Solution: Leap year

Disclaimer

The solution shown below aims to train you using conditional checks. In real life, you can learn the methods of the bulit-in `Date` class. We will learn it in the second part of the course.

Solution

The algorithm of the detection if a year is leap:

1. if year is not divisible by 4 => common year
2. if year is not divisible by 100 => leap year
3. if year is not divisible by 400 => common year
4. else => leap year

Note that you can use the built-in types for working with date and time, so we’ll return to this task in the second part of the course.

Code 1

The first solution reproduces the above steps literary:

```my \$year = prompt 'Year: ';

if \$year % 4 {
say 'Common year';
}
elsif \$year % 100 {
say 'Leap year';
}
elsif \$year % 400 {
say 'Common year';
}
else {
say 'Leap year';
}
```

🦋 Find the program in the file leap-year.raku.

Code 2

In the second solution, the logic is packed to a single expression. Parentheses are not always needed by added here for clarity.

```my \$year = prompt 'Year: ';

say ((\$year %% 400) || ((\$year %% 4) && (\$year % 100))) ?? 'Leap year' !! 'Common year';
```

🦋 Find the program in the file leap-year-formula.raku.

Output

Try both programs with the most critical cases, for example, 1900, 2000, 2020, 2021.

``````\$ raku exercises/conditional-checks/leap-year.raku
Year: 1900
Common year

\$ raku exercises/conditional-checks/leap-year.raku
Year: 2000
Leap year
``````
``````\$ raku exercises/conditional-checks/leap-year-formula.raku
Year: 2020
Leap year

\$ raku exercises/conditional-checks/leap-year-formula.raku
Year: 2021
Common year
``````

`%%` vs `%`

Note that in Boolean contexts, the `%%` and `%` operators are complementary. You can use this fact to chose one of the operators to avoid explicit comparison with zero.

```say ?(100 % 3);    # True
say 100 % 3 != 0;  # True

say ?(100 %% 3);   # False
say 100 %% 3 != 0; # Flase
```