TypeError: cannot determine truth value of Relational when using sympy piecewise

Question

在我的研究中，我找不到任何与分段函数中的关系值相关的示例。尽管我使用 uni_dis 方法收到了 lower 和 upper 值，但我卡在将这些值传递给分段函数的过程中。什么原因？

from sympy import Symbol, Piecewise
import sympy as sym
import sympy.plotting as syp
import math

a = Symbol('a')
b = Symbol('b')
x = Symbol('x')
function = 1 / abs(a-b)

def uni_dis(lower, upper):
    if lower > upper:
        lower, upper = upper, lower
    uniform = Piecewise((0, x < lower), (0, x > upper), (function.subs({a:lower, b:upper}), x >= lower and x <= upper))
    syp.plot(uniform.subs((x,-10,10), title="uni_dis"))

uni_dis(231, 675)

我的报错信息如下：

TypeError                                 Traceback (most recent call last)
<ipython-input-3-c28135b22fc4> in <module>
----> 1 uni_dis(231, 675)

<ipython-input-2-e4a205990c2a> in uni_dis(lower, upper)
      2     if lower > upper:
      3         lower, upper = upper, lower
----> 4     uniform = Piecewise((0, x < lower), (0, x > upper), (function.subs({a:lower, b:upper}), x >= lower and x <= upper))
      5     syp.plot(uniform.subs((x,-10,10), title="uni_dis"))

C:\ProgramData\Anaconda3\lib\site-packages\sympy\core\relational.py in __nonzero__(self)
    374 
    375     def __nonzero__(self):
--> 376         raise TypeError("cannot determine truth value of Relational")
    377 
    378     __bool__ = __nonzero__

TypeError: cannot determine truth value of Relational

Answer 1

您应该将 x >= lower and x <= upper 修改为 (x >= lower) & (x <= upper)。问题在于，使用 SymPy 对象的逻辑评估可以 return 另一个无法推导为 True 或 False 的符号逻辑。您可以看到 (x >= lower).__class__ return 如何成为另一个 sympy 不等式实例。

Answer 2

更正逻辑表达式后，出现“TypeError: 'Symbol' object is not subscriptable”错误。经过一些研究，我了解到绘图方法可以与分段方法一起使用 here。就是这样。

代码的更正版本：

def uniform_dist(lower, upper):
    if lower > upper:
        lower, upper = upper, lower
    syp.plot(Piecewise((0, x < lower), (0, x > upper), (f.subs({a:lower, b:upper}), (x >= lower) & (x <= upper))), (x,-10,10), title="uniform distribution")

TypeError: cannot determine truth value of Relational when using sympy piecewise

TypeError: cannot determine truth value of Relational when using sympy piecewise

python

piecewise

uniform-distribution