GNU bug report logs - #14905
rationalize not returning the simplest rational

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Göran Weinholt <goran <at> weinholt.se>
To: bug-guile <at> gnu.org
Subject: rationalize not returning the simplest rational
Date: Fri, 19 Jul 2013 13:18:41 +0200

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Hello schemers,

the procedure (rationalize x1 x2) from (rnrs) takes a number x1 and
returns the simplest rational that doesn't differ from x1 by more than
x2. The implementation in guile has this behavior:

scheme@(guile-user)> (rationalize #e-0.67 1/4)
$1 = -2/3

The problem here is that -1/2 is simpler than -2/3 (since 1<2 and 2<3)
and it also doesn't differ from #e-0.67 by more than 1/4 (even though
the difference is larger than for -2/3, it is still less than 1/4). So
the result from the expression above should be -1/2.

Tested with Guile 2.0.9.40-824b-dirty.

Regards,

-- 
Göran Weinholt <goran <at> weinholt.se>
"On your own and though you feel alone you are alive.
Don't you know it's just another day." -- You Are Alive, Fragma

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Message #10 received at 14905-done <at> debbugs.gnu.org (full text, mbox):

From: Mark H Weaver <mhw <at> netris.org>
To: Göran Weinholt <goran <at> weinholt.se>
Cc: 14905-done <at> debbugs.gnu.org
Subject: Re: bug#14905: rationalize not returning the simplest rational
Date: Sun, 21 Jul 2013 06:56:03 -0400

Göran Weinholt <goran <at> weinholt.se> writes:

> the procedure (rationalize x1 x2) from (rnrs) takes a number x1 and
> returns the simplest rational that doesn't differ from x1 by more than
> x2. The implementation in guile has this behavior:
>
> scheme@(guile-user)> (rationalize #e-0.67 1/4)
> $1 = -2/3
>
> The problem here is that -1/2 is simpler than -2/3 (since 1<2 and 2<3)
> and it also doesn't differ from #e-0.67 by more than 1/4 (even though
> the difference is larger than for -2/3, it is still less than 1/4). So
> the result from the expression above should be -1/2.

Fixed in 620c13e8fc02060e0af8fa38398ee4de745d41e9.

    Thanks!
      Mark

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