Why 0.1 + 0.2 == 0.3 in D?

assert(0.1 + 0.2 != 0.3); // shall be true

is my favorite check that a language uses native floating point arithmetic.

C++

#include <cstdio>

int main()
{
   printf("%d\n", (0.1 + 0.2 != 0.3));
   return 0;
}

Output:

1

http://ideone.com/ErBMd

Python

print(0.1 + 0.2 != 0.3)

Output:

True

http://ideone.com/TuKsd

Other examples

• Java: http://ideone.com/EPO6X
• C#: http://ideone.com/s14tV

Why is this not true for D? As understand D uses native floating point numbers. Is this a bug? Do they use some specific number representation? Something else? Pretty confusing.

D

import std.stdio;

void main()
{
   writeln(0.1 + 0.2 != 0.3);
}

Output:

false

http://ideone.com/mX6zF

---

UPDATE

Thanks to LukeH. This is an effect of Floating Point Constant Folding described there.

Code:

import std.stdio;

void main()
{
   writeln(0.1 + 0.2 != 0.3); // constant folding is done in real precision

   auto a = 0.1;
   auto b = 0.2;
   writeln(a + b != 0.3);     // standard calculation in double precision
}

Output:

false
true

http://ideone.com/z6ZLk

It's probably being optimized to (0.3 != 0.3). Which is obviously false. Check optimization settings, make sure they're switched off, and try again.