数据结构.ppt
CHAPTER 2ALGORITHM ANALYSIS,【Definition】An algorithm is a finite set of instructions that,if followed,accomplishes a particular task.In addition,all algorithms must satisfy the following criteria:(1)Input There are zero or more quantities that are externally supplied.(2)Output At least one quantity is produced.(3)Definiteness Each instruction is clear and unambiguous.(4)Finiteness If we trace out the instructions of an algorithm,then for all cases,the algorithm terminates after finite number of steps.(5)Effectiveness Every instruction must be basic enough to be carried out,in principle,by a person using only pencil and paper.It is not enough that each operation be definite as in(3);it also must be feasible.,1/15,Note:A program is written in some programming language,and does not have to be finite(e.g.an operation system).An algorithm can be described by human languages,flow charts,some programming languages,or pseudo-code.,Example Selection Sort:Sort a set of n 1 integers in increasing order.,From those integers that are currently unsorted,find the smallest and place it next in the sorted list.,Where and how are they stored?,Where?,for(i=0;i n;i+)Examine listi to listn1 and suppose that the smallest integer is at listmin;Interchange listi and listmin;,Sort=Find the smallest integer+Interchange it with listi.,Algorithm in pseudo-code,2/15,1 What to Analyze,Machine&compiler-dependent run times.Time&space complexities:machine&compiler-independent.,Assumptions:instructions are executed sequentially each instruction is simple,and takes exactly one time unit integer size is fixed and we have infinite memory,Typically the following two functions are analyzed:Tavg(N)&Tworst(N)-the average and worst case time complexities,respectively,as functions of input size N.,If there is more than one input,these functions may have more than one argument.,3/15,1 What to Analyze,Example Matrix addition,/*rows+1*/,/*rows(cols+1)*/,/*rows cols*/,T(rows,cols)=2 rows cols+2rows+1,Q:What shall we do if rows cols?,A:Exchange rows and cols.,4/15,ExampleIterative function for summing a list of numbers,/*count=1*/,/*count+*/,/*count+for last execution of for*/,/*count+*/,/*count+*/,Tsum(n)=2n+3,ExampleRecursive function for summing a list of numbers,/*count+*/,/*count+*/,/*count+*/,Trsum(n)=2n+2But it takes more time to compute each step.,1 What to Analyze,5/15,1 What to Analyze,Is it really necessary to count the exact number of steps?,Uhhh.I dont think so.,Why not?,Because it drives me crazy!,So its too complicated sometimes.But does it worth the effort?,Take the iterative and recursive programs for summing a list for example-if you think 2n+2 is less than 2n+3,try a large n and youll be surprised!,I see.Then whats the point of this Tp stuff?,Good question!Lets ask the students.,6/15,2 Asymptotic Notation(,o),The point of counting the steps is to predict the growth in run time as the N change,and thereby compare the time complexities of two programs.So what we really want to know is the asymptotic behavior of Tp.,Suppose Tp1(N)=c1N2+c2N and Tp2(N)=c3N.Which one is faster?,No matter what c1,c2,and c3 are,there will be an n0 such that Tp1(N)Tp2(N)for all N n0.,I see!So as long as I know that Tp1 is about N2 and Tp2 is about N,then for sufficiently large N,P2 will be faster!,7/15,2 Asymptotic Notation,【Definition】T(N)=O(f(N)if there are positive constants c and n0 such that T(N)c f(N)for all N n0.,【Definition】T(N)=(g(N)if there are positive constants c and n0 such that T(N)c g(N)for all N n0.,【Definition】T(N)=(h(N)if and only if T(N)=O(h(N)and T(N)=(h(N).,Note:2N+3=O(N)=O(Nk1)=O(2N)=We shall always take the smallest f(N).2N+N2=(2N)=(N2)=(N)=(1)=We shall always take the largest g(N).,【Definition】T(N)=o(p(N)if T(N)=O(p(N)and T(N)(p(N).,8/15,2 Asymptotic Notation,Rules of Asymptotic Notation,If T1(N)=O(f(N)and T2(N)=O(g(N),then(a)T1(N)+T2(N)=max(O(f(N),O(g(N),(b)T1(N)*T2(N)=O(f(N)*g(N).,If T(N)is a polynomial of degree k,then T(N)=(N k).,logk N=O(N)for any constant k.This tells us that logarithms grow very slowly.,Note:When compare the complexities of two programs asymptotically,make sure that N is sufficiently large.For example,suppose that Tp1(N)=106N and Tp2(N)=N2.Although it seems that(N2)grows faster than(N),but if N 106,P2 is still faster than P1.,9/15,2 Asymptotic Notation,10/15,11/15,2 Asymptotic Notation,n,12/15,Example Matrix addition,/*(rows)*/,/*(rows cols)*/,/*(rows cols)*/,T(rows,cols)=(rows cols),2 Asymptotic Notation,13/15,2 Asymptotic Notation,General Rules,FOR LOOPS:The running time of a for loop is at most the running time of the statements inside the for loop(including tests)times the number of iterations.,NESTED FOR LOOPS:The total running time of a statement inside a group of nested loops is the running time of the statements multiplied by the product of the sizes of all the for loops.,CONSECUTIVE STATEMENTS:These just add(which means that the maximum is the one that counts).,IF/ELSE:For the fragmentif(Condition)S1;else S2;the running time is never more than the running time of the test plus the larger of the running time of S1 and S2.,14/15,2 Asymptotic Notation,RECURSIONS:,Example Fibonacci number:Fib(0)=Fib(1)=1,Fib(n)=Fib(n1)+Fib(n2),long int Fib(int N)if(N=1)return 1;else return Fib(N 1)+Fib(N 2);,/*O(1)*/,/*O(1)*/,/*O(1)*/,/*T(N)*/,/*T(N 1)*/,/*T(N 2)*/,T(N)=T(N 1)+T(N 2)+2,Fib(N),Proof by induction,Q:Why is it so bad?,15/15,
