An Example of Program Understanding

Abstract

What does it mean to understand a program? What sorts of questions can be answered about a program? What background knowledge is required to answer them? What tools can help the process? To answer questions like these, we will look at an example of program understanding in action. Imagine the following scenario: You are assigned responsibility for maintaining a program you have never seen before. It is written in the FORTRAN language and is concerned with finding the roots of function. We will assume that you know the FORTRAN language but are not an expert at it. That is, you still have to occasionally look at the reference manual to answer questions about the language. We will also assume that you have a computer science background, either through formal education or by on-the-job experience, so that you know how to design and compose programs similar to the one you are about to maintain. Finally, we will assume that you have a passing acquaintance with numerical analysis, possibly from a course you took. Hence, you are familiar with the idea of finding a root of a function, but you would have to look up an actual algorithm in order to write a root-finding program yourself. As you are responsible for long-term maintenance of the program, you want to understand it better. So you decide to systematically read it. This is distinct from the situation where you have a specific task to accomplish, such as finding a bug, adding a new feature, or updating the program to conform to a change in the language or operating environment. In these cases, instead of systematic reading, you might direct your efforts to accomplishing the specific task. Here, we will assume that you are going to make a single, sequential pass through the program text, with perhaps a few side trips to answer small questions as they arise. The following program text is taken directly from its source [1]. We have added three digits of line numbers in the left margin for expository purposes; they are not part of the program itself. We will annotate each line in the program as we come to it. The idea is to express some idea of what that line is there for. Sometimes we will raise questions, sometimes we will generate hypotheses that need to be confirmed, and sometimes we will speculate about the application domain. The intent is to get a feel for the kinds of knowledge required to understand a program on a line-by-line basis.