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I wanted to plot y=(x+2)(x−1)(x−2) for x going from −3 to 3 using a dashed red line. When I wrote the following code, nothing shows up.

import numpy as np
import matplotlib.pyplot as plt


def graph(formula, x_range):

    x = np.array(x_range)
    y = eval(formula)
    plt.plot(x, y)
    plt.show()

    graph('((x-3) * (x-2))', range(-3,3))
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1 Answer 1

up vote 1 down vote accepted

Make sure graph(..) call is outside the graph function definition (IOW, indent correctly):

import numpy as np
import matplotlib.pyplot as plt

def graph(formula, x_range):
    x = np.array(x_range)
    y = eval(formula)
    plt.plot(x, y, 'r--')   # `r--` for dashed red line
    plt.show()

graph('((x-3) * (x-2))', range(-3,3))  # <----

UPDATE

It's not a good idea to use eval. Instead you can pass a function in this case.

def graph(formula, x_range):
    x = np.array(x_range)
    y = formula(x)  # <-----
    plt.plot(x, y, 'r--')
    plt.show()

graph(lambda x: (x-3) * (x-2), range(-3,3))  # <---
share|improve this answer
    
@karel, Thank you for pointing that. I updated the answer accordingly. BTW, I was in the middle of editing the answere to not to use eval. –  falsetru 2 hours ago
    
@karel thanks for pointing that out, really helped me :) –  ssr 1 hour ago
    
that was really helpful. I am new to numpy and matplotlib and this solve so many of my basic questions. Thanks! –  ssr 1 hour ago
    
@ssr, Please post a separate question. –  falsetru 40 mins ago

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