Re: Floating-Point arithmetic in dlang - Difference to other languages
On Tue, Dec 03, 2019 at 09:22:49AM +, Jan Hönig via Digitalmars-d-learn
wrote:
> Today i have stumbled on Hacker News into: https://0.30004.com/
>
> I am learning D, that's why i have to ask.
>
> Why does
>
> writefln("%.17f", .1+.2);
>
> not evaluate into: 0.30004, like C++
> but rather to: 0.2
>
> Many other languages evaluate to 0.30004 as well.
In short, because they use 64-bit floats for intermediate values
(`double`), whereas D defaults to 80-bit intermediates (`real`).
The use of a more precise intermediate value is also indicated by the
fact that 0.2 is closer to the real answer (error of
1e-17) than 0.30004 (error of 4e-17).
If you explicitly specify to use `double`, then you should get the same
answer as C++.
double a=.1, b=.2;
writefln("%.17f", a+b);
T
--
MSDOS = MicroSoft's Denial Of Service
Re: Floating-Point arithmetic in dlang - Difference to other languages
On Tuesday, 3 December 2019 at 09:22:49 UTC, Jan Hönig wrote:
Today i have stumbled on Hacker News into:
https://0.30004.com/
I am learning D, that's why i have to ask.
Why does
writefln("%.17f", .1+.2);
not evaluate into: 0.30004, like C++
but rather to: 0.2
Many other languages evaluate to 0.30004 as well.
D lang could have a very good sprintf replacement.
Re: Floating-Point arithmetic in dlang - Difference to other languages
On Tuesday, 3 December 2019 at 09:52:18 UTC, mipri wrote: Most other languages give you the double result for very reasonable historical reasons Not only historical, it is also for numerical reasons. You can get very unpredictable results if you do compares and compiletime evalution is different from runtime evaluation.
Re: Floating-Point arithmetic in dlang - Difference to other languages
On Tuesday, 3 December 2019 at 09:22:49 UTC, Jan Hönig wrote:
not evaluate into: 0.30004, like C++
but rather to: 0.2
You get the same in C++ with:
#include
int main()
{
printf("%.17f",double(0.1L + 0.2L));
}
Re: Floating-Point arithmetic in dlang - Difference to other languages
On Tuesday, 3 December 2019 at 09:22:49 UTC, Jan Hönig wrote:
Today i have stumbled on Hacker News into:
https://0.30004.com/
I am learning D, that's why i have to ask.
Why does
writefln("%.17f", .1+.2);
not evaluate into: 0.30004, like C++
but rather to: 0.2
Many other languages evaluate to 0.30004 as well.
You can get this result in D as well:
$ rdmd --eval 'double a = .1, b = .2; writefln("%.17f", a+b)'
0.30004
https://dlang.org/spec/float.html explains it: real (80bit,
'long double' in C) floats are used in your first calculation,
and doubles are used in my revised example.
Most other languages give you the double result for very
reasonable historical reasons, described here:
https://retrocomputing.stackexchange.com/questions/9751/did-any-compiler-fully-use-intel-x87-80-bit-floating-point/9760#9760
D's behavior is a minor 'miss': https://nwcpp.org/Oct-2019.html
... I wanted to include a C example that gives the D result,
but it seems to be trickier to force 80 bit intermediates.
Floating-Point arithmetic in dlang - Difference to other languages
Today i have stumbled on Hacker News into:
https://0.30004.com/
I am learning D, that's why i have to ask.
Why does
writefln("%.17f", .1+.2);
not evaluate into: 0.30004, like C++
but rather to: 0.2
Many other languages evaluate to 0.30004 as well.
