Le 16/06/2012 12:16, Sergiu Ivanov a écrit : >> It can be useful to have a distinction between quantities and units for >> several reasons, e.g.: from the "philosophical" point a view, an unit is >> something really fixed and defined, whereas a quantity is just a >> temporary object. Moreover, we can then define quantity with >> uncertainties with are clearly not units. In the same way, logarithmic >> units or units with offset (like °C) are not simple "number" as a >> quantity. So it could be useful to have a difference. >> On the other hand, I think that constant should really be viewed as the >> almost the same thing as an unit (we can subclass to say that the status >> is a little different). > > Yes, I guess I agree. I'm not happy with a philosophical distinction, > obviously, however, quantities with uncertainty do indeed show that a > quantity is something more than a unit. And I totally approve of the > idea of subclassing quantities from units. > > I'm still not sure what point you are trying to make with logarithmic > units or units with offset, however. In fact, to me it looks like an > extra argument that quantities and numbers *are* similar ;-) That is, > you can do very much the same operations on units as you can on > numbers. > > Sergiu >
If you want another example that let feel that unit and quantity are two different concepts is that 0.001 km or 1.6 km are units (m and miles), but if you get 2 km, then you have no corresponding units. And if you say that it could be an unit, so are all the numbers times an unit but in this case you have too much freedom. Units which are logarithmic or with an offset can not be exactly combined as if they where number if there are also "normal" units. For example in thermodynamics you should take a great care when using °C. Harold -- 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.
