Basic arithmetic issue

[Suedede]

julia> 10.3 * 3
30.900000000000002

julia> 10.3 * 3 == 30.9
false

When I type 10.3 * 3 I get 30.900000000000002 instead of 30.9, I'm no expert but I don't get this when I type it into R and I suspect something is wrong.

[ViralBShah]

That is indeed the right answer. Here is matlab:

>> format long
>> 10.3*3     

ans =

  30.900000000000002

This should have been filed at https://github.com/JuliaLang/julia/issues.

[Suedede]

Allright then, thanks for being so polite and giving me a quick short answer.

[StefanKarpinski]

This is a weird place to file such a bug since it has nothing to do with Gadfly. A little more elaboration – R looks like its answer is exactly 30.9 because it rounds trailing digits when it prints numbers, effectively lying about their exact value. Checking equality with 30.9, however, reveals that the value is not actually the same:

> 10.3*3
[1] 30.9
> 10.3*3 == 30.9
[1] FALSE
>

Julia, on the other hand, prints all the digits required to reconstruct a floating point number exactly, so you can see that this value is not exactly 30.9. All these systems are converting the input 30.9 to the closest representable IEEE 754 64-bit floating-point number (30.9 is not exactly representable in binary), and then doing standard IEEE arithmetic to get this result.