Ternary operator

Leap
Leap in C# 
public static bool IsLeapYear(int year)
{
    return year % 100 == 0 ? year % 400 == 0 : year % 4 == 0;
}

A conditional operator, also known as a "ternary conditional operator", or just "ternary operator", 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 remainder operator. If the year is evenly divisible by 100, then the expression is true, and the ternary operator 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 operator 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 operator 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.

Shortening

When the body of a function is a single expression, the function can be implemented as an expression-bodied member, like so

public static bool IsLeapYear(int year) =>
    year % 100 == 0 ? year % 400 == 0 : year % 4 == 0;

or

public static bool IsLeapYear(int year) => year % 100 == 0 ? year % 400 == 0 : year % 4 == 0;
11th Dec 2024 · Found it useful?

Other Approaches to Leap in C#

Other ways our community solved this exercise
public static bool IsLeapYear(int year)
{
    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
}
Boolean chain

Use a chain of Boolean expressions. 

return (year % 4, year % 100, year % 400) switch
{
    (_, 0, 0) => true,
    (_, 0, _) => false,
    (0, _, _) => true,
    _ => false,
};
switch on a tuple

Use a switch on a tuple made from Boolean expressions. 

public static bool IsLeapYear(int year)
{
    return (new DateTime(year, 2, 28)).AddDays(1.0).Day == 29;
}
DateTime addition

Use DateTime addition. 

public static bool IsLeapYear(int year)
{
    return DateTime.IsLeapYear(year);
}
Built-in method

Use the built-in method.