On 08/12/2011 02:44 AM, Aaron Meurer wrote:
Unfortunately, relational assumptions have not yet been implemented,
though they have been planned for a long time (see
http://code.google.com/p/sympy/issues/detail?id=1047). I don't know
how dynamic of assumptions you want to be able to put on theta. If
you just want to be able to assume that cos is positive, you might be
able to substitute it with a positive symbol and then back substitute
once the square root is reduced.
Aaron Meurer
On Thu, Aug 11, 2011 at 5:25 PM, Alan Bromborsky<abro...@verizon.net> wrote:
On 08/11/2011 07:04 PM, Tomo Lazovich wrote:
I just tried this in isympy and as long as you specify both real and
positive, it seems to work:
In [1]: r = Symbol('r', real=True, positive=True)
In [2]: sqrt(r*r)
Out[2]: r
In [3]: r = Symbol('r', real=True)
In [4]: sqrt(r*r)
Out[4]: │r│
On Thu, Aug 11, 2011 at 6:01 PM, Alan Bromborsky<abro...@verizon.net>
wrote:
I have a symbol r which should be assumed real and greater than zero.
What I want is for sqrt(r*r) to evaluate to r. Can this be currently done
in sympy. I am trying to clean up some old code that has a lot of hacks in
it (curvilinear coordinates).
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That worked fine. Now for another similar and more difficult problem.
Evaluate sqrt(r**2*cos(theta)**2) where -pi/2<theta<pi/2. This should
evaluate to r*cos(theta). Are symbol assumptions flexible enough yet to
handle this case (again curvilinear coordinates)?
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My solution is as follows -
def reduce_sqr(expr):
args = expr.args
if len(args) == 0:
return(sqrt(expr))
else:
if str(type(expr)) == "<class 'sympy.core.power.Pow'>":
args = expr.args
if args[1] == 2:
e_mag = args[0]
else:
e_mag = sqrt(expr)
else:
e_mag = 1
for arg in args:
if str(type(arg)) == "<class
'sympy.core.power.Pow'>":
x = arg.args
if len(x) == 2 and x[1] == 2:
e_mag *= x[0]
else:
return(sqrt(expr))
return(e_mag)
For the special case of reducing squares (normalizing basis vectors) for
curvilinear coordinates it should be OK. One question I have is when I
do something like -
str(type(expr)) == "<class 'sympy.core.power.Pow'>"
I would like to say -
type(expr) == xxx
but I don't know what xxx should be.
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