从 numpy 中的向量动态创建矩阵

Question

I'm trying to create a matrix of shape Nx3 where N is not known at first. This is what I'm basically trying to do:

    F = np.array([[],[],[]])
    for contact in contacts:
        xp,yp,theta = contact
        # Create vectors for points and normal
        P = [xp, yp, 0]
        N = [np.cos(theta), np.sin(theta), 0]
        # Calculate vector product
        cross_PN = np.cross(P,N)
        # f = [mz, fx, fi]
        mz = cross_PN[2]
        fx = N[0]
        fy = N[1]
        f = np.array([mz, fx, fy])
        F = np.vstack([F, f])

But this code doesn't work. I can do similar thing in Matlab very easily, but that is not the case in Python using numpy.

Any help is greatly appreciated. Thank you

I would like to create a matrix by adding new rows, but in the beginning the matrix is empty. That is why I receive the error: "along dimension 1, the array at index 0 has size 0 and the array at index 1 has size 3"

Answer 1

Based on your comments and suggestion, I guess the correct and more efficient way to do this is the following:

L = len(contacts)
F = np.zeros((L, 3))
for ii in range(L):
    xp,yp,theta = contacts[ii]
    # Create vectors for points and normal
    P = [xp, yp, 0]
    N = [np.cos(theta), np.sin(theta), 0]
    # Calculate vector product
    cross_PN = np.cross(P,N)
    # f = [mz, fx, fi]
    mz = cross_PN[2]
    fx = N[0]
    fy = N[1]
    fi = np.array([mz, fx, fy])
    F[ii] = fi