Plotting exponential function python

Question

I get a linear graph when trying to plot exponential function:

import math
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)

graph('100*(np.power(0.8, x))', (0,100))

what am I doing wrong? output_image

Answer 1

You really should NOT BE USING EVAL. However, leaving that issue aside, the problem is you are passing a tuple of two values as the argument for the x_range parameter. This is creating a x with the value: array([ 0, 100]) , and if you create the corresponding y's, you'll only have two points so of course you'll get a line. You want to use np.arange instead of np.array . However, if you pass a tuple to your graph function you are going to need to unpack the tuple when you pass it to np.arange. So this should work:

def graph(formula, x_range):
   x = np.arange(*x_range)
   y = eval(formula)
   plt.plot(x, y)

Seriously, though, instead of eval why not just pass a function?

def graph(func, x_range):
   x = np.arange(*x_range)
   y = func(x)
   plt.plot(x, y)

graph(lambda x: 100*(np.power(0.8, x)), (0,100))

Results: