how to get element of solution of solveset

[chaowen guo]

Hi:

In sympy1.0, it is recommended to use solveset to solve equation, but how to get the element of the solution?

For example:

import sympy
x=sympy.symbols('x',real=True)
e,m=sympy.symbols('e,m',positive=True)
sympy.solveset(sympy.sqrt(2*e*m-(x*m)**2),x)

I want to get the first and second solution individually to do further calculation, but I can not directly use [0] and [1], so how to get individual element?

[Denis Akhiyarov]

Solveset is not indexable, so try converting to list. What surprised me is that it is not like regular python set:

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In [1]: import sympy


In [2]: sympy.__version__
Out[2]: '1.0'


In [3]: x=sympy.symbols('x',real=True)
   ...: e,m=sympy.symbols('e,m',positive=True)
   ...: sympy.solveset(sympy.sqrt(2*e*m-(x*m)**2),x)
Out[3]: {-sqrt(2)*sqrt(e)/sqrt(m), sqrt(2)*sqrt(e)/sqrt(m)}


In [4]: _
Out[4]: {-sqrt(2)*sqrt(e)/sqrt(m), sqrt(2)*sqrt(e)/sqrt(m)}


In [5]: sols=_


In [6]: type(sols)
Out[6]: sympy.sets.sets.FiniteSet


In [7]: sols.pop()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-7-188fe3e26bd2> in <module>()
----> 1 sols.pop()


AttributeError: 'FiniteSet' object has no attribute 'pop'


In [8]: list(sols)
Out[8]: [sqrt(2)*sqrt(e)/sqrt(m), -sqrt(2)*sqrt(e)/sqrt(m)]

[Aaron Meurer]

SymPy set objects are immutable, like every other SymPy object, so pop
could not work. However, iterating over the set does work. For finite
sets, the easiest way is to convert to a list. For countable infinite
sets, converting to a list won't work obviously, but you can use
iter(s) and call next() on it. Iteration is generally implemented so
that it hits every element (for instance, iterating over S.Integers
gives 0, 1, -1, 2, -2, ...). For uncountable sets like intervals, all
you can't iterate, but you can check containment using "in", or
intersect with other sets.

Aaron Meurer

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[Aaron Meurer]

Also be aware that FiniteSets are unordered so the order of the result
from list() may differ between sessions.

Aaron Meurer

[Chris Smith]

To make it canonical, you could use `list(ordered(finite_set))`.