[Trilinos-Users] precision double in for Teuchos XMLObject ostream

[Markus Berndt]

It looks like this fix described by Mark (see below in the quoted email)
was included in 10.12. However, for Array<double> and Array<float> the
problem persists. 

I believe that the function

std::string ArrayView<T>::toString() const

needs to be updated/specialized such that for T=double and T=float high
enough precision is used to represent a number accurately in the XML
file.

- Markus


On Mon, 2012-03-05 at 04:51 +0000, Hoemmen, Mark wrote:
> On Sat 03 Mar 2012 at 16:12, George Pau <gpau at lbl.gov> wrote:
> > I have a code that read in an XML file, save it as a ParameterList
> > and then write it to a new XML file again.  What I notice is that
> > the precision of parameter of type double drops in the new XML file.
> > For example, even though I specified 7 decimal points in the
> > original XML file, my new XML file shows the same value to only 5
> > decimal points.  Is there any way that I can maintain the precision
> > in my new XML file?
> 
> I took a brief look at the conversion code, and it seems like the
> issue is with Teuchos::ToStringTraits.  This traits class implements
> conversion to strings (via Teuchos::toString()) for different types T.
> The default implementation of ToStringTraits creates an
> std::ostringstream, writes the object of type T to the ostringstream,
> and returns the std::string version of the ostringstream.  Teuchos
> does not override ToStringTraits for floating-point types, so
> toString() uses the default implementation in that case.
> 
> This would explain why setting the precision of floating-point data in
> the output file, as you do, has no effect.  The XML output code first
> converts the floating-point value to a string (via toString()), then
> writes the string.  This bypasses the precision you set for the output
> file.
> 
> This is really a bug that we should fix, so thank you for pointing
> this out!  If you need a workaround, you could simply specialize
> Teuchos::ToStringTraits for T = double, using an excessive precision.
> I like to use round(52 * log10(2)) + 1 = 17 for double-precision real
> values (please feel free to correct me if I've gotten this wrong).
> 
> namespace Teuchos {
> template<>
> class ToStringTraits<double> {
> public:
>   static std::string toString (const double& t) {
>     std::ostringstream os;
>     os.setf (std::ios::scientific);
>     os.precision (17);
>     os << t;
>     return os.str();
>   }
> };
> 
> There is, in fact, an algorithm, due to Guy Steele (yes, Java's Guy
> Steele) et al., for idempotent printing of finite-length
> floating-point values.  One of these days it would be nice for us to
> take the time to implement that algorithm and test it carefully.  For
> now, cranking up the precision works fine.
> 
> Best,
> mfh
> 
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-- 
Markus Berndt -- LANL CCS-2 -- (505) 665-4711