def get_centroid(aircraft):
    """Return the coordinates of the centroid."""
    stringer_area = aircraft.stringer.area
    cap_area = aircraft.spar.cap_area

    caps_x = [value for spar in aircraft.spar.x for value in spar]
    caps_z = [value for spar in aircraft.spar.z for value in spar]
    stringers_x = aircraft.stringer.x
    stringers_z = aircraft.stringer.z

    denominator = float(
        len(caps_x) * cap_area + len(stringers_x) * stringer_area)

    centroid_x = float(
        sum([x * cap_area
             for x in caps_x]) + sum([x * stringer_area for x in stringers_x]))
    centroid_x = centroid_x / denominator

    centroid_z = float(
        sum([z * cap_area
             for z in caps_z]) + sum([z * stringer_area for z in stringers_z]))
    centroid_z = centroid_z / denominator

    return (centroid_x, centroid_z)


def get_inertia_terms(self):
    """Obtain all inertia terms."""
    stringer_area = self.stringer.area
    cap_area = self.spar.cap_area

    # Adds upper and lower components' coordinates to list
    x_stringers = self.stringer.x
    z_stringers = self.stringer.z
    x_spars = self.spar.x[:][0] + self.spar.x[:][1]
    z_spars = self.spar.z[:][0] + self.spar.z[:][1]
    stringer_count = range(len(x_stringers))
    spar_count = range(len(self.spar.x))

    # I_x is the sum of the contributions of the spar caps and stringers
    # TODO: replace list indices with dictionary value
    I_x = sum(
        [cap_area * (z_spars[i] - self.centroid[1])**2 for i in spar_count])
    I_x += sum([
        stringer_area * (z_stringers[i] - self.centroid[1])**2
        for i in stringer_count
    ])

    I_z = sum(
        [cap_area * (x_spars[i] - self.centroid[0])**2 for i in spar_count])
    I_z += sum([
        stringer_area * (x_stringers[i] - self.centroid[0])**2
        for i in stringer_count
    ])

    I_xz = sum([
        cap_area * (x_spars[i] - self.centroid[0]) *
        (z_spars[i] - self.centroid[1]) for i in spar_count
    ])
    I_xz += sum([
        stringer_area * (x_stringers[i] - self.centroid[0]) *
        (z_stringers[i] - self.centroid[1]) for i in stringer_count
    ])
    return (I_x, I_z, I_xz)