1

Computer Science of Graphics and Games

MONT 105S, Spring 2009Session 4ConditionalsTurtles

2

General Form for a Conditional

if condition: # condition is true or falsePython code A # Executed if condition is true

else:Python code B # Executed if condition is false

Only one of the two blocks of code is executed.

3

General Form for using elif in a Conditional

if condition1: # condition is true or falsePython code A # Executed if condition1 is true

elif condition2:Python code B #Executed if condition1 is false and 2 is true

else:Python code C # Executed if conditions 1 and 2 are false

Only one of the blocks of code is executed.

4

Example Decision Tree

"Enter your height: "

print "Tall" height >= 70?

print "Above average"

true false

print "Short"

height >= 75?

true false

5

An Alternate Decision Treefor if..elif..else

"Enter your height: "

print "Tall" print "Above average" print "Short"

height >= 75 height >= 70Otherwise(height < 70)

Conditions are printed next to the branches.Paths are tested one at a time from left to right.First condition that is true is the path that is followed.Only one path is executed.

6

Program Example

# Program height.py

height = input("Enter your height in inches: ")if height >= 75: print "Wow! You are tall!" print "Please join our basketball team."elif height >= 70: print "You are above average height."else: print "You are shorter than average."print "Thank you for participating in our survey."

7

Multiple decisions

"Do you like to ski?"

"Downhill or cross country?" "You should learn!"

true false

true false

"Try Wachusett" "Try the local trails"

In Python, we use a nested conditional to program this decision tree.

skiAns equals "yes"?

response equals "downhill"?

8

# Program: downhill.py

skiAns = raw_input("Do you like to ski? ")if skiAns == "yes": response = raw_input("Downhill or cross country? ") if response == "downhill": print "Try Wachusett!" else: print "Try the local trails."else: print "You should learn!"

Nested Conditional Example

9

Recall Classes and Objects

Python is an object oriented programming language.All items of data are objects.

Objects have properties: Things that describe the objectsObjects have methods: Things an object can do.

A class describes a group of objects that have the same methods and set of properties.

Example: A car classProperties: color, number of doors, body typeMethods: accelerate, brake, steer

My car is an object that is a member of the car class.

10

The Turtle Class

A turtle in python is an object from the Turtle class.

A turtle can move around a graphics window.

A turtle has a pen that can be up or down. If the pen is down, the turtle draws as it moves.

Turtle properties:x positiony positiondirection (that it's facing)pen position (up or down)pen color

11

Turtle Methods

Turtle methods:

forward(distance) Move forward <distance> pixelsbackward(distance) Move backward <distance> pixelsright(angle) Turn right <angle> degreesleft(angle) Turn left <angle> degreesup( ) Lift pen updown( ) Put Pen downgoto(x, y) Move to position x, ycircle(radius) Draw a circle with a given radius.

12

Turtle Example#Use a turtle to draw a triangle.

from turtle import Pen as Turtle #import from library

yertle = Turtle( ) #Create a new Turtle object; name it yertleyertle.forward(100) #Move yertle forward by 100 pixelsyertle.left(120) #Turn yertle left by 120 degreesyertle.forward(100) #Move yertle forward by 100 pixelsyertle.left(120) #Turn yertle left by 120 degreesyertle.forward(100) #Move yertle forward by 100 pixelsyertle.left(120) #Turn yertle left by 120 degrees