> 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/f/factorial.md).

# Factorial

Recursive:

```ruby
func factorial_recursive(n) {
    n == 0 ? 1 : (n * __FUNC__(n-1))
}
```

Catamorphism:

```ruby
func factorial_reduce(n) {
    1..n -> reduce({|a,b| a * b }, 1)
}
```

Iterative:

```ruby
func factorial_iterative(n) {
    var f = 1
    {|i| f *= i } << 2..n
    return f
}
```

Built-in:

```ruby
say 5!
```