There is a bug in the Json library in UE

[goodriver]

1. Parsing float within JSON in C++yields values that do not match the data
   

   

2. In the blueprint, the data obtained and sent using the blueprint plugin[Json Blueprint Utilities] is incorrect
   

   

[pladux]

That’s not a bug. It’s a floating point issue. https://floating-point-gui.de/

[goodriver]

But this plugin does not read the float correctly

[pladux]

What do you mean?

[goodriver]

In JS language, you can correctly obtain number

Extracting Float from Json using UE’s library still has some deviations

The best way is to extract String from Json

[Chatouille]

This is normal C++ behavior.

The small deviation shouldn’t have any impact most of the time, but it’s something to be aware of. In most UE functions dealing with float/double, there is a Tolerance parameter (with a sensible default value) to mitigate the deviation, for example in FVector comparisons :

If you are comparing floats/doubles directly you should also take that into account :

float A = 0.1f + 0.3f;

// Don't do this
if (A == 0.4f)
{
    //...
}

// Do this instead
if (FMath::IsNearlyEqual(A, 0.4f))
{
    //...
}

// Or this
if (FMath::Abs(A-0.4f) <= UE_KINDA_SMALL_NUMBER)
{
    //...
}

When it comes to displaying floats and doubles to screen, use a format to automatically handle rounding to the amount of decimals you need. Having a deviation at the 10th decimal isn’t gonna impact the number you display with 2-3 decimals.

[goodriver]

Have the bugs been fixed?