Re: [go-nuts] Inconsistent rounding with float printf ?

2019-12-06 Thread Robert Engels 
You can use github.com/robaho/fixed

> On Dec 6, 2019, at 10:19 AM, Michael Jones  wrote:
> 
> 
> Agree with Ian. 
> 
> Solutions are: change expectations, use decimal floating point, or use a 
> base-independent decimal representation. The latter implies scaled integers.
> 
> Quick, ugly, and typed on one hand from bed, but here it is: 
> https://play.golang.org/p/fBztRY6qHP0
> 
> 999000/1000 = 999.0
> 999050/1000 = 999.1
> 999100/1000 = 999.1
> 999150/1000 = 999.2
> 999200/1000 = 999.2
> 999250/1000 = 999.3
> 999300/1000 = 999.3
> 999350/1000 = 999.4
> 999400/1000 = 999.4
> 999450/1000 = 999.5
> 999500/1000 = 999.5
> 999550/1000 = 999.6
> 999600/1000 = 999.6
> 999650/1000 = 999.7
> 999700/1000 = 999.7
> 999750/1000 = 999.8
> 999800/1000 = 999.8
> 999850/1000 = 999.9
> 00/1000 = 999.9
> 50/1000 = 1000.0
> -50/1000 = -1000.0
> -00/1000 = -999.9
> -999850/1000 = -999.9
> -999800/1000 = -999.8
> -999750/1000 = -999.8
> -999700/1000 = -999.7
> -999650/1000 = -999.7
> -999600/1000 = -999.6
> -999550/1000 = -999.6
> -999500/1000 = -999.5
> -999450/1000 = -999.5
> -999400/1000 = -999.4
> -999350/1000 = -999.4
> -999300/1000 = -999.3
> -999250/1000 = -999.3
> -999200/1000 = -999.2
> -999150/1000 = -999.2
> -999100/1000 = -999.1
> -999050/1000 = -999.1
> 
>> On Fri, Dec 6, 2019 at 2:31 AM Ian Davis  wrote:
>>> On Fri, 6 Dec 2019, at 9:25 AM, Christophe Meessen wrote:
>>> I have noticed that printf performs an apparently inconsistent rounding of 
>>> floating point values.
>>> 
>>> I divide a big number by 1000 and printf the resulting value with "%.1f".
>>> Here is the code: https://play.golang.org/p/e7dD3c6IHq2
>> 
>> I think you are just seeing the usual problems of floating point 
>> representation. 
>> 
>> You may wonder why 999450/1000=999.45, 999500/1000=999.50 and 
>> 999550/1000=999.55 all format as 999.5. The answer is that the internal 
>> representation of the three results cannot correspond to the mathematical 
>> result you expect.
>> 
>> This link shows the internal representation of each answer: 
>> https://play.golang.org/p/bBTNCdsAttR
>> 
>> You can see that 999550/1000 = 
>> 999.5499954525264911353588104248046875 which is printed as 999.5
>> 
>> 
>>> I would expect the rounding rule to be "round away from zero" as defined 
>>> here: https://math.stackexchange.com/a/2252888/33796
>>> In this case 0.5 is rounded to 1 (or 0.05 to 0.1) and -0.5 to -1 (or -0.05 
>>> to -0.1). 
>> 
>> The strconv and fmt packages use round to even as a rule. Use math.Round to 
>> round away from zero.
>> 
>> -- Ian
>> 
>> 
>> 
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> 
> 
> -- 
> Michael T. Jones
> michael.jo...@gmail.com
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Re: [go-nuts] Inconsistent rounding with float printf ?

2019-12-06 Thread Michael Jones 
Agree with Ian.

Solutions are: change expectations, use decimal floating point, or use a
base-independent decimal representation. The latter implies scaled integers.

Quick, ugly, and typed on one hand from bed, but here it is:
https://play.golang.org/p/fBztRY6qHP0

999000/1000 = 999.0
999050/1000 = 999.1
999100/1000 = 999.1
999150/1000 = 999.2
999200/1000 = 999.2
999250/1000 = 999.3
999300/1000 = 999.3
999350/1000 = 999.4
999400/1000 = 999.4
999450/1000 = 999.5
999500/1000 = 999.5
999550/1000 = 999.6
999600/1000 = 999.6
999650/1000 = 999.7
999700/1000 = 999.7
999750/1000 = 999.8
999800/1000 = 999.8
999850/1000 = 999.9
00/1000 = 999.9
50/1000 = 1000.0
-50/1000 = -1000.0
-00/1000 = -999.9
-999850/1000 = -999.9
-999800/1000 = -999.8
-999750/1000 = -999.8
-999700/1000 = -999.7
-999650/1000 = -999.7
-999600/1000 = -999.6
-999550/1000 = -999.6
-999500/1000 = -999.5
-999450/1000 = -999.5
-999400/1000 = -999.4
-999350/1000 = -999.4
-999300/1000 = -999.3
-999250/1000 = -999.3
-999200/1000 = -999.2
-999150/1000 = -999.2
-999100/1000 = -999.1
-999050/1000 = -999.1


On Fri, Dec 6, 2019 at 2:31 AM Ian Davis  wrote:

> On Fri, 6 Dec 2019, at 9:25 AM, Christophe Meessen wrote:
>
> I have noticed that printf performs an apparently inconsistent rounding of
> floating point values.
>
> I divide a big number by 1000 and printf the resulting value with "%.1f".
> Here is the code: https://play.golang.org/p/e7dD3c6IHq2
>
>
> I think you are just seeing the usual problems of floating point
> representation.
>
> You may wonder why 999450/1000=999.45, 999500/1000=999.50 and
> 999550/1000=999.55 all format as 999.5. The answer is that the internal
> representation of the three results cannot correspond to the mathematical
> result you expect.
>
> This link shows the internal representation of each answer:
> https://play.golang.org/p/bBTNCdsAttR
>
> You can see that 999550/1000 = 999.5499954525264911353588104248046875
> which is printed as 999.5
>
>
> I would expect the rounding rule to be "round away from zero" as defined
> here: https://math.stackexchange.com/a/2252888/33796
> In this case 0.5 is rounded to 1 (or 0.05 to 0.1) and -0.5 to -1 (or -0.05
> to -0.1).
>
>
> The strconv and fmt packages use round to even as a rule. Use math.Round
> to round away from zero.
>
> -- Ian
>
>
>
> --
> You received this message because you are subscribed to the Google Groups
> "golang-nuts" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to golang-nuts+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
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> 
> .
>


-- 

*Michael T. jonesmichael.jo...@gmail.com *

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Re: [go-nuts] Inconsistent rounding with float printf ?

2019-12-06 Thread Ian Davis 
On Fri, 6 Dec 2019, at 9:25 AM, Christophe Meessen wrote:
> I have noticed that printf performs an apparently inconsistent rounding of 
> floating point values.
> 
> I divide a big number by 1000 and printf the resulting value with "%.1f".
> Here is the code: https://play.golang.org/p/e7dD3c6IHq2

I think you are just seeing the usual problems of floating point 
representation. 

You may wonder why 999450/1000=999.45, 999500/1000=999.50 and 
999550/1000=999.55 all format as 999.5. The answer is that the internal 
representation of the three results cannot correspond to the mathematical 
result you expect.

This link shows the internal representation of each answer: 
https://play.golang.org/p/bBTNCdsAttR

You can see that 999550/1000 = 999.5499954525264911353588104248046875 
which is printed as 999.5


> I would expect the rounding rule to be "round away from zero" as defined 
> here: https://math.stackexchange.com/a/2252888/33796
> In this case 0.5 is rounded to 1 (or 0.05 to 0.1) and -0.5 to -1 (or -0.05 to 
> -0.1). 

The strconv and fmt packages use round to even as a rule. Use math.Round to 
round away from zero.

-- Ian


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[go-nuts] Inconsistent rounding with float printf ?

2019-12-06 Thread Christophe Meessen 
I have noticed that printf performs an apparently inconsistent rounding of 
floating point values.

I divide a big number by 1000 and printf the resulting value with "%.1f".
Here is the code: https://play.golang.org/p/e7dD3c6IHq2

I would expect the rounding rule to be "round away from zero" as defined 
here: https://math.stackexchange.com/a/2252888/33796
In this case 0.5 is rounded to 1 (or 0.05 to 0.1) and -0.5 to -1 (or -0.05 
to -0.1). 

Here is what I see with golang 1.13.4:

999000/1000 = 999.0
999050/1000 = 999.0 // expected 999.1
999100/1000 = 999.1
999150/1000 = 999.1 // expected 999.2
999200/1000 = 999.2
999250/1000 = 999.2 // expected 999.3
999300/1000 = 999.3
999350/1000 = 999.4 
999400/1000 = 999.4
999450/1000 = 999.5 
999500/1000 = 999.5
999550/1000 = 999.5 // expected 999.6
999600/1000 = 999.6
999650/1000 = 999.6 // expected 999.7
999700/1000 = 999.7
999750/1000 = 999.8 
999800/1000 = 999.8
999850/1000 = 999.9 
00/1000 = 999.9
50/1000 = 1000.0 
-50/1000 = -1000.0 
-00/1000 = -999.9
-999850/1000 = -999.9  
-999800/1000 = -999.8
-999750/1000 = -999.8
-999700/1000 = -999.7
-999650/1000 = -999.6 // expected -999.7
-999600/1000 = -999.6
-999550/1000 = -999.5 // expected -999.6
-999500/1000 = -999.5
-999450/1000 = -999.5
-999400/1000 = -999.4
-999350/1000 = -999.4
-999300/1000 = -999.3
-999250/1000 = -999.2 // expected -999.3
-999200/1000 = -999.2
-999150/1000 = -999.1 // expected -999.2
-999100/1000 = -999.1
-999050/1000 = -999.0 // expected -999.1


Is this actual rounding the result of a rule or is it pseudo random ? 

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