Let's celebrate Pi day in clojure!

Exactly 140 characters to print 9 digits of $\pi$ (`3.141592654`

)

```
(let[r reduce N(rest(range))t take](clojure.pprint/cl-format nil"~11f"
(r +' -2(map #(/(r *' 2(t % N))(r *'(t %(take-nth 2 N))))(t 99 N)))9))
```

There are two parts of a problem:

- Calculate rational for $\pi$ using arctangent approximation

$$ \pi = 2 \cdot (1 + \frac{1}{3} + \frac{1 \cdot 2}{3 \cdot 5} + \frac{1 \cdot 2 \cdot 3}{3 \cdot 5 \cdot 7} + ...) $$

```
(defn pi
"Calculate PI by arctangent algorithm"
[& {:keys [iterations limit]
:or {iterations 99 limit 9}}]
(->> (range 1 (inc iterations))
(map #(/ (reduce *' (take % (iterate inc 1)))
(reduce *' (take % (filter odd? (iterate inc 1))))))
(reduce +')
(* 2)
(dec)
(dec)))
```

- Convert rational to decimal. Handled automatically by awesome
`clojure.pprint/cl-format`

```
(clojure.pprint/cl-format nil "~4f" 22/7) ;;=> "3.14"
```

To achieve 140 characters barrier, some functionality was relaxed

- Tweak number
`99`

to control number of iterations for $\pi$ calculation (if you have a high number of iterations, be patient - wait) - Tweak number
`9`

to increase number of digits (that makes sense if you improved number of iterations)

**P.S.** Calculating $\pi$ and some other irrationals is available in
numberto