4 - Arrays

Summary

In this chapter, we extend the simple imperative language and the methods for reasoning about its programs to include one-dimensional arrays with integer subscripts. Although more elaborate and varied forms of arrays are provided by many programming languages, such simple arrays are enough to demonstrate the basic semantical and logical properties of arrays.

There are two complementary ways to think about arrays. In the older view, which was first made explicit in early work on semantics by Christopher Strachey, an array variable is something that one can apply to an integer (called a subscript) to obtain an “array element” (in Strachey's terminology, an “L-value”), which in turn can be either evaluated, to obtain a value, or assigned, to alter the state of the computation. In the newer view, which is largely due to Hoare but has roots in the work of McCarthy, an array variable, like an ordinary variable, has a value — but this value is a function mapping subscripts into ordinary values. Strachey's view is essential for languages that are rich enough that arrays can share elements. But for the simple imperative language, and especially for the kind of reasoning about programs developed in the previous chapter, Hoare's view is much more straightforward.

Abstract Syntax

Clearly, array variables are a different type of variable than the integer variables used in previous chapters.