Dynamically Add and Remove Units

[billdenney]

I work with data which uses concentration in both mass and molar units (like ng/mL and nmol/L). I'd like to enable conversion between the mass and molar units. While it's possible to add a .xml units file before loading the library, keeping track of all the files would be complex.

What do you think of adding a two functions to the library like:

ud.map.unit("g", 350, "mole")
ud.reload.system()

Where the first command would add a mapping from mass units to molar units with a conversion of 350 g = 1 mole. And, the second command would reload the unit system to the system default (removing any mapped units).

[jameshiebert]

Hi @billdenney, sorry for the really slow reply on this. My preference is to keep Rudunits2 as a thin wrapper around the C udunits library. So generally, I would not be keen to add functionality to the library that isn't already in the base library (that kind of breaks the thin wrapper model).

That said, I think that this functionality does exist in the base library, described here. Note that they do say:

you should not normally need to add any new units to a unit-system

but they provide the functionality to do so, so why not?

I don't know that I have the capacity to work on this now, but if you're still interested, I'd be willing to consider it as a pull request if you put one together. You should just be able to follow the existing C code in the package, which basically just puts an R wrapper around the C library calls and adds a bit of error checking. What do you think?

[billdenney]

I'm not much of a C programmer, but I'll see what I can do.