Teuchos_MatrixMarket_SetScientific.hpp

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#ifndef __Teuchos_MatrixMarket_SetScientific_hpp
#define __Teuchos_MatrixMarket_SetScientific_hpp

#include <Teuchos_as.hpp>
#include <Teuchos_ScalarTraits.hpp>
#include <string>


namespace Teuchos {
  namespace MatrixMarket {
    namespace details {
      template<class Scalar>
      class SetScientific {
      public:
        typedef Scalar scalar_type;

        SetScientific (std::ostream& out) :
          out_ (out), originalFlags_ (out.flags())
        {
          typedef Teuchos::ScalarTraits<scalar_type> STS;
          typedef typename STS::magnitudeType magnitude_type;
          typedef Teuchos::ScalarTraits<magnitude_type> STM;

          // Print floating-point values in scientific notation.
          out << std::scientific;

          // We're writing decimal digits, so compute the number of
          // digits we need to get reasonable accuracy when reading
          // values back in.
          //
          // There is actually an algorithm, due to Guy Steele (yes,
          // Java's Guy Steele) et al., for idempotent printing of
          // finite-length floating-point values.  We should actually
          // implement that algorithm, but I don't have time for that
          // now.  Currently, I just print no more than (one decimal
          // digit more than (the number of decimal digits justified
          // by the precision of magnitude_type)).
          //
          // We need to use STM's log10() rather than (say) std::log10
          // here, because STM::base() returns a magnitude_type, not
          // one of C++'s standard integer types.
          const magnitude_type numDecDigits = STM::t() * STM::log10 (STM::base());

          // Round and add one.  The cast to int should not overflow
          // unless STM::t() is _extremely_ large, so we don't need to
          // check for that case here.
          const magnitude_type one = STM::one();
          const magnitude_type two = one + one;
          // Cast from magnitude_type to int, since std::ostream's
          // precision() method expects an int input.
          const int prec = 1 + Teuchos::as<int> ((two*numDecDigits + one) / two);

          // Set the number of (decimal) digits after the decimal
          // point to print.
          out.precision (prec);
        }

        ~SetScientific () {
          out_.flags (originalFlags_);
        }

      private:
        std::ostream& out_;

        std::ios_base::fmtflags originalFlags_;
      };

    } // namespace details
  } // namespace MatrixMarket
} // namespace Teuchos

#endif // __Teuchos_MatrixMarket_SetScientific_hpp