Problems with numerical evaluation in sympy after init_session()

[Marek Kaluba]

Hi all,

recently I started using sympy (and immediately fall for it), so this mail may be simply my lack of understanding how sympy internally works.

However I found a _strange_ (to me) issue with numerical evaluation:

(ipython console)

from sympy import *

N(1/3)

0 //integer division, ok

N(Rational(1)/Rational(3),20)

0.33333333333333333333 //numerical approximation of 1/3, ok

init_session() //for nice latex printing and default variables -- I guess this is what init_session() does??

[...] //standard output

pprint(N(1/3))

0.333333333333333 // why not 0?

pprint(N(1/3,100))

0.3333333333333333148296162562473909929394721984863281250000000000000000000000

000000000000000000000000

// what is happening here??

I'd be grateful if anyone can explain this to me...

Cheers,

Mark

[Mateusz Paprocki]

Hi,

On 20 October 2014 19:18, Marek Kaluba <abu...@gmail.com> wrote:
> Hi all,
>
> recently I started using sympy (and immediately fall for it), so this mail
> may be simply my lack of understanding how sympy internally works.
> However I found a _strange_ (to me) issue with numerical evaluation:
>
> (ipython console)
> from sympy import *
> N(1/3)
> 0 //integer division, ok
> N(Rational(1)/Rational(3),20)
> 0.33333333333333333333 //numerical approximation of 1/3, ok
>
> init_session() //for nice latex printing and default variables -- I guess
> this is what init_session() does??
> [...] //standard output
>
> pprint(N(1/3))
> 0.333333333333333 // why not 0?

This is because init_session() issues `from __future__ import
division`, so now 1/3 is equivalent to float(1)/3.

> pprint(N(1/3,100))
> 0.3333333333333333148296162562473909929394721984863281250000000000000000000000
> 000000000000000000000000
> // what is happening here??

As 1/3 is a float in this case, thus SymPy can't compute it's
numerical value to the expected precision. You can use
init_session(auto_int_to_Integer=True) get the desired behavior, where
1/3 is understood as Integer(1)/3. In general, handling rationals in
SymPy can be tricky, so if unsure, use type(1/3) to get feedback what
you are dealing with.

Mateusz

> I'd be grateful if anyone can explain this to me...
>
> Cheers,
> Mark
>

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[Marek Kaluba]

Well I tried auto_int_to_Integer=True, with no effect:

from sympy import init_session

init_session(auto_int_to_Integer=True)

IPython console for SymPy 0.7.4.1 (Python 2.7.7-64-bit) (ground types: gmpy)

These commands were executed:

>>> from __future__ import division

>>> from sympy import *

>>> x, y, z, t = symbols('x y z t')

>>> k, m, n = symbols('k m n', integer=True)

>>> f, g, h = symbols('f g h', cls=Function)

Documentation can be found at http://www.sympy.org

WARNING: Hook shutdown_hook is deprecated. Use the atexit module instead.

type(1/3)

Out[3]: float

pprint(1/3)

0.333333333333

[Aaron Meurer]

On Mon, Oct 20, 2014 at 1:11 PM, Marek Kaluba <abu...@gmail.com> wrote:
> Well I tried auto_int_to_Integer=True, with no effect:

This is a known issue https://github.com/sympy/sympy/issues/2839.

I recommend just using S(1)/3. This will work even if you write a
Python script with your code (int_to_Integer is only for interactive
use). If you do want the interactive use, you'll need to use isympy -i
for now.

And by the way, if you want integer division, I recommend always using
//, like 1//3. This will work even in Python 3 or Python 2 with from
__future__ import division.

Aaron Meurer

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