Python, part 8

Tk graphics, continued.

Polygons. A polygon is a closed shape--which a line or a curve is not. You
can change the color inside a polygon, whereas a simple line has no inherent
inside and outside. Polygons are specified by 3 or more points, meaning 3 or
more pairs of (x,y) coordinates. We can have degenerate polygons where all
the points are on a straight line--we no longer have a real inside, and all we
have is a line and an outside.

myCanvas.create_polygon(10, 15, 75, 100, 50, 50) tells Tk that we have
3 points: (10,15), (75,100), and (50,50). If you want something with 12 points
you'd need 12 pairs of coordinates. fill, outline, width are available as before.

TEXT in graphics. myCanvas.create_text(100, 100, "Go Vols!") would put
the specified text horizontally, centered on the point (100,100)--NOT beginning
on that point. Options for text include font which lets you pick the font style
and size (e.g. font=("courier", 8)) and also anchor which lets you specify the
point of the bounding box--n,nw,e, se, s, sw,w, nw--north, northwest, etc. nw
is the upper left-hand corner of the box.

RANDOM NUMBERS in Python. These can be fun--we'll see shortly how we
might combine these with Tk graphics.

       import random                          this brings in the library
       number = random.randrange(1, 100)     generates a random number between
                                                                            1 and 99 (one less than the max 100
                                                                            we specified)

       DiceRoll = random.randrange(1, 7) + random.randrange(1, 7)
               this gives you a two-dice total from 2 to 12

RANDOM LINES in Tk
Let's use Tk to draw some connected lines at random. We'll start at (0,0)--
the upper left-hand corner, and draw a line to the random coordinates (x1,y1)
where x1 and y1 are between 1 and 400. We'll then draw a line from (x1,y1) to
(x2,y2), also picked at random. We'll do 4 lines this way, and we'll have arrows
at the far end of the lines.   To do exactly 4 lines, we can use a for loop. In the
middle of the process we'll be wanting to draw a line from (oldx,oldy) to
(newx,newy):

         newx = random.randrange(1, 401)
         newy = random.randrange(1, 401)
         myCanvas .create_line(oldx, oldy, newx, newy, arrow="second")

This draws the new line. But how to get the next line?

        oldx = newx
        oldy = newy
and then we repeat the 3 lines above. Not too hard! So:

        oldx = 0
        oldy = 0             this starts the process off the way we want
        for i in range(4):
               newx = random.randrange(1, 401)
               newy = random.randrange(1, 401)
               myCanvas.create_line(oldx, oldy, newx, newy, arrow="last")
               oldx = newx
               oldy = newy
                                                                               forlines

Remember that this goes into the Tk code--and remember to also have
import random--put it before the Tk import stuff. Try the code. Run
it again! What has happened? The lines go to different points--the x-y
coordinates are picked at random--so this should not be a surprise.
We can try a variation with a while loop--recalling that you need some
way to break out of the loop! We might keep drawing new lines until
x > 350, for example. Don't go overboard here with x > 395--you might
get an awful lot of lines!

        oldx = 0
        oldy = 0
        while(oldx <= 350):
               newx = random.randrange(1,401)
               newy = random.randrange(1,401)
               myCanvas.create_line(oldx,oldy,newx,newy,arrow="last")
               oldx = newx
               oldy = newy
                                                                    randwhilelines

Notice that the last/final arrow has the arrowhead near the right of our
canvas--which is what we expect, if you think about it.

We can also use the random-number generator to pick colors at random
for us:
      WhichColor = random.randrange(1,6)
       if WhichColor == 1:
                myColor = "purple"
      elif WhichColor == 2:
                myColor = "green"
     elif WhichColor == 3:
               myColor = "red"
     elif WhichColor == 4:
              myColor = "blue"
     else:
              myColor = "yellow"

And then when you draw a rectangle (or whatever) you can have as
one of the parameters    fill=myColor