How to specify setprecision rounding

by

There is also std::fesetround from <cfenv>, which sets the rounding direction:

#include <iostream>
#include <iomanip>
#include <cmath>
#include <cfenv>

int main () {
    double runtime = 0.756247615801;

    // Set rounding direction and output with some precision:
    const auto prev_round = std::fegetround();
    std::fesetround(FE_DOWNWARD);    
    std::cout << "desired: " << std::setprecision(6) << runtime << "\n";

    // Restore previous rounding direction and output for testing:
    std::fesetround(prev_round);
    std::cout << "default: " << std::setprecision(6) << runtime << "\n";
}

(note that these are not the kind of comments I recommend, they are just for tutoring purposes)

Output:

desired: 0.756247
default: 0.756248

Important note, though: I did not find any mention in the standard, that the operator<< overloads for floating types have to honour the rounding direction.

More Related Contents:

  1. What is the most effective way for float and double comparison?
  2. Random float number generation
  3. What is the difference between float and double?
  4. How can I write a power function myself?
  5. Sin and Cos give unexpected results for well-known angles
  6. Efficient implementation of log2(__m256d) in AVX2
  7. long double vs double
  8. Difference in floating point arithmetics between x86 and x64
  9. What is a subnormal floating point number?
  10. Floating point format for std::ostream
  11. Change floating point rounding mode
  12. Emulate “double” using 2 “float”s
  13. Why did Microsoft abandon long double data type? [closed]
  14. C++: How to round a double to an int? [duplicate]
  15. How to detect double precision floating point overflow and underflow?
  16. Compare two float variables in C++
  17. A fast method to round a double to a 32-bit int explained
  18. Set back default floating point print precision in C++
  19. Floating point Endianness?
  20. How computer does floating point arithmetic?
  21. How to correctly and standardly compare floats?
  22. Convert a float to a string
  23. Math precision requirements of C and C++ standard
  24. C floating point precision [duplicate]
  25. Why do sin(45) and cos(45) give different results? [duplicate]
  26. C++ floating point to integer type conversions
  27. How to write a Float Mat to a file in OpenCV
  28. Printing double without losing precision
  29. When a float variable goes out of the float limits, what happens?
  30. Does the C++ standard specify anything on the representation of floating point numbers?

Leave a Comment