The CS 5 Herald Goodwill Gesture Goes Awry Claremont (AP) : Seven rooms were damaged in a Harvey Mudd College dormitory Tuesday evening after a misguided attempt to cheer up sleep-deprived students. “We were approached by a group of three penguins who wanted to sing Christmas carols,” explained a witness. “They promised that it would help us study.” Instead, the raucous squawking of the untrained and untalented birds quickly led to a violent dispute between supporters and detractors. One student attempted to encase the singers in foam rubber, but a second set fire to the material in hopes of freeing the animals. The resulting explosion unleashed a conflagration that spread to North Dorm, where there was extensive damage. However, losses in North were estimated at only $35.47, due to the advanced age of the furniture there.

“Forced” Methods Some methods are required by language Must always have __init__ Good to have one of __repr__ or __str__ Others not required, but very common copy (to do deep copies) show (to print self)

Construction and Destruction Constructor ( __init__ ) builds an instance Called when object created Best to initialize everything to reasonable values Destructor ( __del__ ) rarely needed Unlike C++ Unpredictable when (or if) it's called Best to avoid

__repr__ and __str__ __str__ implements the str (…) built-in Automatically called by print __repr__ should give Python syntax E.g., Date(11, 26, 2007) Automatically called when interpreter is returning a result If __str__ undefined, __repr__ used Quick shortcut: only do the latter

Class Globals ("statics") How to put debug in your Date class? Don't want to depend on user's debug Might not exist User might not care about Date debugging Solution: variable that's global within class Only one copy across all objects Also useful for things like counting objects

A Self-Counting Class class Counter: instances = 0 def __init__(self): self.id = Counter.instances Counter.instances += 1 def unique(self): return self.id

Computational Classes It’s often useful to define classes that provide mathematical objects Infinite-precision numbers Rational numbers Matrices Modular fields Writing code using these classes can be painful: y = add(mult(a,mult(x,x)),add(mult(b,x),c))

Operator Overloading Overloading means giving an operator (or function) a definition that depends on context Example: a/b means something different depending on whether a and b are int s or float s Some languages (including Python) let users overload operators at will y = a*x*x + b*x + c

Overloading in Python Most Python operators can be overloaded Just define a special function in a class Annoying exception: plain assignment Many finicky rules Names aren’t always obvious Some operators have “forward” and “reverse” versions Some operators derive from others by default

Basic Overloading class MyMath: def __init__(self, …): … def __add__(self, rhs): # Only works if rhs is a MyMath result = self.value + rhs.value return result def __sub__(self, rhs): … # etc.

Important Details Overloaded operators should return a result Should not modify “self” or right-hand side Often useful to have copy method Exception: Ops in “i” series modify “self” Possibility of rhs not being same type Many packages don’t handle this case well

Overloaded Operator Naming + add - sub * mul / div / truediv (future) // floordiv % mod ** pow + pos - neg abs int float complex == eq != ne <= le >= ge < lt > gt Remember, every name needs two underscores before and after it!

The r… and i… series radd, rsub, etc. implement reversed operators If x is special and y is built-in (e.g., integer), then x+y invokes x.add(y), and y+x invokes y.radd(x) However, if radd isn’t defined, Python will try y.add(x) iadd, isub, etc. implement incremental operators E.g., x.iadd(y) implements x += y Modifies x Should return None (i.e., no explicit return value)

The Alien’s Life Advice Ask questions when you don’t get it! It makes you seem human! OK, maybe you won’t get a date with Xohptzl, but you’ll learn more!

Worksheet: Designing a Class Design class Rational, to store rational numbers (integer divided by integer) Choose reasonable string representation Enumerate data, "special" functions, things it should be able to do E.g., a.add(b) adds b to a, modifying a I looove rationals! They’re so logical!

More On Rationals Should there be any “extra” methods beyond the obvious mathematical ones?

Rules for Connect Four Tic-Tac-Toe with stacking 7x6 board Checkers slide down onto top of column Four in a row (including diagonal) wins

A C4Board Class class C4Board: width = 7 height = 6 def __init__(self): board = [width*[0] for i in range(height)] …

Artificial Intelligence Catch-all term for anything that “appears human” Eliza Chess programs Expert systems Many approaches N-ply lookahead: good for games

Looking into the Future, Part I Basic idea Guess a move See whether it makes your position better Repeat for many guesses, pick best result What defines “better”? Simple answer: use board evaluation function

Evaluating a Connect-4 Board What are some ways to numerically rate a Connect-4 board? If opponent has won, 0.0 If I have won, 1.0 Otherwise… ?

Looking Into the Future, Part II Problem: evaluating a board is HARD Solution: look at where the board leads For each possible move: Make the move in “trial mode” For each possible opponent's response: Make that move in “trial mode” Evaluate that board instead (how?) Return value of opponent's best option This is the “worst case” for us Our best move is “best of the worst cases”

Limiting Recursion When do we stop? That's easy: When we find path that leads to a win No matter what opponent does (e.g., Nim) When all paths lead to losses (sigh) When we have explored all possible moves

The Move Tree X O X O X O X O X X O X O X O X O X O X X O X X O X O X X O X X O X X O X X

Explosive Growth Connect-4 move tree has < 7 42 nodes Roughly 311×10 33 Tree size depends on two parameters: Branching factor per move, B Maximum moves in game, N Approximately B N possibilities Chess: B≈35, N≈150 Backgammon: B≈300, N≈50

Limiting Growth Simplest strategy: stop looking ahead after evaluating N moves (“plies”) Alternative: stop after t seconds elapsed Better: prune move tree Don't follow a path if first move is way worse Don't explore if already found better choice Alpha/Beta pruning

The Curse of the AI Researcher AI is always defined as “something a lot smarter than what my computer can do” Corollary: if my computer can do it, it's not artificial intelligence ⇒ Every time we achieve AI, people decide it's not really AI!