Lecture 2. More Algorithm Analysis, Math and MCSS By: Sarah Buchanan

Transcription

1 Lecture 2 More Algorithm Analysis, Math and MCSS By: Sarah Buchanan

2 Announcements Assignment #1 is posted online It is directly related to MCSS which we will be talking about today or Monday. There are 3 solutions of different complexity to MCSS, just like there will be 3 solutions of different complexity, make sure you keep that in mind! Consider the overflow issue since you will be multiplying large amounts of integers. Given the range of numbers and the input size, and the fact that I expect experimental analysis, I would recommend reading about and using the java BigInteger class. DUE 9/8, by midnight on webcourses, 10% off 1 day late, 20% 2 days late, 0 after that.

3 Summary We are going to go over more mathematical techniques for analyzing algorithms. Then we are going to put them into practice! We will go over Big-Oh, Theta, and Omega And if we have time we will discuss MCSS and experimental analysis.

4 Math Review Last time we went over infinite geometric series and how to derive that formula. Infinite geometric series example, where r < 1: S = a 1 + a 1 r + a 1 r 2 + a 1 r 3 + S = a 1 /(1-r) Solve, More standard summations we will use: S = = a 1 (1-r n )/(1-r) Solve,

5 More Summations we will Use = n(n+1)/2 First n terms of an arithmetic series, = n(a 1 + a n ) /2 Solve, = An arithmetic series is a sequence of numbers such that the different between two successive members is a constant. Use the subtraction technique shown last class to solve

6 Math Review - Logs Logs the log function is the inverse of an exponent, if b a = c then by definition log b c = a You can never take the log of 0 or any negative value. Why? Since a positive number raised to any exponent can never be negative, and only logs with positive bases are computed. Rules: log b a + log b c = log b ac log b a c = c log b a log b a log b c = log b a/c log b a = log c a/log c b b log c a = a log c b b a /b c = b a-c b a b c = b a+c (b a ) c = b ac Solve

7 Math Review - Logs Key observation Logarithms grow slowly = 1024 (log = 10) 2 20 = x10 6 (log 2 1x ) 2 30 = x10 9 (log 2 1x ) This means that the performance of an O(N log N) algorithm is much closer to that of an O(N) algorithm than an O(N 2 ) algorithm, even for large amounts of input. In general, any time we repeatedly halve or multiply a quantity in some way, a lot is involved in the analysis.

8 Examples on the board We re going to solve all of the examples given before on the board.

9 Sorted List Matching Problem Approach #1 Lets compare 3 different solutions to this problem and their runtimes. Problem: Given 2 sorted lists of names, output the names common to both lists. Obvious way to do this: For each name on list #1: a) Search for the current name in list #2. b) If the name is found, output it. So this isn t leveraging the fact that we know the list is sorted, it would take O(n) to do (a) and (b), multiplied by the n names in list#1 gives a total of O(n 2 )

10 Sorted List Matching Problem Approach #2 Let s use the fact that the lists are sorted! For each name on list #1: a) Search for the current name in list #2. b) If the name is found, output it. For step (a) use a binary search. From CS1, we know that this takes O(log n) time. (Because what a binary search does is compares the target to the middle item in the list. If the target is the same, you re done. If it s before the middle, repeat this procedure on the items before the middle. If it s after the middle, repeat on the items after the middle. THUS, we are dividing are search space in half each time.)

11 Sorted List Matching Problem Approach #2 Continued For the moment, lets assume that both lists are of equal size, then we can say that the size of list#2 is ½ the total input size, N. Then technically our search would take O(log N /2) time. HOWEVER, using our log rules, we find that log 2 N= log 2 (N /2) +1 Thus, our running time is simply O(log 2 N) BUT this is just for 1 loop iteration of finding the first name in list #1. How many loop iterations are there? N/2 THUS, the total running time of Approach #2 is O(N log 2 N)

12 Sorted List Matching Problem Approach #3 Can we do better? We still haven t used the fact that list #1 is sorted! Can we exploit this fact so that we don t have to do a full binary search for each name? List #1 List #2 Albert Brandon Carolyn Dan Elton Cari Carolyn Chris Fred Graham

13 Sorted List Matching Problem Approach #3 Formal Version of the algorithm: 1) Start 2 markers, one for each list, at the beginning of both lists. 2) Repeat the following until one marker has reached the end of its list: a) Compare the two names that the markers are pointing at. b) If they are equal, output the name and advance BOTH markers one spot. If they are NOT equal, simply advance the marker pointing to the name that comes earlier alphabetically one spot.

14 Sorted List Matching Problem Approach #3 Algorithm Run-Time Analysis For each loop iteration, we advance at least one marker. The max number of iterations then, would be the total number of names on both list, N. For each iteration, we are doing a constant amount of work. Essentially a comparison, and/or outputting a name. Thus, our algorithm runs in O(N) time an improvement. Can we do better? No, because we need to at least read each name in both lists, if we skip names, on BOTH lists we cannot deduce whether we could have matches or not. Even if we could skip the names in one list we would read all the names in one list giving O(N/2) = O(N).

15 Order Notation: Big-Oh, Theta, and Omega Since we want to simply count the number of simple statements an algorithm runs in terms of Big-Oh notation, we need to learn the formal definitions of Big-Oh, Big- Omega, and Big-Theta, so we properly apply these technical terms.

16 Big-Oh Definition of O(g(n)) f(n) = O(g(n)) iff for all n n 0 (n 0 is a constant) f(n) c g(n) for some constant c Example Let f(n) = 2n + 1 and g(n) = n Does f(n) = O(g(n))? Proof: Let c = 3, n 0 = 2 Then, f(n) = 2n + 1 and cg(n) = 3n Thus we need to show, (2n + 1) 3n for all n 2 We know 2n + 1 2n + n, since n > 1 = 3n

17 Big-Oh Simple rule for polynomial functions: If f(n) is a polynomial of degree k, then f(n) = O(n k ) Example, an 4 + bn 3 + cn 2 = O(n 4 ) Is 2n + 1 = O(n 10 )? Yes, try c = 1, n 0 = 2. Big-Oh is an upper bound. It simply guarantees that a function is no larger than a constant times a function g(n), for O(g(n)). We will usually use Big-Oh notation when we are describing a worst case running time.

18 Big Omega Big Omega is just about the opposite of Big Oh. Definition: f(n) = Ω(g(n)) iff for all n n0 (n0 is a constant) f(n) c g(n) for some constant c. The only difference is the inequality sign!

19 Big Omega Example, Let f(n) = n 2-3, g(n) = 10n f(n) = Ω(g(n))?? Proof: Let c =.1, n 0 = 3 Then, f(n) = n 2 3, c g(n) = n Thus, we need to show n 2 3 n, since n 3 = n(n 1) n(2), since n 3, n-1 2 n

20 Big Omega Ω establishes a lower bound for a function. f(n) has to grow at least as fast as g(n) to within a constant factor. For example, if I said that an algorithm runs in Ω(n) This means whenever you run an algorithm with an input of size n, the number of small instructions executed is AT LEAST cn. Where c is some positive constant.

21 Big Theta Definition f(n) = (g(n)) iff f(n) = O(g(n)) and f(n) = Ω(g(n)) This means that g(n) is both an upper AND lower bound of f(n) within a constant factor. As n grows large, f(n) and g(n) are within a constant factor. Thus if we can show an algorithm runs in : O(f(n)) time Ω(f(n)) time AND We can conclude that both the WORST case running time and BEST case running time are proportional to f(n). If this is the case, we CONDLUDE the algorithm runs in (f(n)) time.

22 Order Notation Summary O is like Ω is like is like = (since f(n) = O(g(n)) iff f(n) cg(n)) Finally, another way to think about each of these is that they describe a class of functions. If f(n) = O(n), it s like saying f(n) ϵ O(n) Meaning f(n) can be any number of functions, where f(n) is proportionate to n or smaller.

23 Experimental Analysis We just covered methods used to determine the theoretical run-time of an algorithm. BUT, sometimes an algorithm is too difficult to analyze theoretically. OR, Sometimes we would like to verify that an algorithm is actually running as fast as we expect. In these cases we can experimentally gauge the run-time of an algorithm.

24 Experimental Analysis Let T(N) be the empirical (observed) running time of the code and the claim is made that T(N) ϵ O(F(n). The techniques is to compute a series of values T(N)/F(N) for a range of N (commonly spaced out by factors of 2). Depending on the values of T(N)/F(N) we can determine how accurate our estimation for F(N) is according to: F(N) = is a close answer( ) if the values convert to a + constant is an overestimate if the values converge to zero is an underestimate if the values diverge.

25 Example 1 The table contains results from running an instance of an algorithm assumed to be cubic. Decide if the Big-Theta estimate, Θ(N 3 ) is accurate. Run N T(N) F(N) = N 3 T(N)/F(N) ms ms ms 1.25x ms ms 1.25x The calculated values converge to a positive constant ( ) so the estimate of Θ (n 3 ) is an accurate estimate.

26 Example 2 The table contains results from running an instance of an algorithm assumed to be quadratic. Decide if the Big-Theta estimate, Θ (N 2 ) is accurate. Run N T(N) F(N) = N 2 T(N)/F(N) ms ms ms ms ms The values diverge, so the code runs in Ω(N 2 ), and has a larger theta bound.

27 MCSS public static int MCSS(int [] a) { int max = 0, sum = 0, start = 0, end = 0; } // Cycle through all possible values of start and end indexes // for the sum. for (i = 0; i < a.length; i++) { for (j = i; j < a.length; j++) { sum = 0; // Find sum A[i] to A[j]. for (k = i; k <= j; k++) sum += a[k]; if (sum > max) { max = sum; start = i; // Although method doesn't return these end = j; // they can be computed. } } } return max; {-2, 11, -4, 13, -5, 2} i is the starting point, j is the ending point i = 0, j = 0, k = 0 etc. sum+=-2 j = 1, k = 0 sum += -2 k = 1 sum += 11 j = 2, k = 0 sum += -2, k = 1 sum+= 11 k = 2 sum+= -4 As we can see we are repeatedly starting the sum over and doing much more work than we need to!

28 MCSS public static int MCSS(int [] a) { int max = 0, sum = 0, start = 0, end = 0; // Cycles all possible values of start and end indexes // for the sum. for (i = 0; i < a.length; i++) { } for (j = i; j < a.length; j++) { } sum = 0; // Find sum A[i] to A[j]. for (k = i; k <= j; k++) sum += a[k]; if (sum > max) { } return max; max = sum; start = i; // Method doesn't return these end = j; // they can be computed. 3 loops: i loop executes N times j loop executes N-1 times k loop executes N-1 times, in the worst case when i = 0. Thus, this gives a rough estimate that the algorithm is O(N 3 )

29 MCSS Continued next class