The paper analyzes the author's experience with modifying large, real-world programs written by other programmers. It finds that Brooks' domain and programming knowledge based hypothesis-test-refine paradigm explains the author's approach to understanding programs and the differences in performance in comparison with his students. Zvegintov's 9-step process of change is found to be a good first level decomposition of the (physical) tasks performed when making corrective changes to a software system.
The paper also makes some new observations. Besides modularity and levels
of abstractions, the organization of source code in hierarchy of directories also
has influence on the ease of locating code segments relevant to a change request.
The functionality of a program is not only understood from its documentation
but also by executing it and inferring relations between its inputs and outputs; an
approach analogous to concept identification. When introducing a new function in
an existing program, a programmer attempts to find sub problems that have been
solved by other parts of the program so as to mimic their solutions. Quite often
this means copying large code segments. However, when deleting a function,
the code implementing it is not destroyed, only execution paths leading to it are
disconnected; leaving behind dead-code. The replicated and dead code segments
are major contributors to the difficulty in understanding and modifying programs.
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