1 / 27

The CS 5 Herald

The CS 5 Herald. Goodwill Gesture Goes Awry.

agnes
Download Presentation

The CS 5 Herald

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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!

More Related