dodecagon.py

# Draws a dodecadon with the colour of each sector alternating red and blue.
#
# Written by Eric Martin for COMP9021

from turtle import *

edge_length = 100
angle = 30

def draw_triangle(i, colour):
    color(colour)
    begin_fill()
    goto(vertices[i])
    goto(vertices[i + 1])
    # A shortcut for: go to the origin.
    home()
    end_fill()

vertices = []
penup()
# Determine the positions of the 12 vertices of the dodecagon
# by going to each of them (moving a distance of 100 from the origin,
# in a direction of 0, -30, -60, -90,... degrees), recording the position,
# and appending that position to the list vertices.
# So vertices is eventually the list
# [(x_0, y_0), (x_1, y_1), (x_2, y_2),..., (x_{11}, x_{11})] where
# (x_i, y_i) are the x and y coordinates of the (i+1)st vertice.
for i in range(12):
    right(i * angle)
    forward(edge_length)
    vertices.append(pos())
    home()
# Add to the end of the list vertices the coordinates of the first vertice
# in order to "close" the picture. So vertices becomes
# [(x_0, y_0), (x_1, y_1), (x_2, y_2),..., (x_{11}, x_{11}), (x_0, y_0)].
vertices.append(vertices[0])
    
pendown()
for i in range(12):
    # If i is odd because the division of i by 2 yields a remainder of 1
    # then make colour 'red'; otherwise make it 'blue'.
    if i % 2:
        colour = 'red'
    else:
        colour = 'blue'
    draw_triangle(i, colour)