Course of Raku / Essentials / Conditional checks / Exercises / Leap year

# 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:

- if year is not divisible by 4 => common year
- if year is not divisible by 100 => leap year
- if year is not divisible by 400 => common year
- 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
```

## Course navigation

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