Python - Find consecutive dates in a list of dates

Given a list of dates, the task is to write a Python program to check if all the dates are consecutive in the list.

Input : [datetime(2019, 12, 30), datetime(2019, 12, 31), datetime(2020, 1, 1), datetime(2020, 1, 2), datetime(2020, 1, 3), datetime(2020, 1, 4)]

Output : True

Explanation : All dates are consecutive, from 30 Dec 2019 to 4 January 2020.

Input : [datetime(2019, 12, 29), datetime(2019, 12, 31), datetime(2020, 1, 1), datetime(2020, 1, 2), datetime(2020, 1, 3), datetime(2020, 1, 4)]

Output : False

Explanation : Non consecutive dates.

Method #1 : Using days() + loop

In this, we check consecutive dates by checking days difference from the previous date using days(). The iteration of all dates is done using a loop.

# Python3 code to demonstrate working of
# Test if dates are consecutive
# Using days() + loop
from datetime import datetime, timedelta

# initializing list
test_list = [datetime(2019, 12, 30), datetime(2019, 12, 31), 
             datetime(2020, 1, 1), datetime(2020, 1, 2),
             datetime(2020, 1, 3), datetime(2020, 1, 4)]

# printing original list
print("The original list is : " + str(test_list))

# using loop for iterating all elements
res = True
for idx in range(1, len(test_list)):

    # checking for 1 day time difference
    if (test_list[idx] - test_list[idx - 1]).days != 1:
        res = False
        break

# printing result
print("Are dates consecutive : " + str(res))

Output:

The original list is : [datetime.datetime(2019, 12, 30, 0, 0), datetime.datetime(2019, 12, 31, 0, 0), datetime.datetime(2020, 1, 1, 0, 0), datetime.datetime(2020, 1, 2, 0, 0), datetime.datetime(2020, 1, 3, 0, 0), datetime.datetime(2020, 1, 4, 0, 0)]

Are dates consecutive : True

Time Complexity : O(n)

Space Complexity : O(1)

Method #2 : Using all() + days()

Similar to the above method, the only difference here is all() is used to check for each day consecution for a more compact solution.

# Python3 code to demonstrate working of
# Test if dates are consecutive
# Using all() + days()
from datetime import datetime, timedelta

# initializing list
test_list = [datetime(2019, 12, 30), datetime(2019, 12, 31), 
             datetime(2020, 1, 1), datetime(2020, 1, 2),
             datetime(2020, 1, 3), datetime(2020, 1, 4)]

# printing original list
print("The original list is : " + str(test_list))

# using loop for iterating all elements
res = all((test_list[idx] - test_list[idx - 1]).days ==
          1 for idx in range(1, len(test_list)))

# printing result
print("Are dates consecutive : " + str(res))

Output:

The original list is : [datetime.datetime(2019, 12, 30, 0, 0), datetime.datetime(2019, 12, 31, 0, 0), datetime.datetime(2020, 1, 1, 0, 0), datetime.datetime(2020, 1, 2, 0, 0), datetime.datetime(2020, 1, 3, 0, 0), datetime.datetime(2020, 1, 4, 0, 0)]

Are dates consecutive : True

Time Complexity : O(n)

Space Complexity : O(1)

Method #3: Using set()

1. Initialize a new list named 'date_range' containing all dates from the first date in the 'test_list' to the last date in the 'test_list'. To do this, use the date range function and add the time delta of one day to each date.
2. Check if 'date_range' and 'test_list' contain the same dates by converting both lists to sets and comparing them. If they are the same, then the dates in 'test_list' are consecutive.
3. Return True if the dates are consecutive and False otherwise.

from datetime import datetime, timedelta

# initializing list
test_list = [datetime(2019, 12, 30), datetime(2019, 12, 31),
             datetime(2020, 1, 1), datetime(2020, 1, 2),
             datetime(2020, 1, 3), datetime(2020, 1, 4)]

# printing original list
print("The original list is : " + str(test_list))

# using set() to check for consecutive dates
date_range = [test_list[0] + timedelta(days=x)
              for x in range((test_list[-1]-test_list[0]).days + 1)]
res = set(test_list) == set(date_range)

# printing result
print("Are dates consecutive : " + str(res))

The original list is : [datetime.datetime(2019, 12, 30, 0, 0), datetime.datetime(2019, 12, 31, 0, 0), datetime.datetime(2020, 1, 1, 0, 0), datetime.datetime(2020, 1, 2, 0, 0), datetime.datetime(2020, 1, 3, 0, 0), datetime.datetime(2020, 1, 4, 0, 0)]
Are dates consecutive : True

Time Complexity: O(n) - linear time complexity since we loop through the test_list and the date_range only once
Auxiliary Space: O(n) - linear space complexity since we create a new list, 'date_range', containing all dates in the given range