Turtle Commands

Commands are how we tell Tracy the Turtle to do things.

Tracy knows several built-in commands.

Movement

# The forward command makes Tracy move forward a given distance
forward(10)
forward(50)
forward(200)

# The backward command makes Tracy move backward a given distance
backward(10)
backward(50)
backward(200)

# Negative values can also be used to move Tracy forward or backward
backward(-200)
forward(-200)

Drawing Circles

# The circle command makes Tracy draw a circle above her with specified radius
circle(10)
circle(100)

# Another parameter can be used to draw only certain portions of a circle
# This command will draw a semi-circle
circle(10, 180)

# This command will draw a quarter-circle
circle(10, 90)

# Another parameter can be used to control the number of points in the shape
# This command will draw a triangle
circle(10, 360, 3)

# This command will draw a square
circle(10, 360, 4)

Pen

# Lift the pen up
# Tracy will not leave a trail when she moves
penup()

# Put the pen down
# Tracy will leave a trail when she moves
pendown()

# Change the size of the pen

# Normal, line width of 1
pensize(1)

# Bigger, line width of 10
pensize(10)

Rotation

# Tracy will turn 90 degrees to the left
left(90)

# Tracy will turn left 180 degrees and
# face the opposite direction
left(180)

# Tracy will turn left a full 360 degrees
# and do a complete spin
left(360)

# The right command does the same thing,
# but Tracy turns right instead of left
right(90)
right(180)
right(360)

# Negative values can also be used to turn Tracy right or left
right(-90)
left(-90)

Position

Tracy is positioned in an (x, y) coordinate plane like so:

Each point in Tracy's world can be located by an (x, y) coordinate. For example:

We can use the setposition command to send Tracy to a specific location

# Send Tracy to the center of the world
setposition(0, 0)

# Send Tracy to the far right middle of the world
setposition(100, 0)

# Send Tracy to the top right corner of the world
setposition(200, 200)

# Set Tracy's x position to be 100
# The y position stays the same
setx(100)

# Set Tracy's y position to be 100
# The x position stays the same
sety(100)

Speed

We can set Tracy's speed using the speed command

Speed options go from 0 to 10

# Slowest
speed(1)

# Normal
speed(6)

# Fast
speed(10)

# Fastest (no animation at all)
speed(0)

Color

# Draws a red line
color("red")
forward(50)

# Draws a blue line
color("blue")
forward(50)

# Some color options:
color("black")
color("blue")
color("brown")
color("cyan")
color("gold")
color("gray")
color("green")
color("indigo")
color("orange")
color("pink")
color("purple")
color("red")
color("violet")
color("white")
color("yellow")

Filling

# To draw a filled shape, start with this call
begin_fill()

# Draw your shape
for i in range(3):
    forward(150)
    left(120)

# Stop filling once you have drawn the shape
end_fill()

Clear Screen

# To clear the screen of any markings, use clear command
clear()

# This can be used at the end of a code
# This will clear the screen after the square is drawn
for i in range(4):
    forward(50)
    left(90)
clear()

# This can be used throughout code
# This will clear the screen before drawing the square
circle(50)
clear()
for i in range(4):
    forward(50)
    left(90)

Loops

Loops help us repeat commands which makes our code much shorter.

Make sure everything inside the loop is indented one level!

For Loops

Use for-loops when you want to repeat something a fixed number of times.

# Instead of repeating code over and over
forward(10)
left(90)
forward(10)
left(90)
forward(10)
left(90)
forward(10)
left(90)

# Use a for loop to repeat the code!
for i in range(4):
    forward(10)
    left(90)

# You can use the i variable inside the loop
# i starts at 0, and goes up to COUNT-1

# This will make Tracy move forward 0, then 1, then 2
for i in range(3):
    forward(i)

# You can change the value of i inside a for loop by using mathematical expressions
for i in range(3):
    forward(i*10)

# You can also control the value of i by using extended parameters
for i in range(STARTING_VALUE, ENDING_VALUE, INCREMENT):

# This will move Tracy forward 25, 50, and 75 pixels
for i in range(25, 76, 25):
    forward(i)

While Loops

Use while loops when you want to repeat something an unknown number of time or until a condition becomes false.

If there is no point where the condition becomes false, you will create an infinite loop which should always be avoided!

# Keep drawing circles until the count variable is greater than 5
# If count variable is not updated, infinite loop will occur
count = 0
while count <= 5:
    circle(50)
    count = count + 1

# You can also use user input to control a while loop
# This code will continue running while the user answers ‘Yes’
continue = input(“Continue code?: ”)
while continue == “Yes”:
    forward(10)
    left(90)
    continue = input(“Continue code?: ”)

Functions

Writing a function is like teaching Tracy the Turtle a new word.

Naming Functions: You can name your functions whatever you want, but you can't have spaces in the function name. Instead of spaces, use underscores (_) like_this_for_example

Make sure that all the code inside your function is indented one level!

Defining a Function

We define a function to teach Tracy the instructions for the new word.

def name_of_your_function():
    # Code that will run when you make a call to
    # this function.

# EXAMPLES

# Teach Tracy to draw an edge
def draw_edge():
    forward(50)
    left(90)

# Teach Tracy to draw a blue edge
def draw_blue_edge():
    color("blue")
    forward(50)
    left(90)

# Teach Tracy to draw a square
def draw_square():
    for i in range(4):
        draw_edge()

Using Parameters in Functions

We use parameters to alter certain commands in our function.

# Use a parameter to control the radius of the circle being drawn
def draw_circle(radius):
    circle(radius)

# Multiple parameters can be used in one function
# Parameters control the length of square sides and color
def draw_square(length, color_choice):
    color(color_choice)
    for i in range(4):
        forward(length)
        left(90)

Calling a Function

We call a function to get Tracy to actually carry out the new command.

# Call the draw_edge() function once
# Tracy will draw one edge
draw_edge()

# Call the draw_edge() function 3 times
# Tracy will draw 3 edges
draw_edge()
draw_edge()
draw_edge()

# Call the draw_square() function with parameters
# Tracy will draw 3 squares with different sizes and colors
draw_square(50, “red”)
draw_square(100, “blue”)
draw_square(75, “orange”)

Variables

We use variables to store values that can be used to control commands in our code. The variable values can also be altered throughout the code.

# Make a variable to keep track of something
count = 0

# Update the value of the variable using mathematical expressions
# This will add one to the value of the count variable
count = count + 1

# Variable values can also be altered inside control structures
# The value of the variable count will increase by 1 on every loop iteration
for i in range(4):
    forward(10)
    count = count + 1

User Input

We can use input from the user to control our code.

# Ask the user for input and save it to a variable to be used in code
# This will only work if the input is being used as a string
circle_color = input(“What is the color of the circle? ”)

# If input needs to be used as a number, include the term ‘int’
radius = int(input(“What is the radius of the circle? ”))
# This input can then be used to control different parts of the code
color(circle_color)
circle(radius)

If/Else Statements

We can tell Tracy how to make decisions using if/else statements.

Make sure that all the code inside your if/else statement is indented one level!

If Statements

Use an if statement to tell Tracy to do something only when a condition is true. If the condition is false, Tracy will skip the commands indented underneath.

# Tracy will only draw a circle if the count variable is less than 5
if count < 5:
    circle(50)

If/Else Statements

Use an if/else statement to force Tracy to make a decision between multiple conditions. If the first condition is false, Tracy will skip to the next condition until she finds one that is true. If no conditions are true, the commands inside the ‘else’ block will be performed.

# Tracy will draw a circle if count variable is less than 5
if count < 5:
    circle(50)
# Tracy will draw a square if count variable is greater than 5
elif count > 5:
    circle(50,360,4)
# In any other case, Tracy will draw a line
# The only other case that exists outside the boundaries stated is if count = 5
# We do not include a condition for else statements
else:
    forward(50)

Comments

We use comments to leave notes about the code to the reader. Comments are not actually run by Python, they are just there to help us read the code.

We can make multiline comments with """ and single line comments with #

"""
A multi-line comment describes your code
to someone who is reading it.
"""

# Use single line comments to clarify code.