Английский язык для пользователей ПК и программистов - Гольцова Е.В.
ISBN 5-7931-0086-5
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Упражнение 8. Give synonyms for the following: Подберите синонимы к следующим словам:
to decline; rapidly; cost; to compress; frequent; expansion; occur; to remove; inexact; layout; error; normally.
Упражнение 9. Be sure that you remember what the words mean: Убедитесь, что вы помните, что означают эти служебные слова:
although; almost; always; still; therefore; thus; usually; that is; then; others; however; with; without.
Упражнение 10. Read the text I very carefully. Прочтите Текст I очень внимательно. Подчеркните все наречия в тексте и переведите. Обратите внимание на их место в предложении.
Textl
COMPRESSION
Although the cost of a byte of storage has declined rapidly, and is still declining, use of data compression techniques can almost always reduce the effective cost still further by squeezing more data into the same space. Consider a text archive or collection of documents. It may be advantageous to hold it in compressed form to save
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space if access to a particular document is infrequent (therefore expansion of it is performed rarely) but the document may be required quickly(thus it needs to be online) . Compression could also save time (and money) when data is transmitted; for example, compression of source code might reduce the number of diskettes needed to distribute software. In this chapter we examine some methods for compressing text. Lelewer and Hirschberg cover most of them, and others, in their survey.
Data compression relies on there being redundancy in the input. Random strings of characters are not compressible to any great degree, neither are object files. Natural language text is redundant in that not all text units (characters, character pairs, words) occur with equal frequency. Compression tends to remove the redundancy, thus compression of a compressed file is normally not worthwhile. Usually compression without loss of information is required, that is, the input file should be exactly recoverable by application of some corresponding expansion technique. In some cases, an inexact reversal may be acceptable. For example, when expanded, a source program in a free format language may not need to have exactly the same layout as the original.
There are many ways of measuring the degree of compression achieved, the following is a useful one:
length(input) - length (output) - size (X) length (input)
X is any information that we need in addition to the compressed text in order to be able to recreate the original. For example, if the original file is 2000 byte long and is compressed to 1000 bytes, and a 100-byte table is required to expand the file back again, then the degree of compression is 45%. Because of overheads, the "compressed" version of a short file might be larger than the original. In general, compression techniques operate by mapping sections of the input file onto (smaller) sections of the output file. We can classify techniques by the type of the input object replaced (fixed or variable length) and the type of out-put object (fixed or variable length). In the following sections we look at the four categories of methods this leads to. Additional characteristics of a compression method are whether the mapping is adaptive (varies as the input is Английский для пользователей ПК
processed) or static and whether the compression requires one pass or more than one pass over the input file.
Compression is not without disadvantages: reduced redundancy makes a file more vulnerable to storage and transmission errors. Lelewer and Hirschberg discuss the susceptibility to error of the output of various algorithms. They note, for example, that the output from adaptive algorithms is more vulnerable than the output from static algorithms but even when a static algorithm is used, an error can propagate through an entire file.
Упражнение 11. Translate the questions and answer them.
Переведите вопросы и ответьте на них, используя предыдущий Текст (I). Обратите особое внимание на перевод сказуемых (действий).
Что может сократить «стоимость» байта памяти?
На чем основано сжатие информации?
Почему мы говорим, что естественный язык избыточен?
Должен ли входной файл быть точно восстанавливаемым?
В каких случаях программе-источнику нет необходимости иметь то
же расположение, что и в оригинале?
Когда и почему сжатый файл может быть больше, чем оригинал? Как вообще осуществляется техника сжатия? Как мы можем классифицировать технику сжатия? Каковы недостатки сжатия?
Упражнение 12. Match the pairs of words: Сопоставьте пары слов:
A. adequate 1. ясно
B. to tit into 2. локализовать
C. inter-office memos 3. двоичное значение
D. suppose 4. достаточно мал, чтобы
E. allocate 5. одновременно
F. at a time 6. отображать
G. binary value 7. 2/3 своего обычного размера
H. is just small enough 8. в большей части текста
I. two-thirds its normal size 9. для более часто встречающихся
секций
J. thus 10. итак, таким образом
