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From: Mark H Weaver <mhw@HIDDEN>
To: David Kastrup <dak@HIDDEN>
Subject: Re: bug#16987: Bad default mantissa width for ~g format string
References: <87lhwg7t0n.fsf@HIDDEN>
Date: Tue, 11 Mar 2014 11:21:36 -0400
In-Reply-To: <87lhwg7t0n.fsf@HIDDEN> (David Kastrup's message of
"Tue, 11 Mar 2014 14:40:40 +0100")
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David Kastrup <dak@HIDDEN> writes:
> scheme@(guile-user)> (format #f "~g" (/ 1e100 3))
> $19 = "3333333333333333000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0 "
>
> That, frankly, is pretty ludicrous. The reserved mantissa width should
> at least be constrained to the actually available precision of the data
> type.
Agreed.
> If it isn't, there is no useful way to output a variable with
> minimal width. Using ~S instead results in the much more reasonable
> "3.333333333333333e99" but it will not convert fractions to a floating
> point representation:
>
> scheme@(guile-user)> (format #f "~g" (/ 10000000000000000000000000000000 3))
> $22 = "3333333333333333600000000000000.0 "
> scheme@(guile-user)> (format #f "~S" (/ 10000000000000000000000000000000 3))
> $23 = "10000000000000000000000000000000/3"
> scheme@(guile-user)>
As a workaround, you can force decimal output with ~S by converting the
exact fraction to an inexact floating point number before passing it to
'format', using 'exact->inexact':
scheme@(guile-user)> (format #f "~S" (exact->inexact (/ 10000000000000000000000000000000 3)))
$4 = "3.3333333333333336e30"
Note that 'exact->inexact' works even if the number passed to it is
already inexact, in which case it's a no-op.
At some point we'll make ~g do the right thing for exact fractions, so
that the last digit printed is a 3, not 6. For now, it can't be helped
because the value is being rounded twice: once in the conversion to
inexact binary, and again when converting from binary to decimal.
Thanks,
Mark
bug-guile@HIDDEN:bug#16987; Package guile.
Full text available.Received: (at submit) by debbugs.gnu.org; 11 Mar 2014 13:40:50 +0000 From debbugs-submit-bounces <at> debbugs.gnu.org Tue Mar 11 09:40:50 2014 Received: from localhost ([127.0.0.1]:32798 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.80) (envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>) id 1WNMvJ-0000GE-UI for submit <at> debbugs.gnu.org; Tue, 11 Mar 2014 09:40:50 -0400 Received: from eggs.gnu.org ([208.118.235.92]:39851) by debbugs.gnu.org with esmtp (Exim 4.80) (envelope-from <dak@HIDDEN>) id 1WNMvF-0000G3-Sz for submit <at> debbugs.gnu.org; Tue, 11 Mar 2014 09:40:46 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from <dak@HIDDEN>) id 1WNMvE-0005yn-KT for submit <at> debbugs.gnu.org; Tue, 11 Mar 2014 09:40:45 -0400 X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on eggs.gnu.org X-Spam-Level: X-Spam-Status: No, score=0.8 required=5.0 tests=BAYES_50,T_RP_MATCHES_RCVD autolearn=disabled version=3.3.2 Received: from lists.gnu.org ([2001:4830:134:3::11]:46882) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from <dak@HIDDEN>) id 1WNMvE-0005yj-He for submit <at> debbugs.gnu.org; Tue, 11 Mar 2014 09:40:44 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41569) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from <dak@HIDDEN>) id 1WNMvD-0008CA-KI for bug-guile@HIDDEN; Tue, 11 Mar 2014 09:40:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from <dak@HIDDEN>) id 1WNMvB-0005yG-J5 for bug-guile@HIDDEN; Tue, 11 Mar 2014 09:40:43 -0400 Received: from fencepost.gnu.org ([2001:4830:134:3::e]:35859) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from <dak@HIDDEN>) id 1WNMvB-0005y9-90 for bug-guile@HIDDEN; Tue, 11 Mar 2014 09:40:41 -0400 Received: from localhost ([127.0.0.1]:43032 helo=lola) by fencepost.gnu.org with esmtp (Exim 4.71) (envelope-from <dak@HIDDEN>) id 1WNMvA-0003s6-TX for bug-guile@HIDDEN; Tue, 11 Mar 2014 09:40:41 -0400 Received: by lola (Postfix, from userid 1000) id 70AB9E278D; Tue, 11 Mar 2014 14:40:40 +0100 (CET) From: David Kastrup <dak@HIDDEN> To: bug-guile@HIDDEN Subject: Bad default mantissa width for ~g format string Date: Tue, 11 Mar 2014 14:40:40 +0100 Message-ID: <87lhwg7t0n.fsf@HIDDEN> MIME-Version: 1.0 Content-Type: text/plain X-detected-operating-system: by eggs.gnu.org: Error: Malformed IPv6 address (bad octet value). X-detected-operating-system: by eggs.gnu.org: Error: Malformed IPv6 address (bad octet value). X-Received-From: 2001:4830:134:3::11 X-Spam-Score: -5.0 (-----) X-Debbugs-Envelope-To: submit X-BeenThere: debbugs-submit <at> debbugs.gnu.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: <debbugs-submit.debbugs.gnu.org> List-Unsubscribe: <http://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe> List-Archive: <http://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit/> List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org> List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help> List-Subscribe: <http://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe> Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org> X-Spam-Score: -5.0 (-----) scheme@(guile-user)> (format #f "~g" (/ 1e100 3)) $19 = "3333333333333333000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0 " That, frankly, is pretty ludicrous. The reserved mantissa width should at least be constrained to the actually available precision of the data type. If it isn't, there is no useful way to output a variable with minimal width. Using ~S instead results in the much more reasonable "3.333333333333333e99" but it will not convert fractions to a floating point representation: scheme@(guile-user)> (format #f "~g" (/ 10000000000000000000000000000000 3)) $22 = "3333333333333333600000000000000.0 " scheme@(guile-user)> (format #f "~S" (/ 10000000000000000000000000000000 3)) $23 = "10000000000000000000000000000000/3" scheme@(guile-user)> -- David Kastrup
David Kastrup <dak@HIDDEN>:bug-guile@HIDDEN.
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