Implementing Interactive Computing in an Object-Oriented Environment

Statistical computing when input/output is driven by a Graphical User Interface is considered. A proposal is made for automatic con- trol of computational flow to ensure that only strictly required com- putations are actually carried on. The computational flow is mod- eled by a directed graph for implementation in any object-oriented programming language with symbolic manipulation capabilities. A complete implementation example is presented to compute and dis- play frequency based piecewise linear density estimators such as his- tograms or frequency polygons. Controlling computation flow in classical programs is not a difficult task: some conditional or case statements would do the job in most cases. When user interaction is needed, the program prompts for it, then it waits for an answer, processes the response and produces output. If we consider statistical computing in a graphical user interface (GUI) environment, things are very different. The user can decide at any moment to change any of the quantities involved in the computation. (S)he can also decide on the desired output: what lines, graphs or windows are to be shown or not. In some cases, the user can decide to change a setting while an animation is running. It can be difficult to know in advance which of the intermediate results are really needed and exactly when they will be needed. Some results can be required by the system when refreshing a