"""
Exercise A.7 from "A primer on..." Fix a bug related
to the indices of the difference equation. 

The bug was in the line defining the for loop, which 
original read:
for i in years[1:]:

This gives an index error since it will run to the last 
element of years, and then we use the index
[i+1] inside the loop. The code below has been 
corrected, so it starts at zero and runs to the
second-to-last element of the years array. 
"""



import numpy as np
import matplotlib.pyplot as plt

x0 = 100    # initial amount
p = 5       # interest rate
N = 40      # number of years

years = range(N+1)
x = np.zeros(N+1)

# Compute solution
x[0] = x0
for i in years[:-1]:  
    x[i+1] = x[i] + (p/100.0) * x[i]

plt.plot(years, x, 'ro')
plt.xlabel('years') 
plt.ylabel('amount')
plt.show()