"""
Exercise 6.9 from "A primer on...
Change the input argument of a function from a list to a dictionary
"""

def triangle_area(vertices):
    #(x1, y1), (x2, y2), (x3, y3) = vertices
    (x1, y1) = vertices[1]
    (x2, y2) = vertices[2]
    (x3, y3) = vertices[3]
    
    return 0.5 * abs(x2 * y3 - x3 * y2 - x1 * y3 + x3 * y1 + x1 * y2 - x2 * y1)


def test_triangle_area():
    """
    Verify the area of a triangle with vertex coordinates
    (0,0), (1,0), and (0,2).
    """
    v1 = (0,0); v2 = (1,0); v3 = (0,2)
    #vertices = [v1, v2, v3]
    vertices = {1:v1, 2:v2, 3:v3} 
    expected = 1
    computed = triangle_area(vertices)
    tol = 1E-14
    success = abs(expected - computed) < tol
    msg = f"computed area= {computed} != {expected} (expected)"
    assert success, msg


test_triangle_area()