Leap

Challenge Overview

Determine whether a given year is a leap year.

Introduction

A leap year (in the Gregorian calendar) occurs:

In every year that is evenly divisible by 4.
Unless the year is evenly divisible by 100, in which case it’s only a leap year if the year is also evenly divisible by 400.

Some examples:

1997 was not a leap year as it’s not divisible by 4.
1900 was not a leap year as it’s not divisible by 400.
2000 was a leap year!

For a delightful, four-minute explanation of the whole phenomenon of leap years, check out [this YouTube video](https://www.youtube.com/watch?v=xX96xng7sAE).

Instructions

Your task is to determine whether a given year is a leap year.

Dig Deeper

Boolean chain

Chain of Boolean expressions

data class Year(val year: Int) {
    val isLeap = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
}

The first boolean expression uses the modulus operator to check if the year is evenly divided by 4.

If the year is not evenly divisible by 4, then the chain will “short circuit” due to the next operator being a logical AND (&&), and will return false.
If the year is evenly divisible by 4, then the year is checked to not be evenly divisible by 100.
If the year is not evenly divisible by 100, then the expression is true and the chain will “short-circuit” to return true, since the next operator is a logical OR (||).
If the year is evenly divisible by 100, then the expression is false, and the returned value from the chain will be if the year is evenly divisible by 400.

year	year % 4 == 0	year % 100 != 0	year % 400 == 0	is leap year
2020	true	true	not evaluated	true
2019	false	not evaluated	not evaluated	false
2000	true	false	true	true
1900	true	false	false	false

The chain of boolean expressions is efficient, as it proceeds from testing the most likely to least likely conditions.

Ternary expression

data class Year(val year: Int) {
    val isLeap = if (year % 100 == 0) year % 400 == 0 else year % 4 == 0
}

A ternary expression uses a maximum of two checks to determine if a year is a leap year.

It starts by testing the outlier condition of the year being evenly divisible by 100. It does this by using the modulus operator. If the year is evenly divisible by 100, then the expression is true, and the ternary expression returns if the year is evenly divisible by 400. If the year is not evenly divisible by 100, then the expression is false, and the ternary expression returns if the year is evenly divisible by 4.

year	year % 100 == 0	year % 400 == 0	year % 4 == 0	is leap year
2020	false	not evaluated	true	true
2019	false	not evaluated	false	false
2000	true	true	not evaluated	true
1900	true	false	not evaluated	false

Although it uses a maximum of only two checks, the ternary expression tests an outlier condition first, making it less efficient than another approach that would first test if the year is evenly divisible by 4, which is more likely than the year being evenly divisible by 100.

plusDays method

`plusDays()` method

import java.time.*

data class Year(val year: Int) {
    val isLeap = LocalDate.of(year, Month.FEBRUARY, 28).plusDays(1).dayOfMonth == 29
}

This approach may be considered a “cheat” for this exercise. By adding a day to February 28th for the year, you can see if the new day is the 29th or the 1st. If it is the 29th, then the year is a leap year. This is done by using the plusDays() and getDayOfMonth() methods of the LocalDate class.

Source: Exercism kotlin/leap

💡 View Solution

object Year {
    fun isLeapYear(year: Int) = year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)
}