Determine A Leap Year With F#

Determine A Leap Year With F#

With this year being a leap year, and today being a leap day, I thought it would be an easy calculation to do in F# to figure out if a given year was a leap year.

To be honest, I seriously thought leap years were easily discernible. Is the current year evenly divisible by 4? Yes — Leap Year. It wasn't until I read a Microsoft Tech Community blog post earlier this week that I learned there were a couple more rules in play.

Many think leap years occur every four years, but the exact algorithm is slightly more complicated:
Is the year evenly divisible by 4? Then it is a leap year. (Examples: 2012, 2016, 2020, 2024)
… Unless it is also evenly divisible by 100. Those are not leap years. (Examples: 1800, 1900, 2100)
… Except those years that are evenly divisible by 400 are leap years. (Examples: 1600, 2000, 2400)

I broke each rule into a function and return the state based on the given year value. Since rule 2 and 3 are directly dependent on each other, I grouped them together for the result.

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let isEvenlyDivisibleByFour (year:int) : bool = year % 4 = 0
let isEvenlyDivisibleByOneHundred (year:int) : bool = year % 100 = 0
let isEvenlyDivisibleByFourHundred (year:int) : bool = year % 400 = 0

let isLeapYear (year:int) : bool =
    let divisibleByFour = isEvenlyDivisibleByFour year
    let divisibleByOneHundred = isEvenlyDivisibleByOneHundred year
    let divisibleByFourHundred = isEvenlyDivisibleByFourHundred year
    divisibleByFour && (not divisibleByOneHundred || divisibleByFourHundred)

isLeapYear 2016;;
isLeapYear 2020;;
isLeapYear 2021;;
isLeapYear 2024;;
isLeapYear 1800;;
isLeapYear 1900;;
isLeapYear 2100;;
isLeapYear 1600;;
isLeapYear 2000;;
isLeapYear 2400;;

Output:

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> isLeapYear 2016;;
val it : bool = true

> isLeapYear 2020;;
val it : bool = true

> isLeapYear 2021;;
val it : bool = false

> isLeapYear 2024;;
val it : bool = true

> isLeapYear 1800;;
val it : bool = false

> isLeapYear 1900;;
val it : bool = false

> isLeapYear 2100;;
val it : bool = false

> isLeapYear 1600;;
val it : bool = true

> isLeapYear 2000;;
val it : bool = true

> isLeapYear 2400;;;;
val it : bool = true

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let years = [1998 .. 2014]
for year in years do
    printf "Is %d a Leap Year? %b\r\n" year (isLeapYear year)

Output:
Is 1998 a Leap Year? false
Is 1999 a Leap Year? false
Is 2000 a Leap Year? true
Is 2001 a Leap Year? false
Is 2002 a Leap Year? false
Is 2003 a Leap Year? false
Is 2004 a Leap Year? true
Is 2005 a Leap Year? false
Is 2006 a Leap Year? false
Is 2007 a Leap Year? false
Is 2008 a Leap Year? true
Is 2009 a Leap Year? false
Is 2010 a Leap Year? false
Is 2011 a Leap Year? false
Is 2012 a Leap Year? true
Is 2013 a Leap Year? false
Is 2014 a Leap Year? false