Custom units having unexpected effects on results

[zcohan]

I'm encountering an issue where custom units are changing how expressions that do not include the new unit are evaluated.

Example:

var result = math.eval('1 km / hour') //answer is 1 km/hour
math.createUnit('mph', {definition: '1.0 mile/hour'})
result = math.eval('1 km / hour') //answer is now 0.6213711922373341 mph

[ericman314]

That's a result of having the unit system set to the default of auto. A unit system defines the particular units results will be displayed in. In auto, the unit system is continually updated each time a unit is created or encountered. Not very functional in a programming sense, but it does help show the user what they probably want to see.

You can disable that using setUnitSystem('si') or similar.

[zcohan]

Thanks @ericman314 , is that a function on the math object, or the unit itself? I can't find any mention of it on mathjs.org.

[ericman314]

I think you can use this:

math.type.Unit.setUnitSystem('si');

[zcohan]

If I set the unit system only once after creating the math object, it doesn't work (I need to set it after creating the unit). So this works:

math.createUnit('mph', {definition: '1.0 mile/hour'})
math.type.Unit.setUnitSystem('si')
math.eval('(10 km/hour)') //answer is 10 km/hour

However, for my purposes the user will be able to add custom units at any time. Of course, I can just set the unit system again after any new unit is created, however it seems there is a bug: if I set unit system twice, it stops applying:

math.type.Unit.setUnitSystem('si')
math.createUnit('mph', {definition: '1.0 mile/hour'})
math.type.Unit.setUnitSystem('si')
math.eval('(10 km/hour)') //answer is 6.21 mph

[zcohan]

Also, setting the unit system to 'si' undoes the fix you made the other day for the 'incorrect custom unit in unit multiplication result' issue:

math.type.Unit.setUnitSystem('si')
math.createUnit('USD')
math.createUnit('EUR', {definition: '1.15 USD'})
math.eval('10 EUR/hour * 2 hours') //23 USD, should be 20 EUR

Maybe auto can be improved such that 1 km / hour = 1 km/hour, even with custom units present? It doesn't seem intuitive to automatically convert into a custom unit, without the conversion being explicitly requested.

[ericman314]

As for mph being used even after setting the unit system to si, I believe createUnit automatically adds the created unit to whatever the current unit system is.

As for EUR vs USD, the intelligent choosing of the unit to format the result in only works in the auto unit system.

It's hard to infer what the user wants to see. For instance, what should the result of 10 km/hour be if you had created the unit kph instead of mph? The current implementation does a good job most of the time. There's some clear improvements that could be made here though, and we might need to step back and rethink how formatting is done.

[zcohan]

I definitely want the intelligent unit choosing behaviour, so I'll have to stick with auto.

Could we then consider 10 km/hour = 6.21 mph a bug with auto?

Personally I don't think it's a great design move to have the existence of custom units override the standard calculation behaviour. I might not be able to offer users the ability to add custom units at all, if as a result answers of unrelated calculations become unpredictably affected.

Furthermore, the (bug? feature?) seems to be only present for compound units:

math.createUnit('pickle', {definition: '5 km'})
math.eval('5 km') // 5 km, not 1 pickle, good!

math.createUnit('giffy', {definition: '5 km/minute'})
math.eval('5 km/minute') // 1.0 giffy, technically true, but not what I expect...

Would you agree that it makes more sense to explicitly request formatting in custom units?

math.eval('5 km/minute in km/minute') // This seems redundant, but it's the only way to avoid being giffied, now that giffies exist.
math.eval('5 km/minute in giffy') // 1.0 giffy, great.

[josdejong]

Thinking aloud: sounds like we want to somehow be able to give specific units some sort of precedence, i.e. give a custom unit like giffy lower precedence than SI units (km/minute). Or somehow have different categories or groups of units, and have a "barrier" to jump from one group (like SI units) to units outside of the group.