"""
Exercise 6.9 from "A primer on...". 
Change an existing function for compuuting
the area of a triangle, from taking a list to a 
dictionary as input parameter.  
"""

#original function, using a list:
def triangle_area(v):
    [x1,y1] = v[0]
    [x2,y2] = v[1] 
    [x3,y3] = v[2]  
    A = abs(0.5*( (x2*y3)-(x3*y2)-(x1*y3)+(x3*y1)+(x1*y2)-(x2*y1) ))
    return A

#usage:
v1 = (0,0);  v2 = (1,0);  v3 = (0,2)
vertices = [v1, v2, v3]
print(triangle_area(vertices))

#modified version, using a dict:
def triangle_area_dict(v):
    [x1,y1] = v[1]
    [x2,y2] = v[2] 
    [x3,y3] = v[3]  
    A = abs(0.5*( (x2*y3)-(x3*y2)-(x1*y3)+(x3*y1)+(x1*y2)-(x2*y1) ))
    return A

vertices = {1:v1, 2:v2, 3:v3} #same vertices, but with keys 1,2,3
print(triangle_area_dict(vertices))

"""
Terminal> python area_triangle_dict.py 
1.0
1.0
"""