EDIT: Nooo, I don't want a behaviour like in Wolfram Alpha. Check this out: http://www.wolframalpha.com/input/?i=%28calorie%2Fcentimeter^2%2F+minute%29%2F%281kelvin-2kelvin%29
By inserting numbers that lead to a negative denominator, suddenly I got units of time^-3! Maybe it's trickier than I thought... On Aug 3, 9:40 am, Stan Schymanski <schym...@gmail.com> wrote: > 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
