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devel / comp.lang.scheme / Re: (expt 9.5 400) and (expt 9.4721 400)

Re: (expt 9.5 400) and (expt 9.4721 400)

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From: no.em...@nospam.invalid (Paul Rubin)
Newsgroups: comp.lang.scheme
Subject: Re: (expt 9.5 400) and (expt 9.4721 400)
Date: Sat, 18 Jun 2022 15:32:31 -0700
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by: Paul Rubin - Sat, 18 Jun 2022 22:32 UTC

Hen Hanna <henhanna@gmail.com> writes:
> is there a way to do this?

The issue is that 9.5**400 is around 1e390 which is larger than the ieee
fp64 maximum value of around 1e308. While there is an extended format
with a wider range, it is probably simplest to use logarithms:

(define (log10 x) (/ (log x) (log 10.0)))

(define x0 (+ 5 (* 2 (sqrt 5)))) ; 9.472...

(define y0 (* 400.0 (log10 x0))) ; log10(x0**400)

(let* ((y1 (floor y0))
(y2 (- y0 y1)))
(display `(,(expt 10 y2) "* 10**" ,y1)))
(newline)

prints (3.794627652179525 * 10** 390.0) when I try it in Guile.

Subject	Author
Re: (expt 9.5 400) and (expt 9.4721 400)	Paul Rubin