> 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/m/modular_inverse.md).

# Modular inverse

Built-in:

```ruby
say 42.modinv(2017)
```

Algorithm implementation:

```ruby
func invmod(a, n) {
  var (t, nt, r, nr) = (0, 1, n, a % n)
  while (nr != 0) {
    var quot = int((r - (r % nr)) / nr);
    (nt, t) = (t - quot*nt, nt);
    (nr, r) = (r - quot*nr, nr);
  }
  r > 1 && return()
  t < 0 && (t += n)
  t
}
 
say invmod(42, 2017)
```

#### Output:

```
1969
```