"""
Exercise A.7 from "A primer on..."
The exercise illustrates a common problem when solving
difference equations, and also similar tasks using for-loops.
The equation can be formulated in different ways, as described
in the exercise, and we need to take care that the lower and
upper bounds of the for-loop match with the formulation inside it.
This code is the working code, after modifying the for-loop bounds. 
"""

import matplotlib.pyplot as plt
import numpy as np

x0 = 100 # initial amount
p = 5 # interest rate
N = 4 # number of years
years = list(range(N+1))

x = np.zeros(len(years))

# Compute solution
x[0] = x0
for i in years[:-1]:    #modified from '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()