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

ackermann should fit, no?
On Feb 23, 2012, at 12:09 AM, Neil Toronto wrote:
>* 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 ⊥
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*