How can I avoid this float number rounding issue in C++?

Question

With below code, I get result "4.31 43099".

  double f = atof("4.31");
  long ff = f * 10000L;
  std::cout << f << ' ' << ff << '\n';

If I change "double f" to "float f". I get expected result "4.31 43100". I am not sure if changing "double" to "float" is a good solution. Is there any good solution to assure I get "43100"?

Answer 1

The problem is that floating point is inexact by nature when talking about decimal numbers. A decimal number can be rounded either up or down when converted to binary, depending on which value is closest.

In this case you just want to make sure that if the number was rounded down, it's rounded up instead. You do this by adding the smallest amount possible to the value, which is done with the nextafter function if you have C++11:

long ff = std::nextafter(f, 1.1*f) * 10000L;

If you don't have nextafter you can approximate it with numeric_limits.

long ff = (f * (1.0 + std::numeric_limits<double>::epsilon())) * 10000L;

I just saw your comment that you only use 4 decimal places, so this would be simpler but less robust:

long ff = (f * 1.0000001) * 10000L;