# Three Ways with Clojure

I’ve been working with Clojure in the last few days, both looking at the Clojure Koans and another resource I’ve discovered 4clojure.com. I’d like to share a nice problem I saw there, and some of the solutions to it which I think expose some of the things I’m beginning to appreciate about the language.

###The Problem

as stated on 4clojure.

Write a function which returns the Nth element from a sequence. (Without using

`nth`

)

`nth`

is the obvious answer to the problem, as it returns the nth element in
a sequence:

```
(= 2 (nth 1 '(1 2 3 4)))
```

We need to get to a function that will do the same, something that will fit in the blank space below:

```
(= (___ '(4 5 6 7) 2) 6)
```

###Solution 1: recurring

```
(fn my_nth [seqn n] (if
(zero? n)
(first seqn)
(my-nth (rest seqn) (dec n))))
```

Here we use recurrance, setting the breaking point as the iteration where `n`

is
zero using `if`

, at which point the function returns the `first`

value of the
sequence. If it’s not, we fire the function again, but this time chopping off
the first member of the sequence (`(rest seqn)`

returns the rest) and
`dec`

rementing the value of `n`

by one. We walk through the sequence, losing
items from the front of the sequence `n`

times, until we get to the index.

We’ve named the function `my_nth`

, but we could easily anonymize it:

```
(fn [seqn n] (if
(zero? n)
(first seqn)
(recur (rest seqn) (dec n))))
```

`recur`

is neat - it executes the expressions that follow it, then rebinds the
values hey produce to the bindings of the recursion point, in this case the
`fn`

method. We then get moved back to that method with the new values, causing
the recusion. Very cool.

Usually recursion is a neat way of writing a short function; here it’s pretty longwinded. We can get smaller…

###Solution 2: taking

```
(fn [seqn n] (last (take (inc n) seqn)))
```

Here we `take`

the first one-more-than-n (`inc`

increments its argument) items
from the sequence, and then take the `last`

one from the end of that new list-
which will be the nth element.

`take`

is used in many of the examples I’ve seen as a way of accessing
a sequence which may be infinite like the Fibonacci series - see some of the
examples over at
Wikibooks.

We can squeeze more succinctness in there using some alternative syntax:

```
#(last (take (inc %2) %1))
```

But if you want to be really succinct:

###Solution 3: ripping off Java

```
.get
```

Clojure gives you access to Java methods and fields through the use of the dot
(`.`

) operator, which takes the form `(.instanceMember instance arguments*)`

.
Here we’re using the `get()`

method from the Java Lists
interface,
which we get to use on these instances as they are, well, Java lists. which
takes one argument - and luckily for us its the index! We’re calling `get(6)`

on the list, which gives us the answer we want.

This is pretty close to cheating, but it goes to show how Clojure’s access to Java gives us a whole other language of libraries and methods to play with.