Wrong result when multiplying a very small number and a very big one using ^

[ronisbr]

Hi guys,

I was implementing a algorithm, comparing to MATLAB version, and I found a very strange behavior in Julia. I need to perform the following multiplication:

4.604716014154575e-19*(10000000000)^2 = -46.047160141545149

First, I was using the ^, but it was providing a wrong answer:

julia> 4.604716014154575e-19*10000000000^2
3.5761512189350633

If I drop the ^ and multiply two times, then the result is correct:

julia> 4.604716014154575e-19*10000000000*10000000000
46.04716014154576

I tested it in Julia v0.6.2 and v0.6.3 in macOS and Linux with the same behavior. I think the operation 10000000000^2 is overflowing Int64 and with ^ this is computed first. On the other hand, using the multiplication, the first operation is 4.604716014154575e-19*10000000000, which does not cause any overflow.

However, I think Julia should be able to handle that just as MATLAB can. At least a warning about the overflow should be printed. Otherwise, sometimes can be very difficult to debug this. What do you think?

[mbauman]

Duplicate of #855

[mbauman]

Note that this works in Matlab because the literal 10000000000 is treated as a floating point number. To duplicate that behavior you can do 4.604716014154575e-19*10000000000.0^2.

[ronisbr]

@mbauman thanks! I will follow that bug.

[ViralBShah]

Its best to ask these questions on discourse.julialang.org.

[ronisbr]

Sorry but it really looked like a bug to me. Actually, IMHO, I think this should be considered a bug.

[StefanKarpinski]

You may not like it but it is not a bug as it is both intentional and thoroughly documented.

[ronisbr]

Ok then...