• Jan =?UTF-8?B?SMO2bmln?= (8/8) Dec 03 2019 Today i have stumbled on Hacker News into:
• mipri (13/21) Dec 03 2019 You can get this result in D as well:
• Ola Fosheim =?UTF-8?B?R3LDuHN0YWQ=?= (4/6) Dec 03 2019 Not only historical, it is also for numerical reasons. You can
• Ola Fosheim =?UTF-8?B?R3LDuHN0YWQ=?= (7/9) Dec 03 2019 You get the same in C++ with:
• Basile B. (2/10) Dec 03 2019 D lang could have a very good sprintf replacement.
• H. S. Teoh (13/25) Dec 03 2019 In short, because they use 64-bit floats for intermediate values

Jan =?UTF-8?B?SMO2bmln?= <hrominium gmail.com> writes:

Today i have stumbled on Hacker News into: 
https://0.30000000000000004.com/

I am learning D, that's why i have to ask.

Why does

     writefln("%.17f", .1+.2);

not evaluate into: 0.30000000000000004, like C++
but rather to: 0.29999999999999999

Many other languages evaluate to 0.30000000000000004 as well.

mipri <mipri minimaltype.com> writes:

On Tuesday, 3 December 2019 at 09:22:49 UTC, Jan Hönig wrote:
 Today i have stumbled on Hacker News into: 
 https://0.30000000000000004.com/

 I am learning D, that's why i have to ask.

 Why does

     writefln("%.17f", .1+.2);

 not evaluate into: 0.30000000000000004, like C++
 but rather to: 0.29999999999999999

 Many other languages evaluate to 0.30000000000000004 as well.

You can get this result in D as well:

   $ rdmd --eval 'double a = .1, b = .2; writefln("%.17f", a+b)'
   0.30000000000000004

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.

Ola Fosheim =?UTF-8?B?R3LDuHN0YWQ=?= <ola.fosheim.grostad gmail.com> writes:

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.

Ola Fosheim =?UTF-8?B?R3LDuHN0YWQ=?= <ola.fosheim.grostad gmail.com> writes:

On Tuesday, 3 December 2019 at 09:22:49 UTC, Jan Hönig wrote:
 not evaluate into: 0.30000000000000004, like C++
 but rather to: 0.29999999999999999

You get the same in C++ with:

#include <cstdio>
int main()
{
     printf("%.17f",double(0.1L + 0.2L));
}

Basile B. <b2.temp gmx.com> writes:

On Tuesday, 3 December 2019 at 09:22:49 UTC, Jan Hönig wrote:
 Today i have stumbled on Hacker News into: 
 https://0.30000000000000004.com/

 I am learning D, that's why i have to ask.

 Why does

     writefln("%.17f", .1+.2);

 not evaluate into: 0.30000000000000004, like C++
 but rather to: 0.29999999999999999

 Many other languages evaluate to 0.30000000000000004 as well.

D lang could have a very good sprintf replacement.