Indeed if you do this:
mpmath.findroot(lambda xi: p.subs(x, Expr._from_mpmath(xi,
mpmath.mp.prec)), 1.0)
[That is, you manually construct sympy objects which reflect the
precision required by mpmath.]
On 27.05.2011 05:57, Mateusz Paprocki wrote:
Hi,
On 27 May 2011 06:31, Aaron S. Meurer <[email protected]
<mailto:[email protected]>> wrote:
Well, it still fails there with the subs. Like I said, it's a bug.
Can you open an issue for it?
It's not necessarily a bug, because you expect subs() (sympy) and
findroot() (mpmath) to work together, but you are skipping the glue
(core/evalf.py or wherever this is). I'm not sure how exactly evalf code
in SymPy works, but I think that manipulation of mp.dps (or .prec)
doesn't affect evalf (the reason to think this way is that each instance
of mpf can have its own dps/prec setting). Poly.eval (also nroots())
work because it uses mpf directly, skipping the glue.
Aaron Meurer
On May 26, 2011, at 9:09 PM, Matthew Emmett wrote:
> Thanks for the quick reply Aaron. I will get the latest git version
> and run with it. BTW, I was running the sample on 0.6.7.
>
> Thanks again,
> Matt
>
> On Thu, May 26, 2011 at 08:28:51PM -0600, Aaron S. Meurer wrote:
>> If I try this with mpmath 0.16 (the version we currently have in
sympy in the latest git master), I get:
>>
>> In [22]: print 'subs: ', mpmath.findroot(lambda xi: p.subs(x,
xi), 1.0)
>> subs:
---------------------------------------------------------------------------
>> ValueError Traceback (most recent
call last)
>>
>> /Users/aaronmeurer/Documents/python/sympy/sympy/<ipython
console> in <module>()
>>
>>
/Users/aaronmeurer/Documents/python/sympy/sympy/sympy/mpmath/calculus/optimization.pyc
in findroot(ctx, f, x0, solver, tol, verbose, verify, **kwargs)
>> 968 '(%g > %g)\n'
>> 969 'Try another starting point or tweak arguments.'
>> --> 970 % (norm(f(*xl))**2, tol))
>> 971 return x
>> 972 finally:
>>
>> ValueError: Could not find root within given tolerance.
(6.67917e-33 > 1.71057e-49)
>> Try another starting point or tweak arguments.
>>
>> Another option for Poly is eval. I get this:
>>
>> In [37]: print 'eval: ', mpmath.findroot(lambda xi: p.eval(x,
xi), 1.0)
>> eval: 0.774596669241483377035853079956479922166584341
>>
>> In [38]: print 'exact: ',
mpmath.sqrt(mpmath.mpf('3.0')/mpmath.mpf('5.0'))
>> exact: 0.774596669241483377035853079956479922166584341
>>
>> In [39]: print 'lambda:', mpmath.findroot(lambda x: 5.0*x**3/2.0
- 3.0*x/2.0, 1.0)
>> lambda: 0.774596669241483377035853079956479922166584341
>>
>> So I'd say there is some kind of bug with subs.
>>
>> Aaron Meurer
>>
>> On May 26, 2011, at 8:21 PM, Matthew Emmett wrote:
>>
>>> Hi everyone,
>>>
>>> I am having trouble combining mpmath's findroot function with
sympy.
>>> Here is a short example of what I am trying to accomplish:
>>>
>>> import sympy
>>> import mpmath
>>>
>>> mpmath.mp.dps = 45
>>>
>>> x = sympy.var('x')
>>> p = (5.0/2.0*x**3 - 3.0/2.0*x).as_poly()
>>>
>>> print 'subs: ', mpmath.findroot(lambda xi: p.subs(x, xi), 1.0)
>>> print 'lambda:', mpmath.findroot(lambda x: 5.0*x**3/2.0 -
3.0*x/2.0,
>>> 1.0)
>>> print 'exact: ', mpmath.sqrt(mpmath.mpf('3.0')/mpmath.mpf('5.0'))
>>>
>>>
>>> The output is:
>>>
>>> subs: 0.77459666924148342341315597798728447146706553
>>> lambda: 0.774596669241483377035853079956479922166584341
>>> exact: 0.774596669241483377035853079956479922166584341
>>>
>>>
>>> That is, when using mpmath.findroot, substituting into a SymPy
>>> polynomial does not produce the same result as using a lambda form
>>> with a handwritten version of the polynomial. Is there a way
to the
>>> SymPy subs method to work as expected?
>>>
>>> Thanks,
>>> Matt
>>>
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