Yes, this makes sense to me. Wolfram seems to treat the expression and the units separately, which makes sense. In your example, any omitted value is seen as 1, so the result is perfectly correct. The expression is evaluated and the units are added after it, but they don't cancel out by subtraction. I don't think it is sensible to treat units as variables if this leads to results like the one I encountered. Then nothing is gained by using the units package and I could just create my own variables called m, K, J, W etc. Is there an easy way to get a behaviour like in Mathematica?
On Aug 2, 9:29 pm, Eviatar <eviatarb...@gmail.com> wrote: > It seems WolframAlpha evaluates the limit of the > expression:http://www.wolframalpha.com/input/?i=%28calorie%2Fcentimeter%5E2%2F+m.... > > On Aug 2, 12:21 pm, Eviatar <eviatarb...@gmail.com> wrote: > > > > > > > > > I don't really see this as a bug. Units are treated as variables, so > > it makes sense. Are you suggesting that 0 * kelvin should be left > > unevaluated, and then not give an error when it is the denominator? > > > On Aug 2, 7:49 am, Stan Schymanski <schym...@gmail.com> wrote: > > > > Dear all, > > > > This is a bug-report or feature request for the units package, taken > > > from sage-support. Basically, the units package does not handle > > > addition and subtraction in a sensible way, as units cancel out when > > > variables with the same units are subtracted from each other. Sage > > > should give an error message when adding or subtracting variables with > > > different units, while leaving the units intact if the variables have > > > the same units. Does anyone have an idea how this could be > > > accomplished? Thanks already! > > > > Below is an example of the problem posted > > > athttp://groups.google.com/group/sage-support/browse_thread/thread/a60c... > > > > sage: udict = {} > > > sage: udict[H_l] = units.energy.calorie/units.length.centimeter^2/ > > > units.time.minute > > > sage: udict[T_a] = units.temperature.kelvin > > > sage: udict[T_l] = units.temperature.kelvin > > > sage: soln = solve(H_l == h_c*(T_a - T_l), h_c)[0]; soln > > > h_c == H_l/(T_a - T_l) > > > sage: soln.subs(udict) > > > Traceback (most recent call last): > > > ... > > > RuntimeError: power::eval(): division by zero > > > > --- > > > This works: > > > sage: (H_l/T_a).subs(udict) > > > calorie/(centimeter^2*kelvin*minute) -- To post to this group, send an email to sage-devel@googlegroups.com To unsubscribe from this group, send an email to sage-devel+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-devel URL: http://www.sagemath.org
