What is a function?

Let’s make a function that draws a square. When we create or define a function, we need a few things: the def command (short for “define”), the name of the function, and a list of parameters or pieces of information the function needs in order to run correctly.

import turtle

def draw_square():
    for i in range(4):
        bob.forward(100)
        bob.right(90)

draw_square()

wn.exitonclick()

This function draws a square, but only with side length of 100. If we add some parameters to the function, we can make the function dynamic. These parameters are similar to user inputs, but are inputs created by the programmer.

Class Activity

Let’s begin by setting up our turtle objects and the draw_square() function. The background color looks like light green and let’s try hot pink for pencolor.

We’ll add one function call to make sure our code is working correctly to this point.

import turtle

def draw_square():
    for i in range(4):
        bob.forward(20)
        bob.left(90)

wn = turtle.Screen()
bob = turtle.Turtle()
wn.bgcolor("light green")
bob.color("hot pink")
bob.pensize(3)

# squares code goes here
draw_square()

wn.exitonclick()

Next, we need to figure out how to create five identical squares that are 20 units apart. This seems like a job for a for-loop.

import turtle

def draw_square():
    for i in range(4):
        bob.forward(20)
        bob.left(90)

wn = turtle.Screen()
bob = turtle.Turtle()
wn.bgcolor("light green")
bob.color("hot pink")
bob.pensize(3)

for i in range(5):
    draw_square()
    bob.penup()
    bob.forward(40)
    bob.pendown()

wn.exitonclick()

Dynamic Functions

We use parameters to make our functions more flexible. Even with turtle functions, we want them to be as dynamic as possible, so we can call the functions with more than one turtle (yes, this is possible).

While we are creating parameters for our function, we’ll also add a length parameter so we can easily change how long the sides of the square are in our function call.

import turtle

# define draw_square function
def draw_square (turtle, length):
  for i in range(4):
    turtle.forward(length)    # note that we are using the parameter `turtle` here, not `bob`
    turtle.right(90)
  
# create Screen and Turtle objects
wn = turtle.Screen()
bob = turtle.Turtle()
joe = turtle.Turtle()     # we make a second turtle simply by repeating the `turtle.Turtle()` command (joe shows up exactly where bob does)

# set initial parameters of turtles
bob.color("blue")
bob.penup()
bob.goto(50, 50)
bob.pendown()
joe.color("red")

draw_square(bob, 50)
draw_square(joe, 100)

Now our function is dynamic: it can draw a square of any sidelength and can draw with any turtle.

Let’s modify the function so it draws a square of a certain color (arguments: length, color)

import turtle

# define draw_square function
def draw_square (turtle, length, turtle_color):
  turtle.color(turtle_color)
  for i in range(4):
    turtle.forward(length)
    turtle.right(90)
  
# create Screen and Turtle objects
wn = turtle.Screen()
bob = turtle.Turtle()
joe = turtle.Turtle()

# set initial parameters of turtles
bob.color("blue")
bob.penup()
bob.goto(50, 50)
bob.pendown()
joe.color("red")

draw_square(bob, 50, "purple")
draw_square(joe, 100, "hot pink")

Class Activity


import turtle

def draw_square(turtle, length):
    for i in range(4):
        turtle.forward(length)
        turtle.left(90)

wn = turtle.Screen()
bob = turtle.Turtle()
wn.bgcolor("light green")
bob.color("hot pink")
bob.pensize(3)

for i in range(5):
    draw_square(bob, i * 20 + 20)      # remember that i actually takes on a different value for each loop iteration
    bob.penup()
    bob.backward(10)        # could we do this with `bob.goto()` and `i`?
    bob.right(90)
    bob.forward(10)
    bob.left(90)
    bob.pendown()

wn.exitonclick()

Active Learning

  • Write a function that returns the area of a circle when it is provided with the radius (use return area).
  • Write a function that returns the area of a rectangle (parameters: length and width)

HW:

  1. Read HTCS, 6.1-6.5
  2. Do exercise 9