Exercise: Leap year – Hands-on Computer Science

Download as a script python-exercise-leap-year.py with the task as comments, to run in VS Code or Positron

Task

A year has 365 days, a leap year one more, the 29th of February. In our calendar a year is a leap year if it is divisible by 4. There is one exception: if it is also divisible by 100, it is not a leap year. And the exception has an exception of its own: if it is divisible by 400, it is one after all. So 2024 is a leap year, 2023 is not, 1900 is not and 2000 is again.

Write a function is_leap_year(year) that returns True if year is a leap year, and False otherwise. A main program main asks for a year and answers with a sentence.

What it should look like

Two runs, once with 2024 and once with 1900:

Which year? 2024
2024 is a leap year.

Which year? 1900
1900 is not a leap year.

Your program

def main():
    year = int(input("Which year? "))
    # Print a fitting sentence with the help of is_leap_year.


def is_leap_year(year):
    # Return True if year is a leap year, otherwise False.
    return False


main()

SHOW HINT 1

"Divisible by 4" means that dividing by 4 leaves no remainder, so in Python year % 4 == 0. "Not divisible by 100" is then year % 100 != 0. First translate each of the three rules into such a comparison before you combine them.

SHOW HINT 2

In words, the whole rule reads: divisible by 4, and in addition either not divisible by 100 or divisible by 400. Because Python works out and before or, the part with or belongs in round brackets. Afterwards, test the function with all four examples from the task.

SHOW THE SOLUTION

def main():
    year = int(input("Which year? "))
    if is_leap_year(year):
        print(f"{year} is a leap year.")
    else:
        print(f"{year} is not a leap year.")


def is_leap_year(year):
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)


main()

The function needs no if, because the condition itself already is the truth value it is meant to return. For 1900 the first part is true, but in the brackets both sides are false, and so the whole result is False. The brackets make the condition read like the rule in words, with the or as the exception to the exception.

For the quick ones

Write a function days_in_month(month, year) that returns how many days a month has, with month as a number from 1 to 12. February has 28 days, 29 in a leap year, April, June, September and November have 30, all others 31. Use is_leap_year in it, and for the months with 30 days, in with a list helps.