12.6. Turtles And Randomness

We can use random numbers to make the results of turtle drawings unpredictable. The program below will pick a random number between 50 and 199 and call it squareSize. That number gets passed to the drawSquare function as the size of the square to draw. Each time you run the program, the square should be a different size.

As a reminder, here are the turtle commands:

Turtle programming reference sheet

def drawSquare(turtleName, size): for side in range(4): turtleName.forward(size) turtleName.right(90) # main part of program from turtle import * import random becca = Turtle() squareSize = random.randrange(50, 200) drawSquare(becca, squareSize)

Note that although we want the size of the square to be random, it is important that all the sides of the square are the same size. That is why we pick the random number in the main part of the program. If we picked a random number on line 3 as we draw each side, each side would be a different length! Try changing line 3 to read turtleName.forward(random.randrange(50, 200)) to see for yourself.

We can also pick a random location for the turtle to start in. Remember that the turtle uses this coordinate system:

The coordinates for the drawing space. Note that the center is x = 0 and y = 0.

Because the square procedure is going to make a square up to 200 wide and tall extending to the right and down from where we start, I don’t want it to start to the right or below the point 0, 0 or it might go off screen. I am fine with it starting to the left of or above that location, but I don’t want it to start right at the edge of the screen. So I want the starting x location to be between -190 and 0 and the starting y to be between 0 and 190. So this version of the program uses goto(x, y) and random numbers to pick a random starting location before drawing each square. We use a loop to draw 5 squares.

def drawSquare(turtleName, size): for side in range(4): turtleName.forward(size) turtleName.right(90) # main part of program from turtle import * import random becca = Turtle() becca.speed(0) for square in range(5): becca.penup() #pick a random starting location startX = random.randrange(-190, 1) startY = random.randrange(0, 191) becca.goto(startX, startY) becca.pendown() squareSize = random.randrange(50, 200) drawSquare(becca, squareSize)

Try the program a few times. Each time you should get different squares.

Now let’s make it a little more interesting by picking a random color inside the loop before we call drawSquare. The colormode has been set to 255, so to pick a random color, you need to call becca.fillcolor(red, green, blue) where red/green/blue are each a value between 0-255. The drawSquare procedure has been changed to call begin_fill and end_fill to fill in the squares.

In the location marked at line 20, add lines of code to make three random values that are each 0-255 and name them red, green, and blue. Then call becca.fillcolor(red, green, blue) to set a random color.

def drawSquare(turtleName, size): turtleName.begin_fill() for side in range(4): turtleName.forward(size) turtleName.right(90) turtleName.end_fill() # main part of program from turtle import * import random becca = Turtle() becca.colormode(255) becca.speed(0) for square in range(5): becca.penup() #pick a random starting location startX = random.randrange(-190, 1) startY = random.randrange(0, 191) becca.goto(startX, startY) becca.pendown() #pick a random color here squareSize = random.randrange(50, 200) drawSquare(becca, squareSize) ===== from unittest.gui import TestCaseGui class myTests(TestCaseGui): def testOne(self): self.assertRegex(self.getEditorText(), r"random.randrange\(0, 25[56]\)", "Testing that you make a random number 0-255.") self.assertRegex(self.getEditorText(), r"becca.fillcolor\([^,]+,[^,]+,[^,]+(,[^,]+)?\)", "Testing that you call becca.fillcolor.") myTests().main()

Optional experiment: If you want to make the squares partially transparent, you can change your call to fillcolor to look like becca.fillcolor(red, green, blue, 0.5). The 0.5 as the fourth parameter says to make the fill be 50% opaque. (Opaque is the opposite of transparent. 20% opaque means 80% transparent and 60% opaque means 40% transparent. The closer to 1.0 that fourth value is, the more solid the color. The closer to 0 it is, the more transparent it will be.)

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