《计算机科学导论》课件Unit 11Data and File Structure.pptVIP

Sequential files Records in a sequential file【顺序文件】 can only be read and written in sequence. To access a specific record within the file, the program must successively retrieve the previous records. An EOF (end-of-file) marker is put after the last record. Sequential files are useful in applications which need to access all records in sequence from beginning to end. For example, if personal information about each employee in a company is stored in a file, you can use sequential access【顺序访问】 to retrieve each record at the end of month to print the paychecks. In this case, sequential access is more efficient and easier than random access【随机访问】. However, indexed files are very inefficient for random access of large data files Indexed files A key【键】 is the unique identifier of data (or record) in the file which is made up of one or more fields. Sequential files may contain a key, but it is not used to locate the address of records. However, the address of the record should be known when accessing a record in a file randomly. An indexed file【索引文件】 contains records ordered by a uniquely identifiable key. For example, an account number might be the key for customer’s record in a bank. An indexed file is made up of a sequential data file and an index. The index itself is a very small file with only two fields: the key of the sequential file and the address of the corresponding record on the disk. Figure 11.17 shows the logical view of an indexed file. Indexed files Figure 11.17 Logical view of an indexed file Inverted file: One of the advantages of indexed file is that it can use alternate indexes, each with a different key. For example, an employee file can be retrieved through employee department or last name rather than employee number. This type of indexed file is said to be an inverted file. Hashed files The index is used to map the key to the address in an indexed file. The hashed file【哈希文件】 does not need an extra file (index). It uses mathematical fun