Lengths being evaluated as unequal (when they should be equal)

jacob_kingery

To explore transforms (specifically rotations), I've been working on a Feature that takes in a string and performs the corresponding moves on a (crude) Rubik's Cube model.  However, there's something weird going on when I try to isolate the cubies on the back side of the cube.  My strategy is to compare certain dimensions of the cubie bounding boxes to the bounding box of the overall cube, and this works for all other sides.

To debug, I have it print out the values it is comparing, followed by the result of the comparison:

// Cubies on the back
if (isIn(side, ['B', 'b']))
{
    print(bounds.maxY);
    print(' =?= ');
    println(maxCorner[1]);
    println(maxCorner[1] == bounds.maxY);
    if (maxCorner[1] == bounds.maxY) {
        cubies = append(cubies, thisCubie);
    }
}

The results of this are what are confusing to me. Whenever the values are unequal, it prints false as expected. However, when the values appear to be equal (and should be equal), it sometimes evaluates them as unequal and sometimes as equal.

...
0.0203497 meter =?= 0.00637966 meter
false
0.0203497 meter =?= 0.0203497 meter
false
...
0.0203497 meter =?= 0.00637966 meter
false
0.0203497 meter =?= 0.0203497 meter
true
...

Any ideas as to what might be going on? FS version is 255.0 if that could be relevant.

kevin_o_toole_1

This is a property of floating point math which is not unique to FS.

As a general rule, using a direct equality comparison on floating point numbers is not what you want. Instead, define a tolerance, and check if two numbers are within that tolerance:

const tolerance = 0.000001 * meter^3;
const volume = evVolume(context, {
       "entities" : qEverything(EntityType.BODY)
});
const expectedVolume = 0.05 * meter^3;
if (abs(volume - expectedVolume) > tolerance)
{
    // Do something
}

kevin_o_toole_1

This is a property of floating point math which is not unique to FS.

As a general rule, using a direct equality comparison on floating point numbers is not what you want. Instead, define a tolerance, and check if two numbers are within that tolerance:

const tolerance = 0.000001 * meter^3;
const volume = evVolume(context, {
       "entities" : qEverything(EntityType.BODY)
});
const expectedVolume = 0.05 * meter^3;
if (abs(volume - expectedVolume) > tolerance)
{
    // Do something
}

ilya_baran

We also have a bugfix coming where even slightly unequal values will be printed as different to avoid precisely this confusion.

jacob_kingery

Ah, thanks.  I was wondering if it could be something along those lines.

Looking through the standard library, I found various tolerantEquals predicates. Would using those work for catching floating point number issues?

On a side note, the tolerantEquals predicate in units.fs has a precondition, but the documentation states that predicates may not have preconditions. Is the phrase 'may not have' supposed to mean 'sometimes will not have' or 'are not allowed to have'?

ilya_baran

Yes, tolerantEquals is designed for this exact thing.

Regarding preconditions, in this case "may not have" means the documentation is both vague and not correct  Will fix.

jacob_kingery

Great, thanks!