# acererak/convex-hull

An algorithm for cycling through the most exterior coordinates in a two-dimensional plane.
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# convex-hull

An algorithm for cycling through the most exterior coordinates in a two-dimensional plane.

The challenge: when given a randomized series of points contained within an array, create a function that contains all coordinates by painting lines to connect only the most exterior points.

For this exercise, I use the "SwamPy" Turtles Graphics package, written by Allen Downey and found here: http://www.greenteapress.com/thinkpython/swampy/.

My solution: I begin by creating a function for the Turtle class to compare two points within a Cartesian point and return a numeric value that communicates which point is greater (or if they are equal).

``````def assess_turn(self,p1,p2):
""" Returns whether p1 is to the left (+1) or to the right (-1)
of p2, when looking at them from the turtle's current
location.  If they are in line, returns 0 instead.  """

return self.location.turn(p1,p2)
``````

I then build my primary convex hull algorithm in the form of a function, corral:

``````def corral(self,points):
self.save()

for p in points:
self.move_to(p)
self.mark()

# FIND THE FIRST POINT ON THE HULL
p0 = min(points)
self.move_to(p0)

self.down()

# create two variables to compare points, with
# p being the turtle's starting location
p = p0
q = 0

# begin cycling through the hull
while q != p0:

# look at each point other than the turtle's current position...
for x in points if x != p:

# ...and compare turns from current position to find the next
# point along the hull.
if self.assess_turn(p,x) == -1:

# assign q the point that is leftmost
q = x

# draw to that point
self.move_to(q)

# update the turtle's position p by changing
# it to be q
p = q

self.up()
self.restore()
``````

To test, open seq.py, corral.py, Stack.py, TurtleCanvas.py, and Turtle.py, and run corral.py in your command line.

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