在本章节中，分析了运行时和渐近行为，有很多比较算法的方法，另外介绍了对不同类型运行时如何渐进比较的直觉，给出了Big-O, Big-Theta, and Big-Omega的定义，以及能够在给定的运行时证明它们。
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1.CSE 373 : Data Structures and Algorithms Pep Talk; Algorithm Analysis Riley Porter Winter 2017
2.Announcements Optional Java Review Section: PAA A102 Tuesday, January 10 th , 3:30-4:30pm . Any materials covered will be posted online . TAs will be around after the session to answer questions. TA office hours will be posted on the website later today HW1 released after class today Extension: Due Tuesday, January 17 th at 11:00PM 2 CSE373: Data Structures & Algorithms Winter 2017
3.Assumed Knowledge (Pep Talk) Objects in Java : fields, methods, encapsulation, inheritance, interfaces, classes Bein g the Client of Data Structures : List, Set, Stack, Queue, Map, Trees Being the implementer of Data Structures: ArrayList , LinkedList , Binary Trees, testing your objects Binary Search, and some awareness of how sorting works : merge sort, selection sort. Some basic analysis about above . Examples: when to use a HashSet vs a TreeSet ? when does adding into an ArrayList become expensive? why does sorting an array help for searching? Winter 2017 3 CSE373: Data Structures & Algorithms
4.NOT Assumed Knowledge Full understanding of the difference between the definitions of ADTs vs Data Structures vs Implementations Big O analysis . Maybe you have some awareness, but we don’t expect mastery of Big O yet. Indepth analysis or mastery of sorting or hashing . Anything at all about Graphs, Heaps, AVL Trees, Union Find, Disjoint Sets, Hashing, Topological Sort, Parallelism Any advanced algorithms, dynamic programming, P vs NP, complexity theory, proofs, induction 4 CSE373: Data Structures & Algorithms Winter 2017
5.Review of last time: Heaps Heaps follow the following two properties: Structure property: A complet e binary tree Heap order property : The priority of the children is always a greater value than the parents (greater value means less priority / less importance) Winter 2017 5 CSE373: Data Structures & Algorithms 15 30 80 20 10 99 60 40 80 20 10 50 700 85 not a heap a heap 450 3 1 75 50 8 6 0 n ot a hea p 10 10
6.Today – Algorithm Analysis Review math for algorithm analysis Exponents and logarithms, floor and ceiling Analyzing code Big-O definition Using asymptotic analysis (continue next time ) Set ourselves up to analyze why we use Heaps for Priority Queues (continue later this week) 6 CSE373: Data Structures & Algorithms Winter 2017
7.Review of Logarithms log 2 x =y if x=2 y (so, log 2 1,000,000 = “a little under 20”) Just as exponents grow very quickly, logarithms grow very slowly Log base B compared to log base 2 doesn’t matter so much In computer science we use base 2 because it works nicely with binary and how a computer does math. we are about to stop worrying about constant factors In particular, log 2 x = 3.22 log 10 x 7 CSE373: Data Structures & Algorithms Winter 2017
8.Review of log properties log(A*B) = log A + log B So log( N k )= k log N log(A/B) = log A – log B log(log x) is written log log x Grows as slowly as 2 2 grows quickly (log x)(log x) is written log 2 x It is greater than log x for all x > 2 It is not the same as log log x 8 CSE373: Data Structures & Algorithms y Winter 2017
9.Review of floor and ceiling 9 CSE373: Data Structures & Algorithms Floor function: the largest integer < X Ceiling function: the smallest integer > X Winter 2017
10.Comparin g Algorithms When is one algorithm (not implementation ) better than another? Various possible answers (clarity, security, …) But a big one is performance : for sufficiently large inputs , runs in less time or less space Large inputs (n) because probably any algorithm is “fine” for small inputs Answer will be independent of CPU speed, programming language , coding tricks, etc . 10 CSE373: Data Structures & Algorithms Winter 2017
11.Comparin g Algorithms Example Given the above list, search for 3, which is better? binary search linear search 11 CSE373: Data Structures & Algorithms Winter 2017 4 2 5 1 8 6 10 9 3 7 1 2 3 4 5 6 7 8 9 10 Given the above list, search for 3, which is better? binary search linear search
12.Analyzing Algorithms As the size of an algorithm’s input grows: How much longer does the algorithm take ( time ) How much more memory does the algorithm need ( space ) Ignore constant factors, think about large input: there exists some input size n 0 , that for all input sizes n larger than n 0 , binary search is better than linear search on sorted input Analyze code to compute runtime, then look at how the runtime behaves as n gets really large (asymptotic runtime) 12 CSE373: Data Structures & Algorithms Winter 2017
13.Analyzing Code “Constant time” operations: Arithmetic, Variable Assignment, Access one Java field or array index, etc Complex operations (approximation ): Consecutive Statements: Sum of time of each statement Conditionals: Time of condition + max( ifBranch , elseBranch ) Loops: Number of iterations ∗ Time for Loop Body Function Calls: Time of function’s body 13 CSE373: Data Structures & Algorithms Winter 2017
14.Example What is the runtime of this pseudocode : x := 0 for i =1 to N do for j=1 to N do x := x + 3 return x 14 CSE373: Data Structures & Algorithms Winter 2017
15.Example Solution What is the runtime of this pseudocode : x := 0 for i =1 to N do for j=1 to N do x := x + 3 return x 1 assignment + (N iterations of loop * (N iterations of loop * 1 assignment and math)) 1 return 1 + (N * (N * 1)) + 1 = N 2 + 2 However, what we care about here is the N 2 part. Let’s look at asymptotic runtimes to see why. 15 CSE373: Data Structures & Algorithms Winter 2017
16.Asymptotic Intuition with Pictures 16 CSE373: Data Structures & Algorithms Winter 2017 Are these the same?
17.Asymptotic Intuition with Pictures 17 CSE373: Data Structures & Algorithms Winter 2017 What about now that we compare them to y=N 2 ?
18.Asymptotic Intuition with Pictures 18 CSE373: Data Structures & Algorithms Winter 2017 What about these? One starts off much lower than the other one, but grows much faster.
19.Asymptotic Notation About to show formal definition, which amounts to saying: Calculate Runtime by analyzing code Eliminate low-order terms Ignore constants and coefficients Examples: 4 n + 5 0.5 n log n + 2 n + 7 n 3 + 2 n + 3 n n log (10 n 2 ) 19 CSE373: Data Structure & Algorithms Winter 2017
20.Examples with Big -O Asymptotic Notation True or F alse ? 3n+10 ∈ O(n) 4+2n ∈ O(1) 20-3n ∈ O( n 2 ) n +2logn ∈ O( logn ) logn ∈ O(n+2logn ) 20 CSE373: Data Structures & Algorithms Winter 2017
21.Examples with Big- O Asymptotic Notation Solutions True or F alse ? 3n+10 ∈ O(n) True (n = n) 4+2n ∈ O(1) False : (n >> 1) 20-3n ∈ O( n 2 ) True: (n ≤ n 2 ) n +2logn ∈ O( logn ) False : (n >> logn ) logn ∈ O(n+2logn) True: ( logn ≤ n+2logn) 21 CSE373: Data Structures & Algorithms Winter 2017
22.Formally Big- O Definition: g( n ) is in O( f( n ) ) if there exist constants c and n 0 such that g( n ) c f( n ) for all n n 0 To show g( n ) is in O( f( n ) ) , pick a c large enough to “cover the constant factors” and n 0 large enough to “cover the lower-order terms” Example: Let g( n ) = 3 n 2 +17 and f( n ) = n 2 c =5 and n 0 =10 is more than good enough This is “less than or equal to” So 3 n 2 +17 is also O ( n 5 ) and O (2 n ) etc. 22 CSE373: Data Structure & Algorithms Winter 2017
23.Big- O We use O on a function f( n ) (for example n 2 ) to mean the set of functions with asymptotic behavior less than or equal to f( n ) So (3 n 2 +17) is in O ( n 2 ) 3 n 2 +17 and n 2 have the same asymptotic behavior What it means: For your runtime, asymptotically, O(function) is the family of functions that defines the upper bound. There is a size of input ( n 0 ) and a constant factor ( c ) you can use to make O(function) strictly larger than your runtime. 23 CSE373: Data Structure & Algorithms Winter 2017
24.Examples using formal definition A valid proof is to find valid c and n 0 : Let g( n ) = 1000 n and f( n ) = n 2. The “cross-over point” is n =1000 So we can choose n 0 =1000 and c =1 Many other possible choices, e.g., larger n 0 and/or c Let g( n ) = n 4 and f( n ) = 2 n. W e can choose n 0 =20 and c =1 24 CSE373: Data Structure & Algorithms Definition: g( n ) is in O( f( n ) ) if there exist constants c and n 0 such that g( n ) c f( n ) for all n n 0 Winter 2017
25.What’s with the c The constant multiplier c is what allows functions that differ only in their largest coefficient to have the same asymptotic complexity Example: g( n ) = 7 n +5 and f( n ) = n For any choice of n 0 , need a c > 7 (or more) to show g( n ) is in O( f( n ) ) 25 CSE373: Data Structure & Algorithms Definition: g( n ) is in O( f( n ) ) if there exist constants c and n 0 such that g( n ) c f( n ) for all n n 0 Winter 2017
26.Big-O: Common Names O (1) constant (same as O ( k ) for constant k ) O ( log n ) logarithmic O ( n ) linear O(n log n ) “ n log n ” O ( n 2 ) quadratic O ( n 3 ) cubic O ( n k ) polynomial (where is k is any constant: linear, quadratic and cubic all fit here too.) O ( k n ) exponential (where k is any constant > 1) Note : “exponential” does not mean “grows really fast”, it means “grows at rate proportional to k n for some k >1 ”. Example: a savings account accrues interest exponentially ( k =1.01? ). 26 CSE373: Data Structures & Algorithms Winter 2017
27.Intuition of Common Runtimes 27 CSE373: Data Structures & Algorithms Winter 2017 Even for small N, these look pretty different very quickly.
28.Intuition of Common Runtimes 28 CSE373: Data Structures & Algorithms Winter 2017 Now y=N and y= logN look a lot more similar in comparison to other runtimes.
29.Intuition of Common Runtimes 29 CSE373: Data Structures & Algorithms Winter 2017 Asymptotically, y=2 N looks way different than the rest and the rest all look roughly the same.