How can I have two for loops in the same list but the second one to start from the next value of the first loop?

Question

I am trying to write a code where i have a list of vectors and Ι have to find the angle between every vector and the rest of them.(I am working on mediapipe's hand landmarks). My code so far is this one:

vectors = [thumb_cmc_vec, thumb_mcp_vec, thumb_ip_vec, thumb_tip_vec, index_mcp_vec, index_pip_vec,
                           index_dip_vec, index_tip_vec, middle_mcp_vec, middle_pip_vec, middle_dip_vec, middle_tip_vec,
                           ring_mcp_vec, ring_pip_vec, ring_dip_vec, ring_tip_vec, pinky_mcp_vec, pinky_pip_vec,
                           pinky_dip_vec, pinky_tip_vec]

                for vector in vectors:
                    next_vector = vector + 1
                    print(vector)
                    for next_vector in vectors:
                        print(next_vector)
                        M = (np.linalg.norm(vector) * np.linalg.norm(next_vector))
                        ES = np.dot(vector, next_vector)
                        th = math.acos(ES / M)
                        list.append(th)
                        print(list)

where M = the multiplication of the norms of the current sets of vectors, ES = the scalar product of the vectors and th = the angle of the vectors. My problem is that the variable next_vector always starts the for loop from the first vector of the list even though I want it to start from the previous loop's next vector in order not to have duplicate results. Also when both of the loops are on the 3rd vector (thumb_ip_vec) I am getting this error th = math.acos(ES / M) ValueError: math domain error . Is there any way to solve this? Thank you!

Answer 1

I think you can iterate through the list indices (using range(len(vectors) - 1)) and access the elements through their indices instead of looping through each element

for i in range(len(vectors) - 1):
    # Iterate from 0 to len(vectors) -1
    vector = vectors[i]
    for j in range(i + 1, len(vectors)):
        # Iterate from index i + 1 to len(vectors)
        next_vector = vectors[j]
        M = (np.linalg.norm(vector) * np.linalg.norm(next_vector))
        ES = np.dot(vector, next_vector)
        th = math.acos(ES / M)
        list.append(th)
        print(list)