TAILIEUCHUNG - text algorithms VOL II

Technology has affected society and its surroundings in a number of ways. In many societies, technology has helped develop more advanced economies (including today's global economy) and has allowed the rise of a leisure class. Many technological processes produce unwanted by-products, known as pollution, and deplete natural resources, to the detriment of the Earth and its environment. Various implementations of technology influence the values of a society and new technology often raises new ethical questions. Examples include the rise of the notion of efficiency in terms of human productivity, a term originally applied only to machines, and the challenge of. | fllgorilhms ĩĩlaxime Crochemore UJo ịcỉecỉi Ryĩĩer 3 21 7 97 Chapter 1 1. Introduction . Texts and their processing One of the simplest and natural types of information representation is by means of written texts. Data to be processed often does not decompose into independent records. This type of data is characterized by the fact that it can be written down as a long sequence of characters. Such linear sequence is called a text. The texts are central in word processing systems which provide facilities for the manipulation of texts. Such systems usually process objects which are quite large. For example this book contains probably more than a million characters. Text algorithms occur in many areas of science and information processing. Many text editors and programming languages have facilities for processing texts. In biology text algorithms arise in the study of molecular sequences. The complexity of text algorithms is also one of the central and most studied problems in theoretical computer science. It could be said that it is the domain where the practice and theory are very close together. The basic textual problem is the problem called pattern matching. It is used to access information and probably many computers are solving in this moment this problem as a frequently used operation in some application system. Pattern-matching is comparable in this sense to sorting or to basic arithmetic operations. Consider the problem of a reader of the French dictionary Grand Larousse who wants all entries related to the word Marie-Curie-Sklodowska . This is an example of a pattern matching problem or string-matching. In this case the word Marie-Curie-Sklodowska is the pattern. Generally we may want to find a string called a pattern of length m inside a text of length n where n is greater than m. The pattern can be described in a more complex way to denote a set of strings and not only a single word. In many cases m is very large. In genetics the pattern can correspond

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