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The CS 5 Herald

The CS 5 Herald. Goodwill Gesture Goes Awry.

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The CS 5 Herald

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  1. 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.

  2. “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)

  3. 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

  4. __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

  5. 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

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

  7. 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))

  8. 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 ints or floats • Some languages (including Python) let users overload operators at will • y = a*x*x + b*x + c

  9. 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

  10. 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.

  11. 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

  12. 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!

  13. 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)

  14. 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!

  15. 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!

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

  17. 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

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

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

  20. 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

  21. 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… ?

  22. 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”

  23. 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

  24. X O X O X O X O X O X X O X O O X X X O X O X X O X X O X O X X O X X O X X The Move Tree

  25. Explosive Growth • Connect-4 move tree has < 742 nodes • Roughly 311×1033 • Tree size depends on two parameters: • Branching factor per move, B • Maximum moves in game, N • Approximately BN possibilities • Chess: B≈35, N≈150 • Backgammon: B≈300, N≈50

  26. 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

  27. 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!

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