def num_of_orthogonal(vectors):
    """ this function returns the number of orthogonal vectors pairs from the
    list"""
    orthogonal_pairs = 0

    # We'll iterate the vectors in the following form, in order to avoid
    # duplications:
    # if the vectors are A, B, C, D we need to check only the next pairs:
    # AB, AC, AD
    # BC, BD
    # CD
    # So we'll use nested loops but each time the main loop go, the nested loop
    # is quicker.
    # Finally, we'll use inner_product function to check the product.

    for i in range(len(vectors) - 1):
        for t in range(i + 1, len(vectors)):
            if inner_product(vectors[i], vectors[t]) is 0:
                orthogonal_pairs += 1
    return orthogonal_pairs