> 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/s/semordnilap.md).

# Semordnilap

```ruby
var c = 0
var seen = Hash()

ARGF.each { |line|
    var r = line.reverse
    ((seen{r} := 0 ++) && (c++ < 5) && say "#{line} #{r}") ->
        || (seen{line} := 0 ++)
}

say c
```

#### Output:

```
$ sidef semordnilap.sf < unixdict.txt
ca ac
dab bad
diva avid
dna and
drab bard
158
```