Program Design Notation

In this chapter we introduce a vehicle for expressing the structure and content of programs as we design them using the process of stepwise refinement. The notation for doing this may be called an abstract programming language because programs written in the notation are not intended to be run directly on a computer, they have to be coded into a real programming language. An abstract programming language performs the same role that in earlier times flowcharting was supposed to do; it is an informal notation to be used during program design. The abstract language used here is an informal extension of the language Pascal. It should be easily readable by anyone familiar with Pascal or any other Algol-like language. The notation is of necessity informal and is introduced by explaining features as and when they occur in the exposition. The essence of structure is organisation. The two dynamically related parts of a program — flow of control and accessing of data — must be organised in a simple and consistent manner. The structuring concepts applicable to each are considered in turn.