1. PYTHON CONDITIONALS AND RECURSION : CHAPTER 5 FROM THINK PYTHON HOW TO THINK LIKE A COMPUTER SCIENTIST

2. BOOLEAN EXPRESSION A Boolean expression is an expression that results in either a True or False. We can test these in easily. For example >>> 5 == 2 #note the double == is used to test for equality False >>> 5 == 5 # Note that 5=5(or x=5) is really bad!! True # Other relational operators include x != yx yx>=y All of these return a True or False Note that True and False are NOT strings

3. LOGICAL OPERATORS There are three logical operators, and, or and not These are used as we do in English >>> True and True True >>> True and False if both true return true else false # False >>> True or False # if either is true return true True >>> not True True >>> True and not False True

4. FURTHER EXAMPLES >>> x=10 >>> y = 0 >>> x>5 and y == 0 True >>> x != 10 or not y == 3 True >>> x>5 and x<=20 # returns True if x is between 5 and 20 True >>> x%2==0 or x%3==0 # divisible by 2 or by 3 return True True

5. CONDITIONAL EXECUTION The first instruction that we will look at that uses conditions is the if statement. Here is the syntax if : statement more_statements # an example if x!=0: y = 20.0/x #makes sure the we are not dividing by 0 print y Execute these instruction only if Is True. Four space Indention is required!

6. ALTERNATIVE EXAMPLE if : Instruction to execute if is True else: Instructions to execute if is False # Example if (x>0): print ‘x is positive’ else: print ‘x is less than or equal to zero’

7. CHAINED CONDITIONALS If x<y: print ‘x is less than y’ elif x>y: print ‘x is greater than y’ else: print ‘x is equal to y’ #You can chain as many of these elif’s as you want # Lets do an example with grades

8. DETERMINE A LETTER GRADE FROM A NUMBER grade= ? #Assign some value here if grade>=90: print ‘I made an A’ #Note: As soon as a true is found elif grade>=80: # the remainder is skipped!! print ‘I made a B’ elif grade>=70: print ‘I made a C’ else: print ‘I better get to studying’

9. WE CAN ALSO PUT AN IF WITHIN AN IF Suppose we want to see if a is greater than b and c if a>b : if b>c: print a, ‘is the largest’ else: if a>c: print a, ‘is the largest’ else: print a, ‘is not the largest’ else: print a, ‘ is not the largest’ Note: This is a contrived example. There is a much easier way to do this! Think about using the and operator!!

10. KEYBOARD INPUT HTTP://DOCS.PYTHON.ORG/2/LIBRARY/FUNCTIONS.HTML Before we look at more if-else examples we need to see how to get input (keyboard) from the person running the python program. raw_input([prompt]) If the prompt argument is present, it is written to screen without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When EOF is read, EOFError is raised.EOFError Example: x = raw_input(‘Enter a number:’) Prompt A string

11. OK, LOOK AT THIS >>> x = raw_input('Enter a number:') Enter a number:23 >>> x+3 #What will be printed? Traceback (most recent call last): File " ", line 1, in x+3 TypeError: cannot concatenate 'str' and 'int' objects WHY? So what can we do to fix it?

12. CONVERT IT TO AN INTEGER OR FLOAT #method 1 >>> x = raw_input('Enter a number:') Enter a number:12 >>> x=int(x) #convert x to an integer >>> x+4 16 #method 2 >>> x = int(raw_input('Enter a number:'))+4 Enter a number:23 27 What if I typed in 3.45 instead of 12

13. HERE IS WHAT HAPPENS >>> x = raw_input('Enter a number:') Enter a number:3.45 >>> int(x) Traceback (most recent call last): File " ", line 1, in int(x) ValueError: invalid literal for int() with base 10: '3.45' Note: float(x) will work!

14. SOME PROBLEMS TO CONTEMPLATE a=float(raw_input(‘Enter a:’)) b=float(raw_input(‘Enter b:’)) c=float(raw_input(‘Enter c:’)) x=float(raw_input(‘Enter x:’)) y = a*x**2+b*x+c print ‘The answer is’,y Enter a:2 Enter b:3 Enter c:4 Enter x:5 The answer is 69.0 >>>

15. BACK TO TURTLE_WORLD One of the problems we noticed with a random walking turtle was that it often off the screen. Let solve this problem and create a fence that the turtle is kept within. We will start by creating a simple move function from swampy.TurtleWorld import * from random import * #Moves turtle to new location #without modifing its heading def MoveTo(t,x,y): “”” t is the turtle, and x,y is the location we want to move to””” t.x=x t.y=y

16. DRAW THE FENCE #Draws the electric fence #with a different turtle and then removes it def DrawFence(s): “”” s is the width of the square fence “”” t=Turtle() # Create a turtle to draw the fence t.delay=.001 #Draw it real fast set_pen_color(t, 'blue') MoveTo(t,-s/2,s/2) # Go to upper left hand corner of the fence for i in range(4): fd(t,s) rt(t,90) die(t) s (0,0)

17. REMEMBER HEADINGS

18. BOUNCE OFF FENCE #Changes the heading if the turtle tries to #move outside the fence. w is the width of the fence def Bounce_off_fence(t,w): b=w/2-3 if t.x>b: #going off  t.x =b t.heading=180 if t.x <-b: #going off  t.x=-b t.heading=0 if t.y>b: #going off top t.y=b t.heading=270 if t.y<-b: #going off bottom t.y=-b t.heading=90 (0,0) 0 90 180 270 headings b

19. THE MAIN PROGRAM RUN TURTLE AROUND IN FENCE world = TurtleWorld() fenceSize=200 DrawFence(fenceSize) bob = Turtle() set_color(bob, 'red') bob.delay=.01 pu(bob) #Turn off the drawing of the path for i in range(1000): # Take 1000 random steps Bounce_off_fence(bob,fenceSize) #remember, this changes its heading fd( bob,3) # the following creates a random number from -45 to +45 # It is used to create the directional wandering movement we see angle = random()*90-45 rt(bob,angle) wait_for_user()

20. HOMEWORK Rewrite the turtle-fence program so that the fence is now a circle that the turtle must stay inside of. 1.You first need to modify the DrawFence(s) to draw a circle of radius r 2.When is above is working modify Bounce_off_fence(t,r) to work with the circle. HINT: remember the distance formula you learned in algebra? Might need http://www.mathsisfun.com/polar-cartesian-coordinates.html 3. The remainder of the code aught to work as is.