Biologically Inspired Computing- Optimisation.ppt

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1、Biologically Inspired Computing: Optimisation,This is mandatory additional material for Biologically Inspired Computing Contents: Optimisation; hard problems and easy problems; computational complexity; trial and error search;,This Material,A formal statement about optimisation, and explanation of t

2、he close relationship with classification.About optimisation problems: formal notions for hard problems and easy ones.Formal notions about optimisation algorithms: exact algorithms and approximate algorithms.The status of EAs (and other bio-inspired optimisation algorithms) in this context.The class

3、ical computing alternatives to EAs,What to take from this material:,A clear understanding of what an optimisation problem is, and an appreciation of how classification problems are also optimisation problems What it means, technically, for an optimisation problem to be hard or easy What it means, te

4、chnically, for an optimisation algorithm to be exact or approximate What kinds of algorithms, technically speaking, EAs are. An appreciation of non-EA techniques for optimisation A basic understanding of when an EA might be applicable to a problem, and when it might not.,Search and Optimisation,Imag

5、ine we have 4 items as follows: (item 1: 20kg; item2: 75kg; item 3: 60kg, item4: 35kg) Suppose we want to find the subset of items with highest total weight,Here is a standard treatment of this as an optimisation problem. The set S of all possible solutions is indicated above, The fitness of a solut

6、ion s is the function total_weight(s) We can solve the problem (I.e. find the fittest s) by working out the fitness of each one in turn. But any thoughts? I mean, how hard is this?,0000 0100 1000 1100 0001 0101 1001 1101 0010 0110 1010 1110 0011 0111 1011 1111,Of course, this problem is much easier

7、to solve than that. We already know that 1111 has to be the best solution. We can prove, mathematically, that any problem of the form “find heaviest subset of a set of k things”, is solved by the “all of them” subset, as long as each thing has positive weight. In general, some problems are easy in t

8、his sense, in that there is an easily provable way to construct an optimal solution. Sometimes it is less obvious than in this case though.,Search and Optimisation,In general, optimisation means that you are trying to find the best solution you can (usually in a short time) to a given problem.,S,s1,

9、s2,s3,We always have a set S of all possiblesolutions,In realistic problems, S is too large to search one by one. So we need to find some other way to search through S.One way is random search. E.g. in a 500-iteration random search, we might randomly choose something in S and evaluate its fitness, r

10、epeating that 500 times.,The Fitness function,Every candidate solution s in the set of all solutions S can be given a score, or a “fitness”, by a so-called fitness function. We usually write f(s) to indicate the fitness of solution s. Obviously, we want to find the s in S which has the best score.Ex

11、amplestimetabling: f could be no. of clashes.wing design: f could be aerodynamic dragdelivery schedule f will be total distance travelled,An Aside about Classification,In a classification problem, we have a set of things to classify, and a number of possible classes. To classify s we use an algorith

12、m called a classifier. So, classifier(s) gives us a class label for s. We can assign a fitness value to a classifier this can be simply the percentage of examples it gets right. In finding a good classifier, we are solving the following optimisation problem: Search a space of classifiers, and find t

13、he one that gives the best accuracy. E.g. the classifier might be a neural network, and we may use an EA to evolve the NN with the best connection weights.,Searching through S,When S is small (e.g. 10, 100, or only 1,000,000 or so items), we can simply do so-called exhaustive search.Exhaustive searc

14、h: Generate every possible solution, work out its fitness, and hence discover which is best (or which set share the best fitness)This is also called Enumeration,However ,In all interesting/important cases, S is much much much too large for exhaustive search (ever). There are two kinds of too-big pro

15、blem:easy (or tractable, or in P)hard (or intractable, or not known to be in P) There are rigorous mathematical definitions of the two types thats what the P is about it stands for the class of problems that can be solved by a deterministic polynomial Turing machine. Thats outside the scope of this

16、module. But, whats important for you to know is that almost all important problems are technically hard.,About Optimisation Problems,To solve a problem means to find an optimal solution. I.e. to deliver an element of s whose fitness is guaranteed to be the best in S.An Exact algorithm is one which c

17、an do this (i.e. solve a problem, guaranteeing to find the best). Is 500-iteration random search an Exact algorithm?,Problem complexity,Problem complexity, in the context of computer science, is all about characterising how hard it is to solve a given problem. Statements are made in terms of functio

18、ns of n, which is meant to be some indication of the size of the problem. E.g.:Correctly sort a set of n numbers Can be done in around n log n stepsFind the closest pair out of n vectors Can be done in O(n2) stepsFind best design for an n-structural-element bridge Can be done in O(10n) steps ,Polyno

19、mial and Exponential Complexity,Given some problem Q, with size n, imagine that A is the fastest algorithm known for solving that problem exactly. The complexity of problem Q is the time it takes A to solve it, as a function of n. There are two key kinds of complexity:Polynomial: the dominant term i

20、n the expression is polynomial in n. E.g. n34, n.log.n, sin(n2.2), etc Exponential: the dominant term is exponential in n. E.g. 1.1n, nn+2 , 2n, ,Polynomial and Exponential Complexity,n,2,3,4,5,6,10,20,50,100,Problems with exponential complexity take too long to solve at large n Note how an exponent

21、ial always eventually catches up, and then rapidly overtakes, a polynomial.,Hard and Easy Problems,Polynomial Complexity: these are called tractable, and easy problems. Fast algorithms are known which provide the best solution. Pairwise alignment is one such problem. Sorting is another.Exponential C

22、omplexity: these are called intractable, and hard problems. The fastest known algorithm which exactly solves it is usually not significantly faster than exhaustive search.,polynomial,exponential,Increasing n,An exponential curve always takes over a polynomial one.,E.g. time needed on fastest compute

23、rs to search all protein structures with 500 amino acids: trillions of times longer than the current age of the universe.,Example: Minimum Spanning Tree Problems,This is a case of an easy problem, but not as obvious as our first easy example.Find the cheapest tree which connects all (i.e. spans) the

24、 nodes of a given graph.Applications: Comms network backbone design; Electricity distribution networks, water distribution networks, etc ,A graph, showing the costs of building each pair-to-pair link,7,12,4,9,6,8,5,14,6,3,What is the minimal-cost spanning tree? (Spanning Tree = visits all nodes, has

25、 no cycles;cost is sum of costs of edges used in the tree),Heres one tree:,7,12,14,3,With cost = 36,Heres a cheaper one,7,4,6,3,With cost 20 ,The problem find the minimal cost spanning tree (aka the MST) is easy in the technical sense.,7,12,4,9,6,8,5,14,6,3,Several fast algorithms are known which so

26、lve this in polynomial time; Here is the classic one: Prims algorithm:Start with empty tree (no edges)Repeat: choose cheapest edge which feasibly extends the tree Until: n 1 edges have been chosen.,Prims step 1:,7,12,4,9,6,8,5,14,6,3,Prims step 2:,7,12,4,9,6,8,5,14,6,3,Prims step 3:,7,12,4,9,6,8,5,1

27、4,6,3,Prims step 4:,7,12,4,9,6,8,5,14,6,3,Prims step 4:,4,5,6,3,Guaranteed to have minimal possible cost for this graph; i.e. this is the (or a) MST in this case.,But change the problem slightly: We may want the degree constrained MST (I.e. the MST, but where no node in the tree has a degree above 4

28、) Or we may want the optimal communication spanning tree which is the MST, but constrained among those trees which satisfy certain bandwith requirements between certain pairs of nodes There are many constrained/different forms of the MST. These are essentially problems where we seek the cheapest tre

29、e structure, but where many, or even most, trees are not actually feasible solutions. Heres the thing: These constrained versions are almost always technically hard. and Real-world MST-style problems are invariably of this kind.,Approximate Algorithms,For hard optimisation problems (again, which tur

30、ns out to be nearly all the important ones), we needApproximate algorithms . These:deliver solutions in reasonable timetry to find pretty good (near optimal) solutions, and often get optimal ones.do not (cannot) guarantee that they havedelivered the optimal solution.,Typical Performance of Approxima

31、te Methods,Evolutionary Algorithms turn out to be the most successful and generally useful approximate algorithms around. They often take a Long time though its worth getting used to the following curve which tends to apply across the board.,Time,Quality,Simple method gets good solutions fast,Sophis

32、ticated method, slow, but better solutions eventually,So ,Most real world problems that we need to solve are hard problems We cant expect truly optimal solutions for these, so we use approximate algorithms and do as well as we can. EAs (and other BIC methods we will see) are very successful approximate algorithms,