import numpy as np
from matplotlib import pyplot as plt

def f(t,y):
    return y

def yexact(t):
    return np.exp(t)

def euler(t0,T,y0,h):
    N = int((T-t0)/h)
    y = np.zeros(N)
    t = np.zeros(N)
    y[0] = y0
    t[0] = t0
    for n in range(1,N):
        y[n] = y[n-1] + h*f(t[n-1],y[n-1])
        t[n] = t[n-1] + h
    return t, y

t0 = 0.0
y0 = 1.0
T  = 5.0

te = np.linspace(t0,T,100)
ye = yexact(te)
plt.plot(te,ye,'--')

h  = 0.2
t,y = euler(t0,T,y0,h)
plt.plot(t,y)

h  = 0.1
t,y = euler(t0,T,y0,h)
plt.plot(t,y)

h = 0.05
t,y = euler(t0,T,y0,h)
plt.plot(t,y)

plt.legend(('Exact','0.2','0.1','0.05'),loc=2)
plt.xlabel('t')
plt.ylabel('y')