Choose Your Own Pyventure/Wandering Grue

Functions Case Study: Wandering Grue Let's say you'd like to have a wandering Grue in your Mysterious House that randomly pops up in a room (or rooms). One way to do this is to make a function that decides whether the Grue is in the room or not: Test Frameowrk Later in our complete code, we will see code similar to: eaten=grue() if eaten: print "Sadly, you were torn limb-from-limb by the Grue and suffered a slow, painful death." else: print "Congratulations, you have not been eaten by the Grue! May you have a long happy life." Here we introduce a new python data type: boolean. Python booleans can take two values True and False. These code fragments are equivalent: if x > 3: print "yep!" if x > 3 is True: print "yep!" if bool(x>3) is True: print "yep!" The "if" syntax implies if predicate is True. In Python, most things evaluate to True, except: None, False, 0 (0.0, etc.), empty strings, zero-length lists, dicts, and few other oddities http://docs.python.org/library/stdtypes.html#truth-value-testing. Variation 1: not very random def grue_always(): ''' this grue always appears. returns true''' return True Our not very random grue always appears. Exercise: Make the reverse -- a grue that never appears. The signature of the function should be grue_never() -> False Variation 2: the 50/50 Grue import random ## random is a Python module, as mentioned above. ## We need to import it to access the random() function. ## now to begin the function definition ## Everything inside the function definition is indented! Remember, white space matters! def random_grue(): ''' boolean. a grue that appears 50% of the time ''' ## we want something that will return True 50% of the time. ## one method: get a random float between (0,1), and return True if it's over .5 ## now we need a random number. random() will give us one between 0 and 1 n=random.random() ## the random before the dot tells Python what module to look in for the function, which is the one we imported above if n > 0.5: grue=1 ## 1 == True else: grue=0 ## 0 == False return grue ## returning allows us to capture the value So what does the random_grue() function do? Let's try it. In the Python interpreter: >>> import random >>> def random_grue(): n=random.random() if n>0/5: grue = 1 else: grue = 0 return grue >>> random_grue() 1 The first command is to import the random module, the second is to define the function and the third is to actually call the function. The 1 is the return of the function. You may get a 1 or a 0 depending on the number random() generated. (Hint: try running it several times) > A digression on pseudorandom numbers, and tips for getting the same ones every time! Variation 2: the Moody Grue import random def grue_moody(cutoff=.5): ''' boolean. a grue that appears (100*cutoff)% of the time ''' n=random.random() above_cutoff = n < cutoff return above_cutoff def grue_moody2(cutoff=.5): ''' boolean. a grue that appears (100*cutoff)% of the time ''' return random.random() < cutoff Note that we simplified down the function quite a bit by returning the boolean value directly (especially in 'grue_moody2'), rather than doing any conditional logic to get a 1 or 0. Also notices that we specified a default value for the argument cutoff. Exercises Predict the behaviour of these functions calls. Then try them. grue_moody() grue_moody(-1) grue_moody(1) grue_moody("a") grue_moody([1,2,3]) Fix the code so that it prints an angry message and returns None if n is outside the interval (0,1). try help(grue_moody). What do you see? what are the types of random, random.random, random.random() Variation 3: the location, location, location Grue In our final variation, we want a grue that: has different percentages of appearing based on which page should have zero chance of appearing in the main room "foyer" should have a default chance of appearing of 5% in rooms that aren't otherwise described import random grue_fractions = { 'foyer':0, 'thedark': 1.0, 'nocake': .2 } def location_grue(room=None, base=.05, cutoffs=dict()): ''' (boolean), does a grue appear in the room? room : str room name base : 'cutoff', float between (0,1), for base (room not found) cutoffs: dict of room_name: cutoff (float between 0,1) ''' cutoff = cutoffs[room] return random.random() < cutoff Exercises as written, location_grue has some bugs, and doesn't meet spec. Identify and fix them. try: location_grue('foyer', cutoffs=grue_fractions) what happens if 'room' isn't in 'cutoffs'? research the 'get' method of dictionarys... help({}.get). Use this method to fix the code. References