted() and sort()

Check if a list is Sorted in Python

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