How to Design Programs: An Introduction to Programming and Computing

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How to Design Programs: An Introduction to Computing and Programming [Go to first , previous , next page; contents ; index ] Section 34 Memory for Functions [../icons/plt.gif] Advanced Student No matter how often we use a function with one and the same argument, we always get the same result. Even an accumulator-style function produces the same result every time we apply it to the same argument, as long as the accumulator argument is also the same. Functions simply do not have any memory about their past uses. Many programs, though, must remember something about their past uses. Recall that a program typically consists of several functions. In the past we have always assumed that there is one main function and that all others are auxiliary and invisible to the user. In some cases, however, a user may expect more than one service from a program, and each service is best implemented as a function. When a program provides more than one function as a service to the user, it is common that, for sheer convenince or possibly because we add a graphical user interface, the functions must have memory. Because this point is difficult to grasp in the abstract, we study some examples. The first one concerns a program for managing telephone numbers in an address book. The standard address book software provides at least two services: 1. a service for looking up the phone number of some person; and 2. a service for adding a name and a phone number to the address book. Based on our guidelines, the program provides two functions to the user. The user can apply those functions in DrScheme's Interactions window to appropriate data. Or, we can develop a graphical user interface with text fields and buttons so that the user doesn't need to know anything about programming. Figure 95 displays such an interface. (1 of 5) [2/5/2008 4:56:33 PM]
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