> For the complete documentation index, see [llms.txt](https://trizen.gitbook.io/sidef-lang/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://trizen.gitbook.io/sidef-lang/programming_tasks/h/happy_numbers.md).

# Happy numbers

```ruby
func happy(n) is cached {
    static seen = Hash()

    return true  if n.is_one
    return false if seen.exists(n)

    seen{n} = 1
    happy(n.digits.sum { _*_ })
}

say happy.first(10)
```

#### Output:

```
[1, 7, 10, 13, 19, 23, 28, 31, 32, 44]
```