Re: bug? floating point precision with std.format
On Monday, June 05, 2017 13:23:38 Steven Schveighoffer via Digitalmars-d wrote: > On 6/5/17 12:29 PM, Seb wrote: > > You do realize that you have used "s" in the D version? > > Yes, I thought it was a stand in for "use the type to determine the > specifier", and I mistakenly assumed that would be 'f', since that's > what I've always used for floating point. Apparently it is 'g', which > behaves as I have shown. I always assumed that it just meant "convert to string" and that it did basically the same thing that to!string would do, in which case, doing something like passing a number to it like you did would not be legal. Clearly, I never read the docs. :) - Jonathan M Davis
Re: bug? floating point precision with std.format
On 6/5/17 12:29 PM, Seb wrote:
You do realize that you have used "s" in the D version?
Yes, I thought it was a stand in for "use the type to determine the
specifier", and I mistakenly assumed that would be 'f', since that's
what I've always used for floating point. Apparently it is 'g', which
behaves as I have shown.
This is a bit more interesting:
writefln("%0.3s", f); // 20.7
printf("%0.3s\n", f); // 20.
That is really bad, because %s means interpret the parameter as a char *
string. So the memory pointed at by the bit pattern of 20.6 cast to
a pointer, has the first 3 bytes '2', '0', and '.', and then a null
character (or a bunch of unprintable characters, followed by a null).
Bizarre...
-Steve
Re: bug? floating point precision with std.format
On 6/5/17 12:53 PM, H. S. Teoh via Digitalmars-d wrote:
On Mon, Jun 05, 2017 at 04:29:06PM +, Seb via Digitalmars-d wrote:
On Monday, 5 June 2017 at 15:37:42 UTC, Steven Schveighoffer wrote:
It appears that the precision parameter in std.format differs from its
meaning in printf. Is that expected behavior?
Example:
import std.stdio;
import core.stdc.stdio;
void main()
{
auto f = 20.6;
writeln(f);
writefln("%0.3s", f);
[...]
That should be "%0.3f", not "%0.3s".
If you use the "%s" specifier, precision is interpreted differently,
i.e., as "maximum number of characters", as per "%s" in C's printf.
Interesting. I thought s just stood for "interpret based on the type",
and would automatically switch to floating point 'f'. I see in the docs
now, it uses 'g', something I've never used.
Curious that 'f' isn't used, I thought it would have been the default.
In any case, I have a fix for my code, move along :)
-Steve
Re: bug? floating point precision with std.format
On Mon, Jun 05, 2017 at 04:29:06PM +, Seb via Digitalmars-d wrote:
> On Monday, 5 June 2017 at 15:37:42 UTC, Steven Schveighoffer wrote:
> > It appears that the precision parameter in std.format differs from its
> > meaning in printf. Is that expected behavior?
> >
> > Example:
> >
> > import std.stdio;
> > import core.stdc.stdio;
> >
> > void main()
> > {
> > auto f = 20.6;
> > writeln(f);
> > writefln("%0.3s", f);
[...]
That should be "%0.3f", not "%0.3s".
If you use the "%s" specifier, precision is interpreted differently,
i.e., as "maximum number of characters", as per "%s" in C's printf.
T
--
If it tastes good, it's probably bad for you.
Re: bug? floating point precision with std.format
On Monday, 5 June 2017 at 15:37:42 UTC, Steven Schveighoffer
wrote:
It appears that the precision parameter in std.format differs
from its meaning in printf. Is that expected behavior?
Example:
import std.stdio;
import core.stdc.stdio;
void main()
{
auto f = 20.6;
writeln(f);
writefln("%0.3s", f);
printf("%0.3f\n", f);
}
prints:
20.6667
20.7
20.667
It appears that the precision specifier is dictating the total
number of digits on *both sides* of the decimal place. Whereas,
in C, it's only the number of digits *after* the decimal place.
I'm trying to specify 3 places of precision after the decimal.
How do I do this easily?
I'm having a hard time believing this behavior has never been
reported, but I can't find anything about it in bugzilla.
Tested all the way back to 2.040.
-Steve
You do realize that you have used "s" in the D version?
This works as expected:
writefln("%0.3f", f); // 20.667
printf("%0.3f\n", f); // 20.667
This is a bit more interesting:
writefln("%0.3s", f); // 20.7
printf("%0.3s\n", f); // 20.
