【正文】
rstanding the Application Tasks One of the oftenneglected steps on building software is really understanding the end user’ s jobthe that puter automation is intended to support. Occasionally, this is because the application itself is quite specialized。附錄 A Data Design 1. DATA TYPES AND VALUE SETS Anyone who has ever programmed a puter is accustomed to the fact that you usually have to tell the puter whether the information stored in a particular location is an integer, a real number, a character string, a Boolean va1ue, or whatever. Most puters have one electronic circuit for adding two integers and another for adding two real numbers, and, of course, addition does not make any sense at all, in general, for character strings. Because the pattern of bits in at storage location can usual1y be interpreted as any of these, the puter must be told what is intended. This specification is known as the type of the data. Identifying a data object as being of a particular type has the effect of defining an internal representation for the object. It also specifies which operations may be performed on that object and their effects. For example, the internal representation of an integer in most modem mainframe puters is a string of 32 bits in 239。s plement format. The external representation is a character string in which only the digits 0 to 9 and the minus sign may appear. Suitable interpretations of all the standard arithmetic and logical operations are also implied when a data object is declared to be of type is also true (but perhaps less obvious) that declaring a data object to be an integer defines a collating system for occurrences of that object. That is, it specifies how the relatively plex operation, sort, should operate. Specific algorithms for converting between interna1 a nd external representations are also implied. 2. KEYS All attributes describe some aspect of an entity. Some attributes perform the additional role of distinguishing one particular entity occurrence from all others of the same type. For example, if care is taken not to assign any employee number to more than one person, then knowing an individual39。 more often, it is becau