[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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