# [racket] Challenge: <impressive function> in 140 chars or less

Inspired by the Game of Life tweet challenge (that was seriously cool,
for a geeky definition of "cool"), I compressed a program that
calculates pi to arbitrary precision. By my count, this is 132
characters, after removing comments and extra whitespace:
;; Computes a converging approximation of (atan x) with alternating
;; signs, if x is in (0,1]
(define (q x n)
(if (= n 0)
x
(- (let ([y (+ 1 (* n 2))])
(/ (expt x y) y))
(q x (- n 1)))))
;; Computes pi to arbitrary precision (higher n is better) using
;; Machin's formula
(define (pi n)
(abs (- (* 16 (q 1/5 n))
(* 4 (q 1/239 n)))))
For example, (real->decimal-string (pi 141) 200) computes pi to 200
decimal places.
What's your favorite not just computable, but tweetable function? :)
Neil ⊥