Programming Praxis

7 Responses to “Leap Year”

1. Steve said

   October 5, 2018 at 3:13 PM

   Klong version

   leap::{:[0=x!4;:[0=x!100;:[0=x!400;1;0];1];0]}
   

   Examples:
   {.p(x,” –> “,leap(x))}'[2018 2016 2000 2100];””
   [2018 –> 0]
   [2016 –> 1]
   [2000 –> 1]
   [2100 –> 0]
   “”
2. Daniel said

   October 5, 2018 at 5:57 PM

   Here are two solutions in Python.

   def is_leap_year1(y):
       return (y % 4 == 0) and ((y % 100 != 0) or (y % 400 == 0))
   
   def is_leap_year2(y):
       if y % 4: return False
       if not (y % 400): return True
       return (y % 100) != 0
   
   for i in range(1, 2019):
       leap1 = is_leap_year1(i)
       leap2 = is_leap_year2(i)
       assert leap1 == leap2
       if leap1:
           print(i)
   

   Output:

   4
   8
   12
   16
   ...
   92
   96
   104
   108
   ...
   392
   396
   400
   404
   408
   ...
   1996
   2000
   2004
   2008
   2012
   2016
   
3. Globules said

   October 6, 2018 at 12:45 AM

   A Haskell version.

   import Text.Printf (printf)
   
   isLeapYear :: Integral a => a -> Bool
   isLeapYear yr = yr `divBy` 4 && not (yr `divBy` 100) || yr `divBy` 400
     where x `divBy` y = x `rem` y == 0
   
   test :: Int -> IO ()
   test yr = printf "%4d -> %s\n" yr $ show $ isLeapYear yr
   
   main :: IO ()
   main = mapM_ test [1, 4, 8, 100, 300, 400, 2018]
   

   $ ./leapyear
      1 -> False
      4 -> True
      8 -> True
    100 -> False
    300 -> False
    400 -> True
   2018 -> False
   
4. V said

   October 7, 2018 at 7:48 AM

   Built-in in Ruby

   require 'date';
   Date.parse("2020-01-01").leap?
   

   => true
5. matthew said

   November 5, 2018 at 12:06 AM

   Here’s a short one in Python, two of the mods can be replaced with bitmasks (if we don’t have to worry about negative years):

   def isleap(n): return not(n%4 or not n%25 and n%16)
   
6. Appable said

   November 9, 2018 at 5:22 AM

   Recently covered this question in an MATLAB introductory course, so here’s a nice version for vector (or scalar) inputs:

   
function leap = is_leap_year(year)
mask = mod(year, 100) == 0;
year(mask) = year(mask) / 100;
leap = mod(year, 4) == 0;
end

7. matthew said

   November 9, 2018 at 7:30 AM

   @Appable: is that like “if year%100 == 0: year //= 100; return year%4 == 0” but for vectorized inputs (‘year(mask)’ is something like ‘the elements of year that have a 1 in mask’)? Interesting.