isbn9789526046266

Class discussion and learning materials can relate new programming constructs to what is needed
to execute a program that uses those constructs. For instance, the topic of frames can be approached
through the observation that any executor of programs needs to keep track of the variables (among other
things) that pertain to function or method calls. The role of the visualization (and VPS) is to illustrate
what techniques the computer uses for this purpose as it runs programs.
Example selection is key to the success of VPS exercises and program visualization in general. Using
simple programs as examples allows cognitive load to be managed while singling out specific aspects of
execution. However, minimal examples alone are often not enough to draw attention to all the critical
variation and to justify the complexity of the execution model presented. For instance, if all the variables
in the visualized example programs have different names, one of the main reasons why the computer uses
multiple frames to keep track of state will not be readily appreciable (and requiring students to create
frames in VPS may seem at least equally pointless).
Another example. The order in which parameters are evaluated may seem to the learner largely an
arbitrary choice of the VPS designer (cf. the ‘red branch’ of our outcome space). If all the parameter
expressions are simply literals, variable names, and arithmetical expressions with no calls and no ‘side
effects’, then evaluation order might indeed not matter. It is important to also visualize example programs
where order does matter. This can lead to a discussion of how the computer needs (and the programming
language provides) an unambiguous definition of how to execute the program. Students can be invited to
ponder interactively whether or not an alternative order or simpler usage of visual elements could work.
(Students may also volunteer such alternatives, as Beth did; see p. 288.) Through such discussions,
students can be led to conclude that the visualization is a way of illustrating the specific principles that
the computer follows so that it can work on the general case, and not just a particular simple kind of
program.
Chapter 18 explores what students do during VPS sessions. We will have more to say there on the
kinds of pedagogy that can encourage students to discern and engage with the conceptual content of
VPS.
17.6.2 Teachers should help students see a purpose to VPS
To encourage transfer from VPS to programming, teachers must help students experience a connection
between computer behavior and useful programming activities.
An important starting point is the way VPS is initially described to students. VPS should be introduced
as a tool for helping the student learn about programming concepts by looking at how the computer deals
with programs. Teaching should stress the idea that the visualization gives an execution-time perspective
on programs, and that adopting such a perspective is frequently useful to the programmer as they reason
about programs.
Novices often focus strongly on the programming language they use. From the start, it should be
stressed that VPS can help them understand the language and therefore how they can use the language
in their own programs.
Some teachers – the author of this thesis, for one – like to explain to their students some of the
pedagogical strategies behind course design, occasionally with reference to learning theory. In the case of
VPS, students could be told that the interactive nature of the visualization is something that is intended
to encourage them to pay careful attention to the visualization and reflect on it, so that they will become
better programmers.
Above all, teachers should go to some trouble to demonstrate explicitly and concretely how
understanding program visualizations can help the students read and write program code. For students
to experience the usefulness of learning about the execution model, they need to be placed in situations
where program animations and VPS exercises interact with other programming tasks in a meaningful
way. In-class activities and open labs can both be designed to emphasize the way the visualization can
help answer pertinent questions. Here are some examples of potentially useful practices (not all of which
297