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* [Bug c++/110749] New: wrong show float
@ 2023-07-20 6:46 alexsyrezerv at mail dot ru
2023-07-20 7:31 ` [Bug c++/110749] " schwab@linux-m68k.org
` (6 more replies)
0 siblings, 7 replies; 8+ messages in thread
From: alexsyrezerv at mail dot ru @ 2023-07-20 6:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110749
Bug ID: 110749
Summary: wrong show float
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: alexsyrezerv at mail dot ru
Target Milestone: ---
Created attachment 55586
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55586&action=edit
To try on different compillers
Code:
```
#include <iostream>
#include <iomanip>
void func_float_point(std::ostream& os, const double& value, const std::size_t&
size) {
os << (0.0 > value ? '-' : '0') << std::setw(size - 1) << std::setfill('0')
<< std::abs(value);
}
int main(int, char**) {
const double a[] = {90.8425, 101.9225};
for(std::size_t i = 0; i < 2; ++i){
func_float_point(std::cout, a[i], 17);
std::cout << std::endl;
}
return 0;
}
```
result
```
butiaev@butiaev-VirtualBox:~/tmp$ make bug
g++ bug.cpp -o bug
butiaev@butiaev-VirtualBox:~/tmp$ ./bug
000000000090.8425
0000000000101.922
butiaev@butiaev-VirtualBox:~/tmp$
```
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110749] wrong show float
2023-07-20 6:46 [Bug c++/110749] New: wrong show float alexsyrezerv at mail dot ru
@ 2023-07-20 7:31 ` schwab@linux-m68k.org
2023-07-20 9:09 ` alexsyrezerv at mail dot ru
` (5 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: schwab@linux-m68k.org @ 2023-07-20 7:31 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110749
Andreas Schwab <schwab@linux-m68k.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
101.9225 cannot be represented exactly, the nearest representable value is
101.92249999999999943.
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110749] wrong show float
2023-07-20 6:46 [Bug c++/110749] New: wrong show float alexsyrezerv at mail dot ru
2023-07-20 7:31 ` [Bug c++/110749] " schwab@linux-m68k.org
@ 2023-07-20 9:09 ` alexsyrezerv at mail dot ru
2023-07-20 10:03 ` redi at gcc dot gnu.org
` (4 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: alexsyrezerv at mail dot ru @ 2023-07-20 9:09 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110749
--- Comment #2 from Aleksei <alexsyrezerv at mail dot ru> ---
Created attachment 55589
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55589&action=edit
GDB SHOW IT
But GDB show it normally
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110749] wrong show float
2023-07-20 6:46 [Bug c++/110749] New: wrong show float alexsyrezerv at mail dot ru
2023-07-20 7:31 ` [Bug c++/110749] " schwab@linux-m68k.org
2023-07-20 9:09 ` alexsyrezerv at mail dot ru
@ 2023-07-20 10:03 ` redi at gcc dot gnu.org
2023-07-20 10:58 ` alexsyrezerv at mail dot ru
` (3 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: redi at gcc dot gnu.org @ 2023-07-20 10:03 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110749
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Your program is using the default precision for printing a double: 6
significant figures.
101.92249999999999943 with 6 digits of precision is 101.922 (because (24 rounds
to 2).
101.92249999999999943 with 7 digits of precision is 101.9225 (because 49 rounds
to 5).
If you use std::setprecision(7) << std::abs(value) you will get 101.9225:
000000000090.8425
000000000101.9225
If you use std::setprecision(17) you will get the same values as GDB shows:
090.842500000000001
000000000101.9225
If you use std::setprecision(18) you'll see that Andreas is correct:
090.8425000000000011
0101.922499999999999
This is not a GCC bug, this is just how floating-point numbers work.
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110749] wrong show float
2023-07-20 6:46 [Bug c++/110749] New: wrong show float alexsyrezerv at mail dot ru
` (2 preceding siblings ...)
2023-07-20 10:03 ` redi at gcc dot gnu.org
@ 2023-07-20 10:58 ` alexsyrezerv at mail dot ru
2023-07-20 11:08 ` alexsyrezerv at mail dot ru
` (2 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: alexsyrezerv at mail dot ru @ 2023-07-20 10:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110749
--- Comment #4 from Aleksei <alexsyrezerv at mail dot ru> ---
ок
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110749] wrong show float
2023-07-20 6:46 [Bug c++/110749] New: wrong show float alexsyrezerv at mail dot ru
` (3 preceding siblings ...)
2023-07-20 10:58 ` alexsyrezerv at mail dot ru
@ 2023-07-20 11:08 ` alexsyrezerv at mail dot ru
2023-07-20 12:10 ` redi at gcc dot gnu.org
2023-07-20 12:10 ` redi at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: alexsyrezerv at mail dot ru @ 2023-07-20 11:08 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110749
--- Comment #5 from Aleksei <alexsyrezerv at mail dot ru> ---
this variant more clear.. 4 symbol after point// may by c++ has modification
for set symbols after decimal point ?
```
void func_float_point(std::ostream& os, const double& value, const
mcx::convert_info_t& cv) {
char buf[buf_size];
snprintf(buf, buf_size - 1, "%0*.*f", cv.size, cv.pos, value);
os << buf;
}
```
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110749] wrong show float
2023-07-20 6:46 [Bug c++/110749] New: wrong show float alexsyrezerv at mail dot ru
` (4 preceding siblings ...)
2023-07-20 11:08 ` alexsyrezerv at mail dot ru
@ 2023-07-20 12:10 ` redi at gcc dot gnu.org
2023-07-20 12:10 ` redi at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: redi at gcc dot gnu.org @ 2023-07-20 12:10 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110749
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
If you use std::fixed as the floatfield format then std::setprecision specifies
the number of digits after the decimal point:
os << (0.0 > value ? '-' : '0') << std::setw(cv.size - 1) << std::setfill('0')
<< std::setprecision(cv.pos) << std::fixed << std::abs(value);
You don't need to handle the -'- sign yourself:
os << std::setw(cv.size) << std::internal << std::setfill('0') << std::fixed <<
std::setprecision(cv.pos) << value;
This matches the output of printf("%0*.*f", cv.size, cv.pos, value)
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110749] wrong show float
2023-07-20 6:46 [Bug c++/110749] New: wrong show float alexsyrezerv at mail dot ru
` (5 preceding siblings ...)
2023-07-20 12:10 ` redi at gcc dot gnu.org
@ 2023-07-20 12:10 ` redi at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: redi at gcc dot gnu.org @ 2023-07-20 12:10 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110749
--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #6)
> You don't need to handle the -'- sign yourself:
Oops, I mean '-' sign.
^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2023-07-20 12:10 UTC | newest]
Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
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2023-07-20 6:46 [Bug c++/110749] New: wrong show float alexsyrezerv at mail dot ru
2023-07-20 7:31 ` [Bug c++/110749] " schwab@linux-m68k.org
2023-07-20 9:09 ` alexsyrezerv at mail dot ru
2023-07-20 10:03 ` redi at gcc dot gnu.org
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