ABSTRACT
Important advances in programming languages have occurred since the early days of assembly languages and op codes, and modern programming languages have significantly simplified the task of writing computer programs. Unfortunately, tools for understanding programs have not achieved the accompanying levels of improvement. Software developers still face difficulties in understanding, analyzing, debugging, and improving code. As an example of the difficulties still evident, consider a developer who has just imple-
mented a new algorithm and is now ready to examine the program’s behavior. After issuing a command to begin program execution, the developer examines output data and/or interactive behavior, attempting to ascertain if the program functioned correctly, and if not, the reasons for the program’s failure. This task can be laborious and usually requires the use of a debugger or perhaps even the addition of explicit checking code, usually through output statements. Furthermore, the process can be quite challenging and the average developer may not be that skilled in fault diagnosis and correction. It would be extremely advantageous to have a tool that allows programmers to “look inside” programs as they execute, examining the changes to data and data structures that occur over time. Such a tool also could provide helpful context by identifying the current execution line, the active program unit, and the set of activations that have occurred to reach this configuration. Tools, such as the one described above, are one form of software visualization [1]. Price,
Baecker, and Small describe software visualization as, “the use of the crafts of typography, graphic design, animation, and cinematography with modern human-computer interaction and computer graphics technology to facilitate both the human understanding and effective use of computer software [2].” Fundamentally, software visualization seeks to take advantage of the visual perception and pattern matching skills of people to assist software development and software understanding. Human beings have very sophisticated visual perception systems that we constantly use to help us think [3]. Software visualization is a
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subfield in the larger area of information visualization [4, 5] that studies how visualizations serve as external cognition aids. Software visualization research and systems fall into two primary categories, program
visualization and algorithm visualization. Program visualization systems present data about and attributes of some existing soft-
ware system. Typically, program visualizations are used in a software engineering context, that is, to help design, implement, optimize, test, or debug software. Thus, program visualization systems often illustrate source code, program data, execution statistics, program analysis information, or program objects such as data structures. The imagery in program visualization systems also is usually displayed with little or no input from the user/viewer. That is, a program (source code or executable) is analyzed by the system, which then automatically produces the graphics when the user requests them. Algorithm visualization, conversely, strives to capture the abstractions and semantics of
an algorithm or program primarily for pedagogical purposes. One often thinks of algorithm visualization as being more “high-level” as compared to program visualization being more “low level.” The term algorithm animation is used interchangeably for this area simply because the displays are so dynamic and algorithm operations appear to animate over time. More specifically, Brown states, “Algorithm animation displays are, essentially, dynamic displays showing a program’s fundamental operations-not merely its data or code [6].” Algorithm visualizations typically are hand-crafted, user-conceptualized views of what is “important” about a program. These types of views usually require some person, often the system developer or the program’s creator, design and implement the graphics that accompany the program. The highly abstract, artificial nature of algorithm animations makes their automatic generation exceptionally difficult. This chapter focuses on one specific subarea of software visualization, the visualization of
data structures. By displaying pictures of data structures, one seeks to help people better understand the characteristics and the use of those structures. How the data in computer programs are manipulated and organized by groups is a key
aspect of successful programming. Understanding how elements of the data relate to other elements is a vital component in being able to comprehend and create sophisticated algorithms and programs. The word “structure” in “data structure” simply reinforces this point. A person learning a new piece of software and seeking to modify it for some added func-
tionality will likely ask a number of different questions. How are data elements organized? Is some kind of sequential structure used or is a more complicated organization present? Which items reference which other items? Understanding the answers to questions such as these helps a person to understand the utility and value of data structures. Providing pictures to express the relationships of data elements clearly then is one of the most useful ways to answer those questions.
