show unit symbols for common units

[jonasrauber]

I think it would be nice to see the unit symbol for common units, e.g. N for Newton instead of kg m s⁻².

julia> using SIUnits

julia> using SIUnits.ShortUnits

julia> 10J
10 J

julia> 10J/s
10 W

julia> kg*m/s
kg m s⁻¹

julia> kg*m/s^2
N

[Keno]

I had thought about this, but was concerned about cases where there was two possible representations, e.g. J or N m. Thoughts?

[jonasrauber]

We could display both, i.e. 10 J or N m, and/or provide the user with a way to overwrite this according to the context. Furthermore, this feature could be optional adding a switch to turn it on or off.

[tomasaschan]

I'd rather do this as a separate function call, than automatically, i.e. something like

julia> using SIUnits, SIUnits.ShortUnits
julia> 10N
10 kg m/s^2
julia> simplifyunits(10 N * m)
10 J
julia> simplifyunits(10 N * m; commons=[N,m])
10 N m

where my idea with the optional parameter (which could probably be better named) is we have a predefined list of "common" units to which we contract, but the user can override it to alter the behavior - e.g. to choose to display N m instead of J.

Doing it this way has the added benefit that it won't alter the behavior of existing code.

[jonasrauber]

When I played around with this, I initially also did it using a separate function. However, it became kind of annoying to always call simplifyunits all the time, because in 99% of the cases, that's what I wanted. That's why I thought the user should be able to choose that units are always simplified when they are printed.

[tomasaschan]

@jonasrauber, sure, but how does the user get to choose which units are used? I see no functionality in this PR that allows that flexibility.

[jonasrauber]

@tlycken no, you are right, that's currently not part of this PR. I thought someone might have a better idea to solve this problem. Do you know any other julia package that gives the user the choice to configure the output of show ?

[mbauman]

This is somewhat similar to the GRISU/show_shortest problem when displaying floats. It'd be really cool to automagically print the shortest representation of the units, but it seems like a traveling-salesman problem to me… especially once you start considering combinations of powers of derived units. But then showall could show the longer base SI representation.

For my purposes, though, I almost always use microvolts or milliamps, so something simple along these lines would make most of what I do more intuitive.

[ma-laforge]

Hi Keno/Everyone,

Along the same theme, I implemented facilities to display numeric values using SI prefixes (M, k, m, u, n, ...)

using SIUnits
using SIUnits.ShortUnits

f=100e6Hz # => 1.0e8 s-1
w=2pi*f # => 6.283185307179586e8 s-1

using NumericIO
println("Angular freq: ", Formatted(w.val,:SI,ndigits=4), " rad/s")
# => Angular freq: 628.3M rad/s

Question
Do you think this would be a good addition to the SIUnits module?

If so, the code can be found under (MIT-lic)
https://github.com/ma-laforge/NumericIO.jl