graphing an equation with matplotlib

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

Your guess is right: the code is trying to evaluate x**3+2*x-4 immediately. Unfortunately you can't really prevent it from doing so. The good news is that in Python, functions are first-class objects, by which I mean that you can treat them like any other variable. So to fix your function, we could do:

import numpy as np   import matplotlib.pyplot as plt    def graph(formula, x_range):       x = np.array(x_range)       y = formula(x)  # <- note now we're calling the function 'formula' with x     plt.plot(x, y)       plt.show()    def my_formula(x):     return x**3+2*x-4  graph(my_formula, range(-10, 11)) 

If you wanted to do it all in one line, you could use what's called a lambda function, which is just a short function without a name where you don't use def or return:

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

And instead of range, you can look at np.arange (which allows for non-integer increments), and np.linspace, which allows you to specify the start, stop, and the number of points to use.

Answer 2

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))