i try to create linear function in an interval and the other interval is 0. and add it to an array
so i tried this code
import numpy as np
import sympy as sp
import matplotlib as plt # This is all the library's i need
import mpmath
x = sp.symbols('x')
n = 10
xx = (np.array([np.linspace(0, 1, n+1)]))
i = 0
N = []
N[0] = sp.Piecewise( ((xx[0, i + 1] - x) / (xx[0, i + 1] - xx[0, i]), xx[0, i] <= x),
((xx[0, i + 1] - x) / (xx[0, i + 1] - xx[0, i]), x <= xx[0, i + 1]),
(0, True)
)
and it always returns
line 511, in __bool__
raise TypeError("cannot determine truth value of Relational")
TypeError: cannot determine truth value of Relational
EDIT: after i add x is positive to symbol i try to make a vector of linear function in the intervals in a loop and i don't know why it refuses to determine the the truth value even tough in the line above it succeed:
import sympy as sp
import matplotlib as plt # This is all the library's i need
import mpmath
x = sp.symbols('x', positive=True)
n = 10
xx = (np.array([np.linspace(0, 1, n+1)]))
i = 0
N = []
a = sp.Piecewise(
((xx[0, i + 1] - x) / (xx[0, i + 1] - xx[0, i]), (xx[0, i]) <= x),
((xx[0, i + 1] - x) / (xx[0, i + 1] - xx[0, i]), (x <= (xx[0, i + 1]))),
(0, True)
)
N.append(a)
for i in range(1, n):
a = sp.Piecewise(
(0, x < xx[0, i - 1]),
((xx[0, i - 1] - x) / (xx[0, i - 1] - xx[0, i]), ((xx[0, i - 1]) <= x)),
((xx[0, i - 1] - x) / (xx[0, i - 1] - xx[0, i]), (x <= (xx[0, i]))),
((xx[0, i + 1] - x) / (xx[0, i + 1] - xx[0, i]), ((xx[0, i]) <= x)),
((xx[0, i + 1] - x) / (xx[0, i + 1] - xx[0, i]), (x <= (xx[0, i + 1]))),
(0, True)
)
N.append(a)
and i get the same Error:
File "", line 23, in File "pythonProjectFEANew\venv\lib\site-packages\sympy\core\relational.py", line 511, in bool raise TypeError("cannot determine truth value of Relational") TypeError: cannot determine truth value of Relational
This is a bug in numpy float64 somehow:
In [48]: xx[0,1]
Out[48]: 0.1
In [49]: type(xx[0,1])
Out[49]: numpy.float64
In [50]: xx[0,1] < x
---------------------------------------------------------------------------
TypeError
sympy/core/relational.py in __bool__(self)
509
510 def __bool__(self):
--> 511 raise TypeError("cannot determine truth value of Relational")
512
513 def _eval_as_set(self):
TypeError: cannot determine truth value of Relational
The error comes from sympy but it's because numpy.float64.__lt__ is for some reason calling bool on the inequality that is returned by x.__gt__. If you reverse the direction of the inequality or convert the numpy.float64 to an ordinary Python float then it will work fine:
In [51]: x > xx[0,1]
Out[51]: x > 0.1
In [52]: float(xx[0,1]) < x
Out[52]: x > 0.1
The bug is that numpy.float64.__lt__ should be returning NotImplemented somewhere but instead does something weird.