import numpy as np 

def euler(f, t0, tend, y0, h):
    # Solve the ODE y' = f(t, y) with Eulers method.
    #
    # Input: f : Function handle to the rhs. of the ODE. 
    #      t0, tend: The integration interval.
    #      y0 : Initial value.
    #      h :  Stepsize.
    # Output: tn : 
    #      yn : The solution at the points tn.
    
    t = t0       # Current solution
    y = y0

    tn = [t0]   # Store the solution
    yn = [y0]

    while t < tend:
        if t + h > tend:
            h = tend - t
            
        # The Euler step
        y = y + h * f(t,y)
        t = t + h

        # Store the solution
        tn.append(t)
        yn.append(y)

    return np.array([tn,yn])