CODEWITHSUNDEEP
sympy
gboffi
gboffi

Reputation: 25023

Conditional expression in Sympy

If I try to have an expression whose value depends on another expression

from sympy import *
x = symbols('x')
y1 = x/cos(x)
y2 = y2 if y1>0 else nan

but an exception is raised

File /usr/lib/python3.10/site-packages/sympy/core/relational.py:511, in Relational.__bool__(self)
    510 def __bool__(self):
--> 511     raise TypeError("cannot determine truth value of Relational")

TypeError: cannot determine truth value of Relational

Is there a chance of having the effect that I'd want?

In case my problem is a X-Y problem, what I ultimately want to do is to plot x/cos(x) only where the function is positive.

UPDATE

I used the useful suggestion of @Oscar Benjamin (that completely answers my original question) but I have other issues when plotting the Piecewiswe function, that I'll expose in another question, as well as a wrong plot when I use the plot_implicit solution they suggested.

Upvotes: 0

Views: 248

Answers (1)

Oscar Benjamin
Oscar Benjamin

Reputation: 14480

What you're looking for is Piecewise:

In [8]: p = Piecewise((x/cos(x), x/cos(x) > 0), (S.NaN, True))

In [9]: p
Out[9]: 
⎧  x           x       
⎪──────  for ────── > 0
⎨cos(x)      cos(x)    
⎪                      
⎩ nan      otherwise

A more direct solution to your problem though would be something like

plot_implicit(Eq(y, x/cos(x)) & (x/cos(x) > 0))

Upvotes: 1

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