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pythonsympyprobability

Sympy: Probability measure of a set

While trying to calculate a probability of a bounded set like this:

from sympy.stats import Normal, P
from sympy import  Interval
n = Normal("n", 0, 1)
P(n in Interval(-1, 1))

I'm getting the following error:

TypeError: did not evaluate to a bool: (-1 <= n) & (n <= 1) How to deal with that?

Solution

  • To use Interval() you have to call its contains() method:

    P(Interval(-1,1).contains(n))

    as the documentation states on using in with Interval() (here):

    As a shortcut it is possible to use the ‘in’ operator, but that will raise an error unless an affirmative true or false is not obtained. [..] The result of ‘in’ is a bool, not a SymPy value.

    Alternatively, you can directly formulate the conditions inside the probability function:

    P((n >-1) & (n<1))