# CSE 417 Algorithms Richard Anderson Winter 2020 Lecture CSE 417 Algorithms Richard Anderson Winter 2020 Lecture 4 Announcements Reading Chapter 2.1, 2.2 Chapter 3 (Mostly review) Start on Chapter 4 Homework Guidelines Submit homework with Canvas Submit problems separately

Deadline is 9:30 AM on Wednesday Describing an algorithm Clarity is most important Pseudocode generally preferable to just English But sometimes both methods combined work best Prove that your algorithm works A proof is a convincing argument Give the run time for your algorithm Justify that the algorithm satisfies the runtime bound You may lose points for style

Homework assignments will (probably) be worth the same amount Five Problems Scheduling Weighted Scheduling Bipartite Matching Maximum Independent Set Competitive Facility Location Maximum Independent Set Given an undirected graph G=(V,E), find a set I of vertices such that there are no

edges between vertices of I Find a set I as large as possible Find a Maximum Independent Set A B C D E

F G H K J O L I

S N T R M P Q U

Verification: Prove the graph has an independent set of size 8 NP-Completeness Hard to find a solution Easy to verify a solution once you have one Other problems like this Hamiltonian circuit Clique Subset sum Graph coloring

Are there even harder problems? Simple game: Players alternating selecting nodes in a graph Score points associated with node Remove nodes neighbors When neither can move, player with most points wins 4 3 2

6 5 2 3 4 1 6

5 6 4 6 5 5 6

4 1 6 2 1 4 2

5 8 7 8 5 4 4

5 4 3 4 Competitive Facility Location Choose location for a facility Value associated with placement Restriction on placing facilities too close together Competitive placement of facilities E.g., KFC and McDonalds

P-Space complete instead of NP-Complete Appear to be much harder No obvious certificate G has a Maximum Independent Set of size 10 Player 1 wins by at least 10 points Complexity theory These problems are P-Space complete instead of NP-Complete Appear to be much harder No obvious certificate G has a Maximum Independent Set of size 10 Player 1 wins by at least 10 points

Summary Five Problems Scheduling Weighted Scheduling Bipartite Matching Maximum Independent Set Competitive Scheduling

What does it mean for an algorithm to be efficient? Definitions of efficiency Fast in practice Qualitatively better worst case performance than a brute force algorithm Polynomial time efficiency An algorithm is efficient if it has a polynomial run time Run time as a function of problem size Run time: count number of instructions executed on an underlying model of

computation T(n): maximum run time for all problems of size at most n Polynomial Time Algorithms with polynomial run time have the property that increasing the problem size by a constant factor increases the run time by at most a constant factor (depending on the algorithm) Why Polynomial Time? Generally, polynomial time seems to capture the algorithms which are efficient

in practice The class of polynomial time algorithms has many good, mathematical properties Polynomial vs. Exponential Complexity Suppose you have an algorithm which takes n! steps on a problem of size n If the algorithm takes one second for a problem of size 10, estimate the run time for the following problems sizes: 12 14

16 18 20 Ignoring constant factors Express run time as O(f(n)) Emphasize algorithms with slower growth rates Fundamental idea in the study of algorithms Basis of Tarjan/Hopcroft Turing Award

Why ignore constant factors? Constant factors are arbitrary Depend on the implementation Depend on the details of the model Determining the constant factors is tedious and provides little insight Why emphasize growth rates? The algorithm with the lower growth rate will be faster for all but a finite number of cases Performance is most important for larger

problem size As memory prices continue to fall, bigger problem sizes become feasible Improving growth rate often requires new techniques Formalizing growth rates T(n) is O(f(n)) [T : Z + R+] If n is sufficiently large, T(n) is bounded by a constant multiple of f(n) Exist c, n0, such that for n > n0, T(n) < c f(n)

T(n) is O(f(n)) will be written as: T(n) = O(f(n)) Be careful with this notation Prove 3n + 5n + 20 is O(n ) 2 Let c = Let n0 = T(n) is O(f(n)) if there exist c, n0, such that for n > n0, T(n) < c f(n)

2 Order the following functions in increasing order by their growth rate a) b) c) d) e) f) g) h) n log4n

2n2 + 10n 2n/100 1000n + log8 n n100 3n 1000 log10n n1/2 Lower bounds T(n) is W(f(n)) T(n) is at least a constant multiple of f(n) There exists an n0, and e > 0 such that T(n) > ef(n) for all n > n0

Warning: definitions of W vary T(n) is Q(f(n)) if T(n) is O(f(n)) and T(n) is W(f(n)) Useful Theorems If lim (f(n) / g(n)) = c for c > 0 then f(n) = Q(g(n)) If f(n) is O(g(n)) and g(n) is O(h(n)) then f(n) is O(h(n)) If f(n) is O(h(n)) and g(n) is O(h(n)) then f(n) + g(n) is O(h(n)) Ordering growth rates For b > 1 and x > 0

logbn is O(nx) For r > 1 and d > 0 nd is O(rn)