Hello, On Thu, Jun 14, 2012 at 7:06 PM, Harold Erbin <[email protected]> wrote: > > There is a real liberty to define unit systems which will be useful, > instead of having hundred of units, and so one can really use only the > system and the units he needs (for example astrophysicists can only use > units with ua, Earth or Sun masses, the theoricists will use natural > system, etc.).
Sounds very cool! > The main gap in this approach is that unit and quantity are almost the > same things, since the factor of the unit can be moved to the quantity, > and vice versa. I'm thinking how to improve this, maybe by keeping in > the factor's unit only what is necessary to give 1 when expressed in > terms of base units, or when it's a special unit (like liter...). Why should there be a difference between a quantity and unit at all? If we keep in mind the analogy with vector spaces (and I actually believe there's yet a stronger correlation), the bases or generating sets of a vector (sub)space consist of vectors, just as the whole vector space. Thus, I think that it is actually good that unit and quantity are almost the same (in fact, a unit should be just a quantity with some additional properties). > I decided to let the possibility to add units or an unit with a > quantity, because it's simpler when using interactive shell (in my > opinion, but I was hesitating a lot) : > m + m = 2*m > 2*m = 3*m > so the unit m is interpreted as the quantity 1*m. And this is just another argument which supports the idea I expressed in the previous paragraph. I think that m and 1*m *are* the same thing. Now, the necessary disclaimer: I do not claim to have an authoritative opinion, so what I say may be entirely wrong :-) In that case I hope that pointing out where I err may be helpful in offering a slightly different view on the situation :-) Sergiu -- You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.
