【導讀】語言情況的考查。本JAVA語言試題庫是一個簡單的應用軟件。Access建立的數(shù)據(jù)庫，用VisualBasic集成開發(fā)環(huán)境開發(fā)完成。特點博得了廣大軟件開發(fā)者的厚愛。VisualBasic語言是可視化的Basic語言，在。VB已逐漸成為簡單易學、功能強大的編程工具。本試題庫包含了：試題的輸入和瀏覽、選題、試題導出四個大的功能模塊。在本論文中詳細介紹了如何設計本試題庫。第一章是緒論，介紹了本課題研究的意義、背景等。第二章介紹了Access和VisualBasic的相關知識。第三章用流程圖、結構圖和數(shù)據(jù)字典描述了試題庫的總體設計。第四章是制作界面的詳細設計。

　　

【正文】 d one after another. An example is a weekly payroll application where employee records are processed sequentially. A second type of file anization is called indexed file anization. Just as in a sequential file, records are stored in an indexed file in an ascending or descending sequence based on the value in the key field of the record. An indexed file, however, also has an index which itself is a file. An index consists of a list containing the values of a key field and the corresponding disk address for each record in a file. In the same way that an index for a book points to the page where a particular topic is covered, the index for a file points to the place on a disk where a particular record is located. The index is updated each time a record is added to or deleted from the file. The index is retrieved from the disk and placed in main memory when the file is to be processed. Record can be accessed in an indexed file both sequentially and randomly. As we previously discussed, sequential retrieval means that the records in a file are retrieved one record after another in the same order that the records are stored. Random retrieval, also called random access or direct access, means the system can go directly to a record without having to read the preceding records. For example, with sequential retrieval, to read the fiftieth record in a file, record 1 through 49 would be read first. With random retrieval, the system can go directly to the fiftieth record. To directly access a record in an indexed file, the index is searched until the key of the record to be retrieved is fond. The address of record (also stored in the index)is then used to retrieve the record directly from the file without reading any other record. An advantage of indexed files is that usually more than one index can be maintained. Each index can be used to access or report records in a particular order. For example, an employee file might have three separate indexes, one for employee number, a second for employee name, and the third for social security number. A disadvantage of indexed file is that searching an index for a record in a large file can take a long time. In addition, maintaining one or more indexes adds to the processing time whenever a record is added or deleted. 西安工業(yè)學院 學士學位論文 第 31 頁 共 51 頁 A direct file or relative file(sometimes called a random file)uses the key value of a record to determine the location on the disk where the record is or will be stored. For example, a program could establish a file that has nine locations where records can be stored. These locations are sometimes called buckets. A bucket can contain multiple records. If the key in the record is a onedigit, then the value in the key would specify the relative location within the file where the record was stored. For example, the record with key 3 would be placed in relative location, or bucket,3,the record with key 6 would b placed in relative locating 6,and so on. Usually, the storage of records in a file is not so simple. For instance, what if the maximum number of records to be stored in a direct file is 100 and the key for the record is a fourdigit number? In this case, the key of the record could not be used to specify the relative or actual location of the record because the fourdigit key could result in up to 9, cases such as these, an arithmetic formula is used to calculate the relative or actual location in the file where the record is stored. The process of using a formula and performing the calculation to determine the location of a record is called hashing. One hashing method is the division/remainder method. With this method, the puter uses a prime number close to, but not greater than the number of records to be stored in the file. A prime number is a number divisible by only itself and example, suppose you have 100 records. the number 97 is the closest prime number to 100 without being greater than key of the record is then divided by 97 and the remainder from the division operation is the relative location where the record is stored. For example, if the record key is 3428,the relative location where the record will be stored in the file is location 33(Figure ). Direct files present one problem you do not encounter with sequential or indexed files. In all three file anization methods, the key in the record must be unique so that it can uniquely identify the record. For example, the employee number, when acting as the key in an employee file, must b unique. Not two employees can have the same number. When a hashing technique is used to calculate a disk address, however, it is possible that two different keys could identify the same location on disk. 西安工業(yè)學院 學士學位論文 第 32 頁 共 51 頁 For example ,employee number 3331 generates the same identify the same relative location(33)as employee number the locations generated from the different key are the same, they are called synonyms. The occurrence of this event is called a collision. A method that is often used to resolve collisions is to place the record that caused the collision in the next available storage location. This location may be in the same bucket(if multiple records are stored in a bucket)or in the next bucket. Once a record is stored in its relative location within a direct file, it can be retrieved either randomly or sequentially. The method normally used with direct files is random retrieved from a direct file by performing three steps. 1. The program obtains the key of the record to be retrieved. The value of the key, such as a part number, is entered by the user or is read from another file record such as a sales invoice. 2. The program determines the location of the record to be retrieved by performing the same hashing process as when the record was initially stored. Thus, to retrieve the recor