【正文】 ctive is to identify and link these key functions and data elements, yielding a logical system design. Start with the system39。s objective is determining what the user (in this case, the store39。t enough。 because the program is but one ponent in a system. A system is a group of ponents that work together to acplish an objective. For example, consider a payroll system. Its objective is paying employees. What ponents are involved? Each day， employees record their hours worked on time cards. At the end of each week, the time cards are collected and delivered to the puter center, where they are read into a payroll program. As it runs, the progra m accesses data files. Finally, the paychecks are printed and distributed. For the system to work, people, procedures, input and output media, files, hardware, and software must be carefully coordinated. Note that the program is but one ponent in a system. Computerbased systems are developed because people need information. Those people, called users, generally know what is required, but may lack the expertise to obtain it. Technical professionals, such as programmers, have the expertise, but may lack training in the user39。s owner) needs. Note that, as the process begins, the user possesses the critical information, and the analyst must listen and learn. Few users are technical experts. Most see the puter as a magic box, and are not concerned with how it works. At this stage, the analyst has no business even thinking about programs, files, and puter hardware, but must municate with the user on his or her own term. The idea is to ensure that both the user and the analyst are thinking about the same thingThus, a clear, written statement expressing the analyst39。s basic functions. The key is keeping track of the stockonhand for each product in inventory. Inventory changes because customers purchase, exchange, and return products, so the system will have to process customer transactions. The store39。 input data enter the system from a source, and output data flow to a destination. Once in the system, the data are manipulated or change by processes, represented by roundcorner rectangles. A process might be a program, a procedure, or anything else that changes or moves data. Data can be held for later processing in data stores, symbolized by openended rectangles. A data store might be a disk file, a tape file, a database, written notes, or even a person39。s major functions and data elements me thodically. Remember that the objective is translating user needs into technical terms. Since the system starts with the user, the first step is defining the user39。s understanding of the system requirements. By reviewing these documents, the user can correct misunderstandings or oversights. Finally, they represent an excellent starting point the next step, design. 23 Design As we enter the design stage, we know what the system must do, and thus can begin thinking about how to do it. The objective is to develop a strategy for solving the problem. At this stage, we are not interested in writing code or in defining precise data structures。 there are dozens of alternative solutions. Let39。s data structure can be planned, How should the file be anized? That depends on how it will be accessed. For example, in some applications, data are processed at regular, predictable intervals. Typically, the data are collected over time and processed together, as a batch. If batch processing is acceptable, a sequential file anization is probably best. It is not always possible to wait until a batch of transactions is collected, however. For example, consider an air defense early warning system. If an unidentified aircraft is spotted it must be identified immediately the idea of waiting until 5 _ 00 . because that39。s major ponents have been identified .we can begin to develop them. Our system includes two programs, several pieces of equipment, and a number of data structures. During implementation, each program is planned and written using the techniques described in Chapter 7. Files are created, and their contents checked. New hardware is purchased, installed, and tested. Additionally, operating procedures are written and evaluated. Once all the ponent parts are ready, the system is tested. Assuming the user is satisfied, the fini