On Oct 15, 8:31 pm, Scott <spectre...@gmail.com> wrote:
> What I really want is sage to give me the answer as:
> q1 = 0.58239...(however many digits)*q2
>
> Is there a way to do that? I don't want an approximate value of the
> entire thing (obviously you can't get that without giving the value of
> q2), but I want it to compute whatever it can (in this case that
> (223317 pi + 416000)/(223317 pi + 416000) = 0.58239...
>
> Note:
> An error in my first post:
> When this is run, the result for sol_q1 is: q1 = (223317 pi + 416000)/
> ((223317 pi + 416000) q2)*q2
> SHOULD BE:
> When this is run, the result for sol_q1 is: q1 = (223317 pi + 416000)/
> (223317 pi + 416000)*q2
> What I want:
> q1 = 0.58239...*q2
I see. So what you want is for things like pi to autoevaluate, or (if
pi wasn't present), for fractions to auto-approximate. That is, if
you got
q1 == (3/4)*q2
you'd want n() to give you
q1 == 0.7500000...*q2
Is that right? Technically, you could access the individual
coefficients in .rhs() (via things like .coefficients and .pyobject, I
think) and apply n() to them all equations, as in the for loop you
mention earlier. That might be a lot of work, I'm not sure.
I suppose that now that our symbolic expressions are all tree-based,
it's conceivable that this could be implemented. I don't know if it's
advisable. Burcin, Mike, others... any thoughts? I have a feeling
that could be dangerous for internal reasons.
- kcrisman
>
> On Oct 15, 5:16 pm, kcrisman <kcris...@gmail.com> wrote:
>
> > Dear Scott,
>
> > Thanks for posting.
>
> > On Oct 15, 6:46 pm, Scott <spectre...@gmail.com> wrote:
>
> > > I know this is really basic, but for some reason I cannot figure this
> > > out and my searching (here, sage website, google, etc) has not yielded
> > > me any results! I just started using sage, literally, yesterday.
>
> > > I am using solve() to solve an equation for q1 in terms of q2. The
> > > solution of this is saved, but when I print it, it prints as an exact
> > > answer, but I want it in decimal/approximate form. I have tried using
> > > the n() function, but I get an error when trying it.
>
> > > Here is an example:
> > > --------------------------------------------------
> > > q1, q2 = var('q1, q2')
> > > r = 0.60
> > > t1 = 0.005
> > > t2 = 0.007
> > > d = 2.0
> > > A1 = (1/2)*pi*r*r
> > > A2 = (1/2)*d*(2*r)
> > > theta1 = 1/(2*A1)*(q1*2*pi*r/t1+(q1-q2)*2*r/t1)
> > > theta2 = 1/(2*A2)*(q2*d/t2+q2*sqrt(d^2+(2*r)^2)/t2+(q2-q1)*2*r/t1)
> > > sol_q1 = solve([theta1==theta2], q1)
> > > print sol_q1
> > > print sol_q1[0].substitute(q2=1).right().n()
> > > --------------------------------------------------
>
> > > When this is run, the result for sol_q1 is: q1 = (223317 pi + 416000)/
> > > ((223317 pi + 416000) q2)*q2
> > > The last line does work to give me the desired value (0.58239....),
> > > but this is definitely not an ideal way of getting it! And of course
> > > this wouldn't work for a more complicated result (since I use q2=1).
>
> > Why not? You should be able to put whatever you want in for q2.
> > Maybe I am misunderstanding the question.
>
> > > I've tried many other combinations of sol_q1.n(), sol_q1[0].n(), etc,
>
> > All these will do nothing if you haven't specified q2, because you are
> > trying to approximate something which has no approximation; you have
> > to substitute all free variables before approximating. I suppose it
> > would be possible to make Sage do everything on coefficients, but I
> > don't think that is the usual functionality. Your solution is
> > actually pretty good; even doing .substitute(q2=1.0) wouldn't help,
> > because we prefer to leave pi unapproximated until necessary. I do
> > agree that it's annoying! But I'm not sure how to make it less so,
> > short of writing another function that evaluated a symbolic equation
> > at a given point - without checking that you had something like
>
> > sage: solve([sin(x-y)==x*y],y)
> > [y == -sin(-x+y)/x]
>
> > where it would cause trouble.
>
> > Hope this helps!
>
> > - kcrisman
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