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著名的BM字符串匹配算法

ISAM enhances the functionality of your programs through its
flexibility. If you add a section to a book, remove a few pages,
or rearrange paragraphs or sections, you have to recreate your
index, since the keywords must appear in relation to each other.
In this case, the relationship of the keywords to each other is
alphabetic order. A ISAM index changes automatian employee, or
change anything in a record, ISAM immediately updates all indexes.
You can create an index on any field, on several fields to be used
together, or on parts thereof, that you want to use as a key. The
keys in indexes allow you quick access to specific records and define
orders for sequential processing of a ISAM file. After you no longer
need an index, you can delete it. Addition and indexes have no effect
on the data records or on other indexes.
You may want a field in field in each record to uniquely identify that
record from all other records in the file. For example, the Employee
Number field is unique if you do not assign the same number to two
different employees, and you never reassign these numbers to other
employees. If you wish to find or modify the record belonging to a
specific employee, this unique field saves the thouble of determining
whether you have the correct record.
If you do not have a unique field, you must find the first record
the matches your key and determine whether the record is the one you
want. If it is not the correct one, you must search again to find others.
If you know that you have a unique field within your records, you
can include this fact in the key description, and ISAM will allow only
unique keys. For example, if you specify that the employee numbers are
unique, ISAM only lets you add records to the file for, or change
numbers to, employee numbers that do not alreadly exist int file.