1. COSC 1306 COMPUTER SCIENCE AND PROGRAMMING
Jehan-François Pâris
Fall 2016 1

2. THE ONLINE BOOK CHAPTER IV TURTLES

3. Turtles?
A Python package allowing novice users to draw simple colorful figures
Python port of the Logo turtle
Seymour Pappert
Used to taech programming concepts yo young children

4. Starting up (I) Import the turtles package import turtle
Create a drawing canvas
mycanvas = turtle.Screen()
A new instance of the Screen object as defined in the turtle package

5. Starting up (II) Create a turtle Alice = turtle.Turtle()
A new instance of the Turtle object as defined in the turtle package
Turtle and turtle are different identifiers
Python is case-sensitive
Can have many turtles on the same canvas
Move the turtle

6. Example import turtle wn = turtle.Screen() # Creates a canvas
alex = turtle.Turtle() # Create a turtle "alex"
alex.forward(50) # Go forward 50 units
alex.left(90) # Turn by 90 degrees
alex.forward(30) # Go forward 50 units
wn.mainloop() # Wait for user to close window

7. Another example import turtle wn = turtle.Screen()
shasta = turtle.Turtle()
shasta.right(90)
shasta.forward(80)
shasta.left(90)
shasta.forward(40)
wn.mainloop()

8. Allowing drawing discontinuities
shasta.penup()
Tell the turtle shasta to start moving without leaving a trace
Think of asking shasta to raise up its pen
shasta.pendown()
Tell the turtle shasta to return to its normal state and leave a trace
Think of asking shasta to put its pen down to the paper

9. Guess what this program draws
import turtle
wn = turtle.Screen()
shasta = turtle.Turtle()
shasta.right(90)
shasta.forward(80)
shasta.left(90)
shasta.forward(40)
shasta.penup()
shasta.pendown()
shasta.forward(80)
shasta.penup()
shasta.backward(40)
shasta.left(90)
shasta.forward(40)
shasta.backward(80)
wn.mainloop()

10. The answer

11. Do exercises in section 4.2
Teaches you to put things in the right order

12. Making it look better Can Add color to canvas wn.bgcolor("lightgreen")
Specify color and thickness of he pen of a given turtle
tess = turtle.Turtle()
tess.color("blue")
tess.pensize(3)

13. Another way to exit wn.exitonclick()
Closes the canvas window when the user clicks his or her mouse

14. Can have multiple turtles
alice = turtle.Turtle()
bob = turtle.Turtle() …
Each with its color and pensize attributes
alice.color("blue")
bob.color("red")
...

15. A small problem
Want to congratulate several students for their project
print("Congratulations, Alice!")
print("Congratulations, Bob!")
print("Congratulations, Carol!") …
Becomes quickly tedious

16. The solution The for loop for x in ["Alice", "Bob", "Carol"] :
print("Congratulations, " + x +"!")
Repeats the statement(s) inside the loop for all entries in the list ["Alice", "Bob", "Carol"] assigning in turn the value of each entry to x

17. What are lists? A truly wonderful construct of Python
Extends the C/C++/Java/C# concept of array
I love it and you will!
Sequence of values inside square brackets separated by commas
["Alice", "Bob", "Carol"]
[0, 1, 2, 3, 4, 5]
[1, 3, 5, 7, 13, 17, 23]

18. The for loop syntax for iterator in list : body of the list . . .
rest of the program

19. Please note For statements end with a column Do not forget it!
All statements that will be repeated
Are said to be inside the loop
Must all be indented by the same numbers of spaces or tabs
Must use them both in a consistent fashion
Python is very very finicky

20. Example (I) for x in ["Alice", "Bob", "Carol"] :
print("Congratulations, " + x +"!")
print("You did very well!")
Will print two lines of congratulations for all three entries in the list
Total of six lines

21. Example (II) for x in ["Alice", "Bob", "Carol"] :
print("Congratulations, " + x +"!")
print("You did very well!") #outside
Will print a single line of congratulations for all three entries in the list then
You did very well!
only once
Total of four lines

22. Example (III) for x in ["Alice", "Bob", "Carol"] :
print("Congratulations, " + x +"!")
print("You did very well!") #outside
Will not print anything because the interpreter will refuse to execute the program

23. Warning When you move pieces of code around in your programs
Very easy to mess with the indentations in your code
Python is unforgiving
Must check and recheck your indentations

24. The good for index in list : loop body
Indentations make your code more concise and more legible
Compare
for index in list : loop body
with
for (weird construct) { loop body }

25. The bad
Python is unforgiving

26. Application: drawing a square
import turtle
wn = turtle.Screen()
alex = turtle.Turtle()
for i in [0, 1, 2, 3]: # repeat 4 times
alex.forward(50)
alex.left(90)
wn.exitonclick()

27. Comments We could have used for i in [1, 2, 3, 4]:
as the values of i are not used inside the loop
It is just a convention to start all iterations with the zero value
Unless you have a reason to do otherwise
Shared with C/C++/Java/C#/Ruby and many other similar languages
Get used to it!

28. Back to Babylonian math
""" Computes square roots
"""
number=float(input("Enter n >= 0: "))
root = number/2
for i in [0, 1, 2, 3, 4] :
inverse = number/root
root = (root + inverse)/2
print ("Its root is", root)

29. A problem What if we need more precision?
Must specify more iterations:
for i in [0, 1, 2, 3, 4, 5, 8] :
Becomes quickly tedious

30. The solution range(n) n is not in the range!
Lets index go through list of n values
Starting at 0
Ending at n – 1
n is not in the range!
Because we like to start at zero

31. A better algorithm """ Computes square roots """
number=float(input("Enter n >= 0: "))
root = number/2
for i in range(10) :
inverse = number/root
root = (root + inverse)/2
print ("Its root is", root)

32. Its output Enter a positive number: 4096 Its root is 1025.0

33. More turtle tricks (I) alex.shape("turtle")
Can change the shape of the turtle
alex.shape("turtle")
Options include "arrow", "blank", "circle", "classic", "square", "triangle", and "turtle"

34. More turtle tricks (II)
Can change the speed of the turtle
alex.speed(5)
1 is slowest and 10 is fastest
Can ask the turtle to stamp a mark at its current location
alex.stamp()

35. A nice turtle program (I)
import turtle
wn = turtle.Screen()
wn.bgcolor("lightgreen")
tess = turtle.Turtle()
tess.shape("turtle")
tess.color("blue")

36. A nice turtle program (II)
tess.penup()
size = 20
for i in range(30):
tess.stamp() # Leave a mark size = size # Increase size tess.forward(size) # Move along
tess.right(24) # Turn her 24o
wn.mainloop()