UNDERSTANDING DATA-STRUCTURES IN A SYSTEMS ANALYSIS AND DESIGN
COURSE
Mohammad A. Rob, University of Houston-Clear Lake
rob@cl.uh.edu
ABSTRACT
Understanding the role of data structures in the data flow diagram is important in
designing the essential components of an information system. Most Systems Analysis and Design
textbooks describe data structures in the data dictionary, and often fail to clarify how data
structures are associated with the various components of a data flow diagram. The purpose of
this paper is to present some illustrations that would not only enhance the understanding of data
structures associated with the data flow diagram, but would also clarify the role of data
structure in the design of the input, output, program modules, and data storage components of an
information system.
INTRODUCTION
Systems Analysis and Design (SAD) is a course that provides an understanding of the
concepts, skills, methodologies, techniques, and tools necessary to develop an information
system. Almost all SAD texts(Shelly, Cashman, & Rosenblatt, 2001; Kendall and Kendall,
2001; Dennis and Wixom, 2000) outline a systems development life cycle (SDLC), which is a
series of steps or activities that are performed in order to develop an information system. These
activities are organized into several phases, and most SAD texts are devoted to explain the
activities of the systems analysis and systems design phases of the SDLC.
The heart of a system is its data, which is entered through a system’s input, and is
subsequently stored in files or databases. Data is typically entered into a system through a set of
inter-related data, termed as data-structure or record. The input data structures are processed and
often broken into several sub-data structures, and subsequently, saved into appropriate files or
database tables. When creating an output, several data structures from various files or database
tables may be combined.
The data flow diagramming technique (Gane & Sarson, 1979) is commonly used in the
systems analysis phase of an SDLC to understand the business processes of a system. Once the
business processes are understood through various levels of data-flow diagrams, a data
dictionary is created that describes all data flows, data stores, processes, and external entities of
the data-flow diagrams. The data-flows and data stores in the data dictionary are further
expanded to describe the data structure associated with each of these components. Each data
structure is further expanded to describe each data element that comprises the data structure.
An understanding of data-structure and its flow through a system is important in
designing the inputs, programs, data storage, and output components of a system. Although the
1
data structures associated with these components are described in the data dictionary, the design
activities are based on the information contained in the data flow diagram. Furthermore, the data
flow diagram does not clarify how data-structures associated with several data-flows are
combined to form a data store, or how data structures from several data stores are combined to
form a data flow. Thus, most students in a SAD course fail to understand the role of data
structures with the input, data storage, program modules, and output design of a system.
Shelly et al. (2001) graphically illustrated the process of creation of data dictionary for
the four components of a data flow diagram. Kendall and Kendall (2001) used a similar diagram
to relate data dictionary with the data flow and data store components of a data flow diagram.
The later also presented a description of data structure using algebraic notations. However, both
texts fail to clarify the complex relationship of data structures associated with the data flow and
data store components of a data flow diagram.
In this paper, we present some graphical illustrations that can enhance the understanding
of data structures associated with the data flow diagram. They would also clarify the role of data
structures in the input, output, program modules, and data storage design of a system.
THE DATA FLOW DIAGRAM AND THE SYSTEM COMPONENTS
The data flow diagram uses several symbols to illustrate the flow of data through various
business processes. It is considered to be the essential tool of a systems development activities,
because it outlines the relationship between the business processes with the input, output, and
data storage components of an information system. Figure 1 illustrates a skeleton data flow
diagram that uses four traditional symbols, namely the external entity, the data flow, the process,
and the data storage. It also shows how various components of the data flow diagram are
conceptually related to the four components of a system. As shown, the input data flows are
related to the input components, the output data flows are related to the output components, the
processes are related to the program modules, and the data stores are related to the data storage
component of the information system.
THE DATA STRUCTURE AND THE DATA FLOW DIAGRAM
As it was mentioned before, the design of the system components is based on the data
structures associated with the various components of the data flow diagram. These data
structures are not visible in the data flow diagram, but they are typically described in a data
dictionary. This causes a complex relationship between the design aspects of a system to the data
flow diagram. Shelly et al. (2001) presented a chapter entitled “Transition to Systems Design,”
but it does not contain any discussion on the relationship between the analysis and design
activities. Dennis and Wixom (2000) presents a brief discussion on balancing the entity
relationship diagrams with the data flow diagrams, but it does not include all components of the
data flow diagram.
In a data flow diagram, each data flow and each data store typically carry a data structure.
In order to illustrate the role of the data structures in the data flow diagram, we present in
Figure2, two sections of a data flow diagram of an order processing system. Part (a) illustrates
2
two data flows (Customer Information and Customer Order) and their associated data structures
originated from an external entity named customer. As shown, each data structure is consisted of
several data elements such as CustomerNumber, CustomerName, CustomerAddress, City, State,
and ZipCode. As can be seen from Figure 1, these data structures are used to design the data
entry and update forms for the Customer Information and Customer Order.
Figure 2(b) illustrates the data structure associated with the data flow, Sales Report.
Again the data structure is consisted of several data elements such as Date, CustomerName,
ItemNumber, NoOfOrder, and Amount.
The illustrations presented in figure 2 extends the traditional data flow diagram to include
the data structures associated with the input and output data flows from and to the external
entities, respectively. These illustrations could be extended to include data structures associated
with other data flows and data store components of the data flow diagram. These data structures
are related to the design of the program modules and data storage components of the system.
CONCLUSION
In conclusion, we have presented some illustrations that can provide a better
understanding of the data structures associated with the various components of a data flow
diagram. We have also illustrated how the data structures are related to the design of the input,
output, program modules, and data storage components of an information system. These tools
can be considered as extensions to the traditional data flow diagram that is used to model a
business system.
REFERENCES
Shelly, G., Cashman, T., and Rosenblatt, H. (2001). Systems Analysis and Design. Boston, MA:
Course Technology.
Kendall, K., and Kendall, J. (2001). Systems Analysis and Design, (5th Ed.). Upper Saddle River,
NJ: Prentice Hall.
Dennis, A. and Wixom, B. H. (2000). Systems Analysis and Design: An Applied Approach. New
York, NY: John Wiley and Sons.
Gane, C. and Sarson, T. (1979). Structured Systems Analysis: Tools and Techniques. Englewood
Cliffs, NJ: Prentice-Hall.
3
4
5