Question

I’m trying to make a function that will graph whatever formula I tell it to.

import numpy as np  
import matplotlib.pyplot as plt  
def graph(formula, x_range):  
    x = np.array(x_range)  
    y = formula  
    plt.plot(x, y)  
    plt.show()  

When I try to call it the following error happens, I believe it’s trying to do the multiplication before it gets to y = formula.

graph(x**3+2*x-4, range(-10, 11))

Traceback (most recent call last):  
  File "<pyshell#23>", line 1, in <module>  
    graph(x**3+2*x-4, range(-10, 11))  
NameError: name 'x' is not defined  

Answer 1

This is because in line

graph(x**3+2*x-4, range(-10, 11))

x is not defined.

The easiest way is to pass the function you want to plot as a string and use eval to evaluate it as an expression.

So your code with minimal modifications will be

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()

and you can call it as

graph('x**3+2*x-4', range(-10, 11))