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Captain's Log, star date 21:34.5...

devel / comp.lang.tcl / Re: Math precision question

SubjectAuthor
* Math precision questionLuc
`* Math precision questionRich
 `* Math precision questionLuc
  `- Math precision questionArjen Markus

1
Math precision question

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From: luc@sep.invalid (Luc)
Newsgroups: comp.lang.tcl
Subject: Math precision question
Date: Wed, 15 Nov 2023 13:54:43 -0300
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 by: Luc - Wed, 15 Nov 2023 16:54 UTC

Hi.

$ set ::tcl_precision 2
2 $ expr {1.08688 - 1.08684}
4e-05
$ set ::tcl_precision 17
17
$ expr {1.08688 - 1.08684}
4.0000000000040004e-05

Why don't I just get 0.00004?

--
Luc
>>

Re: Math precision question

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From: rich@example.invalid (Rich)
Newsgroups: comp.lang.tcl
Subject: Re: Math precision question
Date: Wed, 15 Nov 2023 18:48:57 -0000 (UTC)
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 by: Rich - Wed, 15 Nov 2023 18:48 UTC

Luc <luc@sep.invalid> wrote:
> Hi.
>
> $ set ::tcl_precision 2

Note this big warning from the documentation:

tcl_precision

This variable controls the number of digits to generate when
converting floating-point values to strings. It defaults to 0.
**Applications should not change this value; it is provided for
compatibility with legacy code.**

> 2
> $ expr {1.08688 - 1.08684}
> 4e-05
> $ set ::tcl_precision 17
> 17
> $ expr {1.08688 - 1.08684}
> 4.0000000000040004e-05
>
> Why don't I just get 0.00004?

https://0.30000000000000004.com/

If you want a particular number of decimal digits when converting to a
string, use [format] and the %f format method.

Re: Math precision question

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From: luc@sep.invalid (Luc)
Newsgroups: comp.lang.tcl
Subject: Re: Math precision question
Date: Wed, 15 Nov 2023 22:27:03 -0300
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 by: Luc - Thu, 16 Nov 2023 01:27 UTC

On Wed, 15 Nov 2023 18:48:57 -0000 (UTC), Rich wrote:

>https://0.30000000000000004.com/
>
>If you want a particular number of decimal digits when converting to a
>string, use [format] and the %f format method.
**************************

Well, I'm glad I asked because I didn't know that site
and it's really interesting.

Thank you again.

--
Luc
>>

Re: Math precision question

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Subject: Re: Math precision question
From: arjen.markus895@gmail.com (Arjen Markus)
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 by: Arjen Markus - Thu, 16 Nov 2023 07:53 UTC

On Thursday, November 16, 2023 at 2:27:08 AM UTC+1, Luc wrote:
> On Wed, 15 Nov 2023 18:48:57 -0000 (UTC), Rich wrote:
>
> >https://0.30000000000000004.com/
> >
> >If you want a particular number of decimal digits when converting to a
> >string, use [format] and the %f format method.
> **************************
>
> Well, I'm glad I asked because I didn't know that site
> and it's really interesting.
>
> Thank you again.
>
> --
> Luc
> >>
Classic case of floating-point numbers not being decimal numbers. Think of representing 1/3 as a decimal number. That can only be done approximately. The same is true for many fractions that in decimal are finite, when using floating-point numbers, as these use 2 as the base instead of 10.

Regards,

Arjen

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