在对 numpy 数组使用按位运算符和比较运算符时，内部会发生什么并引发 ValueError？

Question

import numpy as np

arr = np.array([3, 4, 6, 15, 25, 38])


print(arr > 5 & arr <= 20)

"""Output
Traceback (most recent call last):
  File "main.py", line 11, in <module>
    print(arr > 5 & arr <= 20)
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
"""

我知道发生此错误是因为我错过了条件周围的括号。但我想知道表达式的计算顺序以及导致此错误的原因。

Answer 1

&有一个higher precedence than <=, so this is being run as arr > (5 & arr) <= 20 and since comparison operators are chained，这相当于：

(arr > (5 & arr)) and (arr <= (5 & arr))

这会导致众所周知的错误，因为您不能在布尔上下文中使用数组，正如错误消息所解释的那样...（旁注，5 & arr 仅计算一次...）

所以考虑一下，即使这样也会失败：

>>> arr = np.array([1,2,3,4,5])
>>> 0 < arr < 4
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

在对 numpy 数组使用按位运算符和比较运算符时，内部会发生什么并引发 ValueError？

What happens internally and raises ValueError while using bitwise and comparison operators with numpy arrays?

python

numpy

expression

bitwise-operators

comparison-operators