3. Block and program structure

Abstract

So far, we haven’t learned of any possibilities of structuring a program. If we want to solve a larger problem with our current means, then we would soon come to thousands of statements and would find it difficult to know exactly that is going on in our program. Not to mention the hundreds of variables, which all occupy storage at the same time, although they were perhaps only being used successively. What can help us here? Well—humans often get control of complexity by hierarchical structuring. In programming we also speak of gradual refinement. In this chapter we will see that it makes sense to divide a problem into sub-problems, these sub-problems possibly again into sub-problems etc. The solution of such sub-problems in PL/I is performed via appropriate sub-programs, called procedures.