带有运算符 < 的数组的 Pyplot或 > [英] Pyplot of array with an operator < or >

问题描述

我正在尝试使用 pyplot 在 Python 中绘制一个函数，问题可以归结为:

I am trying to plot a function in Python with pyplot, where the problem could be boiled down to this:

import numpy as np
import matplotlib.pyplot as plt

def func(x):
    if x<0:
        return x*x

interval = np.arange(-4.0,4.0,0.1)
plt.plot(interval, func(interval))
plt.show()

抛出以下错误:

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

我怎样才能避免这种情况?

How can I avoid this?

推荐答案

如果要更改零以下的值，可以使用

If you wanted to change the values that are below zero you could use np.where:

import numpy as np

def func(x):
    return np.where(x < 0, x*x, x)

print(func(np.arange(-4, 5)))  # array([16,  9,  4,  1,  0,  1,  2,  3,  4])

如果您只希望值小于零，则可以使用

If you just wanted the values below zero you could use indexing with a boolean array:

def func(x):
    return x[x<0] ** 2 

print(func(np.arange(-4, 5)))  # array([16,  9,  4,  1])

<小时>

更笼统: numpy.array 上的比较运算符只返回一个布尔数组:

---

More general: comparison operators on numpy.arrays just return a boolean array:

>>> arr > 2
array([False, False,  True], dtype=bool)

>>> arr == 2
array([False,  True, False], dtype=bool)

还有例外

ValueError:具有多个元素的数组的真值不明确.使用a.any()或a.all()

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

在你执行 bool(somearray) 时发生.在很多情况下， bool()调用是隐式的(因此发现它可能不是立即明显的).这种隐式 bool() 调用的例子是 if、while、and 和 or>:

happens when you do bool(somearray). In a lot of cases the bool() call is implicit (so it may not be immediatly obvious to spot it). Examples of this implicit bool() call are if, while, and and or:

>>> import numpy as np
>>> arr = np.array([1, 2, 3])
>>> bool(arr)
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

>>> if arr: pass
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

>>> while arr: pass
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

>>> arr and arr
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

>>> arr or arr
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

在您的情况下，如果x<0 是发生异常的原因，因为 x<0 返回一个布尔数组，然后 if 尝试获取该数组的 bool().如上例所示，该异常将引发您的异常.

In your case the if x < 0 was the reason for the exception because x < 0 returns a boolean array and then if tried to get the bool() of that array. Which as shown in the example above throws the exception you got.

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