CODEWITHSUNDEEP
c++c++11floating-point
Jasper Braun
Jasper Braun

Reputation: 109

Does raising by too small double result in 0.0? If not, how to achieve this?

I am dealing with floating point very small in magnitude and would like to round to 0 whenever the floating point cannot be represented by a double because it is too small in magnitude.

This code:

#include <iostream>
#include <cmath>

int main() {
  double x{100.0};
  double y{(-1.0) * 1.7976931348623157e308};
  double z{std::pow(x, y)};
  std::cout << typeid(z).name() << ": " << z << std::endl;
  std::cout << (z == 0) << std::endl;
  return 0;
}

prints

$ ./a.out
d: 0
1

for me (clang version 11.0.0, -std=c++11), as desired.

Question: Do expressions that result in a floating point too small in magnitude to be represented by a double always evaluate to 0 when assigned to a double? (including for other C++ compilers?) If not, how can I achieve this behavior, or test for such an expression being too small in magnitude?

Edit: As @Eljay pointed out, I could test if the expression results in a denormalized double.

The solution then would be to test std::fpclassify(z) == FE_SUBNORMAL and if so to set z equal to zero. This solves my problem.

I should point out that in my question I asked for setting z equal to zero when the expression assigned to it is not representable as double due to floating point underflow. A double that classifies as FE_SUBNORMAL is representable by a double, so technically the answer by @eerorika is correct.

Upvotes: 1

Views: 437

Answers (1)

eerorika
eerorika

Reputation: 238401

Do expressions that result in a floating point too small in magnitude to be represented by a double always evaluate to 0

Not necessarily. The result depends on the rounding mode. For example, if FE_UPWARD is used, then you would get a very small non-zero result - assuming rounding modes are supported by the language implementation.

If not, how can I achieve this behavior

FE_TOWARDZERO rounding mode should behave like that - again, assuming rounding modes are supported.

Upvotes: 1

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