COMP 1000: Think Like a Computer: The PreTeXt Interactive Edition

So far, our programs have either been a series of statements which always execute sequentially, operations that are applied to each item in an iterable, or calls to functions. Yet programs frequently need to be more subtle with their behavior. For example, a messaging app might only set a message’s title bold if it has not been read by the user. Or a video game needs to update the position of all the characters, except the ones that are asleep. These situations represent choices (bold or not bold?, update or not update?) and in programming, this is done with something called a selection or a conditional statement.

In the context of turtle drawings, using this kind of statement will allow us to check conditions and change the behavior of the program accordingly. Run the program below a few times and examine the code.

The code uses turtle stamping to draw a beach scene. We create a canvas and turtle, and pick the pen up so that there are no traces as the turtle moves around. We are using the stamp() function which just imprints the turtle image on the canvas. We use a for loop to stamp turtles in random locations 20 times. The random.rangrange() function is used to get a random x location and a random y location for the turtle each time through the loop, and we move the turtle to that location. We use randrange again to get a random direction (between 0 and 360), and rotate the turtle to face that direction using the setheading() function.

The conditional part comes in when we decide what color the turtle should be before we do the stamping. To emulate turtles heading up a beach to nest, we are colouring pregnant turtles carrying eggs dark green, and turtles that have already laid eggs a lighter green. How do we know which ones to colour light vs. dark? We’re going to assume the turtles heading north are going to lay eggs, so if the angle they are facing is between 0 and 180 degrees they are heading more north than south. If the angle they are facing is greater than 180, then they are heading more south, and we will color them light green. The code on line 24 shows the conditional statement if direction < 180:. This statement looks at the current value of the direction variable, and if it is less than 180, the expression (direction < 180) evaluates to true. In that case the code in the indented block below executes (so amy’s colour would be set to dark green). If the current angle is greater than or equal to 180, the expression evaluates to false and the code in the indented block below the else is executed (so amy’s colour would be set to light green). What’s critical to note here is that amy’s colour is either going to be set to light green or dark green. Each time through this loop, the code on line 25 will execute, or the code on line 27 will execute, it will never be the case that both lines execute during the same loop iteration.

This chapter will further detail how to use conditional statements.