Pythonic way to check if a list is sorted or not


Is there a pythonic way to check if a list is already sorted in ASC or DESC

listtimestamps = [1, 2, 3, 5, 6, 7]

something like isttimestamps.isSorted() that returns True or False.

I want to input a list of timestamps for some messages and check if the the transactions appeared in the correct order.

1/21/2017 12:44:58 PM

Accepted Answer

Actually we are not giving the answer anijhaw is looking for. Here is the one liner:

all(l[i] <= l[i+1] for i in xrange(len(l)-1))

For Python 3:

all(l[i] <= l[i+1] for i in range(len(l)-1))
1/29/2018 2:08:41 PM

I would just use

if sorted(lst) == lst:
    # code here

unless it's a very big list in which case you might want to create a custom function.

if you are just going to sort it if it's not sorted, then forget the check and sort it.


and don't think about it too much.

if you want a custom function, you can do something like

def is_sorted(lst, key=lambda x: x):
    for i, el in enumerate(lst[1:]):
        if key(el) < key(lst[i]): # i is the index of the previous element
            return False
    return True

This will be O(n) if the list is already sorted though (and O(n) in a for loop at that!) so, unless you expect it to be not sorted (and fairly random) most of the time, I would, again, just sort the list.

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