System Analysis and Design
System Implementation
- Mr. Ahmad Al-Ghoul
learning Objectives

explain a structure chart showing
top-down design, modular design,
cohesion, and coupling
Avicenna
System Analysis and Design
System Implemantation
2
Structured Application Development

Structure Charts



Avicenna
Structure charts show the program modules
and the relationships among them
Control module: In a structure chart, a control
module is a higher-level module that directs
lower-level modules, called subordinate
modules.
Subordinate modules: A lower-level module in
a structure chart.
System Analysis and Design
System Implemantation
3
Structured Application Development
Structure Charts
 Module
 A self-contained component of a system, defined by a
function
 One single coordinating module at the root of structure chart
 Single point of entry and exit
 Communicate with each other by passing parameters

Avicenna
library module: In a structure chart, a library module is a module that is
reusable and can be invoked from more than one point in the chart.
System Analysis and Design
System Implemantation
4
Structured Application Development
Data and control passed between structure chart
modules is either a:


Data couple, A diagrammatic representation
of the data exchanged between two
modules in a structure chart
 passing only data, shown as an arrow
with an empty circle.
Control couple


shown as an arrow with a filled-in circle.
In a structure chart, a control couple shows a
message, also called a flag, which one module sends
to another.

Avicenna
A module uses a flag to signal a specific condition or action
to another module
System Analysis and Design
System Implemantation
5
Structured Application
Development


Control modules make the decisions about which
lower-level modules should be executed.
Lower-level modules are functional, performing only
one task.
Systems analysts should keep the number of
couples to a minimum.

Avicenna
The fewer data couples and control flags one has in
the system, the easier it is to change the system.
System Analysis and Design
System Implemantation
6
Structured Application Development
The update customer file module sends an
account overdue flag back to the maintain
customer data module
An example of a structure chart control couple.
Avicenna
System Analysis and Design
System Implemantation
The look up customer
name module
exchanges data with
the maintain customer
data module
An example of a structure
chart data couple.
7
Structured Application
Development

Structure Charts – Notation
Modules
Library modules
Module call
Data
Flag
Avicenna
System Analysis and Design
System Implemantation
8
Structured Application Development
Structure Charts
 Condition:




A line with diamond on one end presents a condition
A specified action or state in a structure chart.
A condition line indicates that a control module determines which
subordinate modules will be invoked, depending on a specific condition
Sort inventory parts is a control module with a condition line that
triggers one of the three subordinate modules
An example of a structure chart condition and condition lines.
Avicenna
System Analysis and Design
System Implemantation
9
Structured Application Development
Structure Charts
 Loop:



A curved arrow represents a loop
In a structure chart, a loop indicates that one or more
modules are repeated.
The get student grades and calculate GPA modules
are repeated for each student
An example of a structure chart loop.
Avicenna
System Analysis and Design
System Implemantation
10
Structured Application Development

In order to choose among alternatives when
dividing systems into modules, it useful to
evaluate the connection between them. If there
are few or no connections between modules,
then it is easier to understand one module
without reference to others. The notion of
module independence can be described in terms
of ‘coupling’ and ‘cohesiveness’. These concepts
were introduced by Edward Yourdon and Larry
Constantine who are concerned with ‘goodness’
of design.
Avicenna
System Analysis and Design
System Implemantation
11
Structured Application Development




Coupling: Coupling measures relationships and
interdependence among modules.
Coupling is the strength of relationships between
modules (the degree to which modules are
interconnected with or related to one another).
The stronger the coupling between modules in a system,
the more difficult it is to implement and maintain the
system, because a modification to one module will then
necessitate careful study, as well as possible changes
and modifications, to one or more other modules.
In practice, this means that each module should have
simple, clean interface with other modules, and that the
minimum number of data elements should be shared
between modules.
Avicenna
System Analysis and Design
System Implemantation
12
Structured Application Development
Loosely coupled: Modules that are
relatively independent. Loosely coupled
modules are easier to maintain and
modify, because the logic in one module
does not affect other modules.
 Tightly coupled: If modules are tightly
coupled, one module refers to internal
logic contained in another module.

Avicenna
System Analysis and Design
System Implemantation
13
Structured Application Development





This figure shows two examples of
coupling
In the tightly coupled example, the
subordinate module, Calculate
Current Charges, depends on a
status flag that is receives from
control module, Update Customer
Balance.
The loosely coupled modules on the
left are logically independent.
Rather than passing flag down to
the subordinate module, the control
module allows the Apply Discount
module to handle discount
processing independently.
Logic errors, if any, are confined to
the Apply Discount module, where
they can be detected and resolvedAn example of loosely coupled and
more easily
tightly coupled structure charts.
Avicenna
System Analysis and Design
System Implemantation
14
Structured Application Development

Cohesion: Cohesion measures a module's scope and
processing characteristics.





Avicenna
Cohesion is the measure of the strength among the elements in
the same module (the degree to which the components of a
module are necessary and sufficient to carry out one, single,
well defined function).
In practice, this means that the systems designer must ensure
that they does not split essential processes into fragmented
modules and the systems designer must ensure that they does
not gather together unrelated processes (represented as
processes on the DFD) into meaningless modules.
The best modules are those that are functionally cohesive.
The worst modules are those that are coincidentally cohesive.
High cohesiveness occurs when all of the module parts
contribute directly to the purpose or function which the module
is supposed to accomplish.
 If you need to make a module more cohesive, you can split it
into separate units, each of which performs a single function
System Analysis and Design
System Implemantation
15
Structured Application Development


In the figure below the task called Check Customer
Status actually consists of two separate tasks: Check
Customer Number and Check Customer Credit Limit.
Notice that the figure with one module is less cohesive
than the figure with the two modules, because it
combines the two tasks into a single module, while the
other figure treats them as separate modules, you
achieve more cohesion and better program quality
An example of structure chart cohesion.
Avicenna
System Analysis and Design
System Implemantation
16
Structured Application
Development

advantages
Structure Charts:




Modularity improves system maintainability
Provides a means for transition from analysis to design
Provides a synchronous hierarchy of modules
disadvantages
Structure Charts:


Avicenna
Does not work well for asynchronous processes such
as networks
Could be too large to be effectively understood with
large programs.
System Analysis and Design
System Implemantation
17
Sequence Summary










A structure chart consists of symbols that represent program
modules, data couples, control couples, conditions, and loops
A rectangle represents a program module, which consists of
program code that accomplishes a specific function
An arrow with an empty circle represents a data couple, which
shows data that one module passes to anther
An arrow with a filled circle represents a control couple, which
shows a message, also called a flag
A line with a diamond on one end represents a condition, which
indicates a control module that determines which subordinate
module are invoked, depending on a specific condition
A curved arrow represents a loop, which indicates that one or more
modules are repeated
Cohesion measures a module’s scope and processing characteristics
A module that performs a single function or task has a high degree
of cohesion, which is desirable
Coupling measures relationships and interdependence among
modules
Modules that are relatively independent are loosely coupled, which
is desirable
Avicenna
System Analysis and Design
System Implemantation
18
Sequence Summary

In this Sequence we have





Avicenna
Defined the terms structure chart, control
module, Subordinate module, library module,
Data couple, control couple, condition, and loop
Described a structure charts – notation
Explained a structured application development
explained a structure charts, cohesion, and
coupling
Described a structure charts advantages and
disadvantages
System Analysis and Design
System Implemantation
19
Reference
[1] System Analysis and Design, Sixth
Edition
Authors: Gary B. Shelly, Thomas J.
Cashman and Harry J. Rosenblatt
Publisher: SHELLY CASHMAN SEWIES.
Avicenna
System Analysis and Design
System Implemantation
20