Introduction to the Analysis of Algorithms
Mathematical studies of the properties of computer algorithms have spanned a broad spectrum, from general complexity studies to specific analytic results. In this chapter, our intent is to provide perspective on various approaches to studying algorithms, to place our field of study into context among related fields and to set the stage for the rest of the book. To this end, we illustrate concepts within a fundamental and representative problem domain: the study of sorting algorithms.
First, we will consider the general motivations for algorithmic analysis. Why analyze an algorithm? What are the benefits of doing so? How can we simplify the process? Next, we discuss the theory of algorithms and consider as an example mergesort, an “optimal” algorithm for sorting. Following that, we examine the major components of a full analysis for a sorting algorithm of fundamental practical importance, quicksort. This includes the study of various improvements to the basic quicksort algorithm, as well as some examples illustrating how the analysis can help one adjust parameters to improve performance.
These examples illustrate a clear need for a background in certain areas of discrete mathematics. In Chapters 2 through 4, we introduce recurrences, generating functions, and asymptotics—basic mathematical concepts needed for the analysis of algorithms. In Chapter 5, we introduce the symbolic method, a formal treatment that ties together much of this book’s content. In Chapters 6 through 9, we consider basic combinatorial properties of fundamental algorithms and data structures. Since there is a close relationship between fundamental methods used in computer science and classical mathematical analysis, we simultaneously consider some introductory material from both areas in this book.
1.1. Why Analyze an Algorithm?
There are several answers to this basic question, depending on one’s frame of reference: the intended use of the algorithm, the importance of the algorithm in relationship to others from both practical and theoretical standpoints, the difficulty of analysis, and the accuracy and precision of the required answer.
The most straightforward reason for analyzing an algorithm is to discover its characteristics in order to evaluate its suitability for various applications or compare it with other algorithms for the same application. The characteristics of interest are most often the primary resources of time and space, particularly time. Put simply, we want to know how long an implementation of a particular algorithm will run on a particular computer, and how much space it will require. We generally strive to keep the analysis independent of particular implementations—we concentrate instead on obtaining results for essential characteristics of the algorithm that can be used to derive precise estimates of true resource requirements on various actual machines.
In practice, achieving independence between an algorithm and characteristics of its implementation can be difficult to arrange. The quality of the implementation and properties of compilers, machine architecture, and other major facets of the programming environment have dramatic effects on performance. We must be cognizant of such effects to be sure the results of analysis are useful. On the other hand, in some cases, analysis of an algorithm can help identify ways for it to take full advantage of the programming environment.
Occasionally, some property other than time or space is of interest, and the focus of the analysis changes accordingly. For example, an algorithm on a mobile device might be studied to determine the effect upon battery life, or an algorithm for a numerical problem might be studied to determine how accurate an answer it can provide. Also, it is sometimes appropriate to address multiple resources in the analysis. For example, an algorithm that uses a large amount of memory may use much less time than an algorithm that gets by with very little memory. Indeed, one prime motivation for doing a careful analysis is to provide accurate information to help in making proper tradeoff decisions in such situations.
The term analysis of algorithms has been used to describe two quite different general approaches to putting the study of the performance of computer programs on a scientific basis. We consider these two in turn.
The first, popularized by Aho, Hopcroft, and Ullman  and Cormen, Leiserson, Rivest, and Stein , concentrates on determining the growth of the worst-case performance of the algorithm (an “upper bound”). A prime goal in such analyses is to determine which algorithms are optimal in the sense that a matching “lower bound” can be proved on the worst-case performance of any algorithm for the same problem. We use the term theory of algorithms to refer to this type of analysis. It is a special case of computational complexity, the general study of relationships between problems, algorithms, languages, and machines. The emergence of the theory of algorithms unleashed an Age of Design where multitudes of new algorithms with ever-improving worst-case performance bounds have been developed for multitudes of important problems. To establish the practical utility of such algorithms, however, more detailed analysis is needed, perhaps using the tools described in this book.
The second approach to the analysis of algorithms, popularized by Knuth , concentrates on precise characterizations of the best-case, worst-case, and average-case performance of algorithms, using a methodology that can be refined to produce increasingly precise answers when desired. A prime goal in such analyses is to be able to accurately predict the performance characteristics of particular algorithms when run on particular computers, in order to be able to predict resource usage, set parameters, and compare algorithms. This approach is scientific: we build mathematical models to describe the performance of real-world algorithm implementations, then use these models to develop hypotheses that we validate through experimentation.
We may view both these approaches as necessary stages in the design and analysis of efficient algorithms. When faced with a new algorithm to solve a new problem, we are interested in developing a rough idea of how well it might be expected to perform and how it might compare to other algorithms for the same problem, even the best possible. The theory of algorithms can provide this. However, so much precision is typically sacrificed in such an analysis that it provides little specific information that would allow us to predict performance for an actual implementation or to properly compare one algorithm to another. To be able to do so, we need details on the implementation, the computer to be used, and, as we see in this book, mathematical properties of the structures manipulated by the algorithm. The theory of algorithms may be viewed as the first step in an ongoing process of developing a more refined, more accurate analysis; we prefer to use the term analysis of algorithms to refer to the whole process, with the goal of providing answers with as much accuracy as necessary.
The analysis of an algorithm can help us understand it better, and can suggest informed improvements. The more complicated the algorithm, the more difficult the analysis. But it is not unusual for an algorithm to become simpler and more elegant during the analysis process. More important, the careful scrutiny required for proper analysis often leads to better and more efficient implementation on particular computers. Analysis requires a far more complete understanding of an algorithm that can inform the process of producing a working implementation. Indeed, when the results of analytic and empirical studies agree, we become strongly convinced of the validity of the algorithm as well as of the correctness of the process of analysis.
Some algorithms are worth analyzing because their analyses can add to the body of mathematical tools available. Such algorithms may be of limited practical interest but may have properties similar to algorithms of practical interest so that understanding them may help to understand more important methods in the future. Other algorithms (some of intense practical interest, some of little or no such value) have a complex performance structure with properties of independent mathematical interest. The dynamic element brought to combinatorial problems by the analysis of algorithms leads to challenging, interesting mathematical problems that extend the reach of classical combinatorics to help shed light on properties of computer programs.
To bring these ideas into clearer focus, we next consider in detail some classical results first from the viewpoint of the theory of algorithms and then from the scientific viewpoint that we develop in this book. As a running example to illustrate the different perspectives, we study sorting algorithms, which rearrange a list to put it in numerical, alphabetic, or other order. Sorting is an important practical problem that remains the object of widespread study because it plays a central role in many applications.