conversion with user_defined units should work

[t-kalinowski]

cells <- symbolic_unit("cells", check_is_parsable = FALSE)
ml <- make_unit(ml)
l <- make_unit(l)
x <- 500

units(x) <- cells/ml
units(x) <- cells/l
# Error: cannot convert cells/ml into cells/L

[edzer]

This can be solved in Rudunits2:

library(units)

# Attaching package: ‘units’

# The following object is masked from ‘package:base’:

#     %*%

library(udunits2)
# udunits system database read

ud.add.dimensionless("cell")

a = set_units(1, cell/l)
b = set_units(1, cell/ml)
a + b
# 1001 cell/L

[Enchufa2]

I think the user wouldn't have to know about udunits2. I would expect to install_symbolic_unit and/or install_conversion_factor and to obtain the same as with ud.add.dimensionless.

[edzer]

I agree, but it requires extensions of package udunits2 before we can do this in units.

[t-kalinowski]

This feature might be incorporated upstream:
pacificclimate/Rudunits2#9

[edzer]

... which has been open for 11 months now. @jameshiebert has pushing Rudunits2 any further still priority for you, or would you be happy if someone else took over?

[Enchufa2]

Oh, I see. I didn't realise before that the API exposed by udunits2 is very limited.

[dholstius]

Thanks for the suggested workaround @edzer. I must be misunderstanding, as I can't find udunits2:::ud.add.dimensionless(), at least not in the current CRAN version (0.13).

FWIW, my use cases are countable units like person or vehicle (for, e.g., person*day or vehicle*mile).

[edzer]

This was added to this fork: https://github.com/r-quantities/Rudunits2 and after that integrated (but renamed) in the udunits branch of the units source. We're now pretty close to releasing that to CRAN.