  Program tracing, or mentally simulating a program on concrete inputs, is an
important part of general program comprehension. Programs involve many kinds of
virtual state that must be held in memory, such as variable/value pairs and a
call stack. In this work, we examine the influence of short-term working memory
(WM) on a person's ability to remember program state during tracing. We first
confirm that previous findings in cognitive psychology transfer to the
programming domain: people can keep about 7 variable/value pairs in WM, and
people will accidentally swap associations between variables due to WM load. We
use a restricted focus viewing interface to further analyze the strategies
people use to trace through programs, and the relationship of tracing strategy
to WM. Given a straight-line program, we find half of our participants traced a
program from the top-down line-by-line (linearly), and the other half start at
the bottom and trace upward based on data dependencies (on-demand).
Participants with an on-demand strategy made more WM errors while tracing
straight-line code than with a linear strategy, but the two strategies
contained an equal number of WM errors when tracing code with functions. We
conclude with the implications of these findings for the design of programming
tools: first, programs should be analyzed to identify and refactor
human-memory-intensive sections of code. Second, programming environments
should interactively visualize variable metadata to reduce WM load in
accordance with a person's tracing strategy. Third, tools for program
comprehension should enable externalizing program state while tracing.
