Python matplotlib ValueError: The truth value of an array with more than one element is ambiguous

Question

I'm trying to plot two functions func1 and func2 using matplotlib and python. I keep getting a ValueError for the following code and have no idea what is wrong. I've searched through related questions, tried a ton of things, and nothing seems to work.

import numpy as np
import matplotlib.pyplot as plt

fig, ax = plt.subplots(1, 1)
plt.xlim(0, 10)
plt.ylim(0, 10)
plt.xlabel('$X$')
plt.ylabel('$Outputs$')
plt.title('Title')

x = np.arange(0, 10, .1)

def func1(X):
    output = max(3*X/7 - 3/7, 0, 12*X/35 - 3/35)
    return output

def func2(X):
    output = max(3*X/7 - 3/7, 0)
    return output

plt.plot(x, func1(x), 'g')
plt.plot(x , func2(x), 'b')

plt.show()

DSM · Accepted Answer · 2017-02-07 23:46:55Z

3

max(2,3) is clearly 3, because 3 > 2.

But when we compare ndarray arguments, we don't get a single scalar result, but an array:

In [23]: np.array([3,1]) > np.array([1,2])
Out[23]: array([ True, False], dtype=bool)

and we can't convert an array of bools into a single value-- should it be True, because there's at least one, or False, because it's not all True? Or, as the error message puts it, "The truth value of an array with more than one element is ambiguous". This means that the builtin max function fails, because it tries to branch on whether a comparison is true or not.

Fortunately, as it looks like you want the pairwise maxima, numpy already has a function which handles that, np.maximum. Replacing the builtin max with np.maximum in your code gives me:

answered Feb 7, 2017 at 23:46

DSM

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1 Comment

user7531647 Over a year ago

This almost worked, but now something very strange is going on. I changed max to np.maximum yet my graph doesn't look like yours, instead it goes through the origin. Any thoughts on why this is? Something weird with my pycharm configurations?

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