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When I run my code I get the error

IndexError: index 0 is out of bounds for axis 0 with size 0 on line 42 for v[0]=v0.

I'm not familiar with python and don't know how to debug this.

""" Density at height y """ def rho(rh, y): rho = rh*math.exp(-y/7249) return rho """ Acceleration of skydiver """ def accl(m, g, rho, v, As, Cs, Ap, Cp): if y[i+1] >= 1200: accl = (1/(2*m) * rho(rh, y) * v**2 * As * Cs - g ) else: accl = (1/(2*m) * rho(rh, y) * v**2 * (As * Cs + Ap * Cp) - g) return accl h0 = 4000 # initial hieght dt = 0.1 #timestep n = int(180/dt) #lists for displacement, time, and velocities v= np.array([]) v[0] = v0 t= np.array([]) t[0] = t0 y= np.array([]) y[0] = h0 #calculation of time, velocity and displacement for i in range(n): t[i+1] = t[i] + dt v[i+1] = v[i] + accl(m, g, rho, v[i], As, Cs, Ap, Cp) * dt y[i+1] = y[i] + v[i+1] * dt if y[i+1] < 0: #ends plot once skydiver has landed break # Plot plt.plot(t, y, label = 'Position m') plt.plot(t, v, label = 'Speed m/s') plt.plot(t, accl, label = 'Acceleration m/s^2', color = 'green') plt.xlim([t[0], t[-1]]) plt.xlabel("Time (seconds)") plt.legend()
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3 Answers 3

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Numpy arrays created from empty lists can't be indexed - they don't have any elements to index into. 

np.array([]).shape # (0,)

If you know the dimensions of v in advance, you can use np.empty():

n = 10 np.empty(n, dtype=float) # array([ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.])

Alternately, just initialize v with v0:

v = np.array([v0])
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v= np.array([]) v[0] = v0

This makes no sense: you are trying to set a value to the first element of an empty array (which has no element!).

Try this instead: v = np.array([v0])

if you want to grow your array, do this:

v = [] # whatever logic to grow v like: for i in range(10): v.append(2*i-1) # now turn it into an np.array: v = np.array(v)
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This will produce an error when he is accessing further indices of v. He needs to initialize v with size n.
@wit221 No: list do not need a size to be predefined.
Yes, I was referring to your solution to use v = np.array([v0]). This will produce an error when accessing the numpy array in v[i+1] = v[i] + ...
1

Replace v = np.array([]) with v = np.zeros(n+1). Your cannot access elements of the numpy array that are outside of its bounds. Your declaration creates an empty numpy array.

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