NOS : Certificate in Computer Applications
| Home | Table of Contents |

Up: Online Course Material

Graphical representation of any process is always better and more meaningful than its representation in words. Moreover, it is very difficult to arrange and organise the large amount of data into meaningful interpretation of the whole. System Analysis and Design makes use of the various tools for representing and facilitating comprehension of the complex processes and procedure involved. In this lesson, we present some details about Flowcharts, data flow diagram (DFD), Decision Tables and Decision Trees.

After going through this lesson you should be able to

• Draw flowchart
• Represent any physical system through DFD
• Prepare decision table
• Display decision tree

The pictorial representation of the programs or the algorithm is known as flowcharts. It is nothing but a diagrammatic representation of the various steps involved in designing a system. Some of the boxes which are used in flowcharts are:

A flowchart consists of a set of ‘flowchart symbols’ connected by arrows. Each symbol contains information about what must be done at that point & the arrow shows the ‘flow of execution’ of the algorithm i.e. they show the order in which the instructions must be executed. The purpose of using flowcharts is to graphically present the logical flow of data in the system and defining major phases of processing along with the various media to be used.

Flowcharts are of three types:

• System flowcharts
• Run flowcharts
• Program flowcharts

System flowchart describes the data flow for a data processing system. It provides a logical diagram of how the system operates. It represents the flow of documents, the operations performed in data processing system. It also reflects the relationship between inputs, processing and outputs. Following are the features of system flowcharts:

• the sources from which data is generated and device used for this purpose
• various processing steps involved
• the intermediate and final output prepared and the devices used for their storage

Figure 30.1 is a sample of system flowchart for the following algorithm:

1. Prompt the user for the centigrade temperature.
2. Store the value in C
3. Set F to 32+(9´ C/5)
4. Print the value of C , F
5. Stop

Figure: 30.1 System Flowchart

Run flowcharts are used to represent the logical relationship of computer routines along with inputs, master files, transaction files and outputs. Figure30. 2 illustrates a run flowchart.

Figure: 30.2 Run Flowchart

A program flowchart represents, in detail, the various steps to be performed within the system for transforming the input into output. The various steps are logical/ arithmetic operations, algorithms etc. It serves as the basis for discussions and communication between the system analysts and the programmers. Program flowcharts are quite helpful to programmers in organising their programming efforts. These flowcharts constitute an important component of documentation for an application.

Figure 30.3 represents a program flowchart for finding the sum of first five natural numbers ( i.e. 1,2,3,4,5).

Fig 30.3 Program Flowchart

Data flow diagrams are the most commonly used way of documenting the process of current & required systems. As their name suggests they are a pictorial way of showing the flow of data into, around & out of a system.

Graphical representation of a system’s data and how the processes transform the data is known as Data Flow Diagram (or DFD). Unlike, flowcharts, DFDs do not give detailed descriptions of modules but graphically describe a system’s data and how the data interact with the system.

DFDs are constructed using four major components

• external entries
• data stores
• processes and
• data flows

(i) External Entities

External entities represent the source of data as input to the system. They are also the destination of system data. External entities can be called data stores out side the system. These are represented by squares.

(ii) Data Stores

Data stores represent stores of data within the system. Examples, computer files or databases. An open-ended box represents a data/store – data at rest or a temporary repository of data.

(iii) Process

Process represents activities in which data is manipulated by being stored or retrieved or transferred in some way. In other words we can say that process transforms the input data into output data. Circles stand for a process that converts data into information.

(iv) Data Flows

Data flows represents the movement of data from one component to the other. An arrow identifies data flow – data in motion. It is a pipeline through which information flows... Data flows are generally shown as one-way only. Data Flows between external entities are shown as dotted lines.

Consider the figure30.4. It is clear from the figure that orders are placed, orders are received, the location of ordered parts is determined and delivery notes are dispatched along with the order.

It does not however tell us how these things are done or who does them. Are they done by computers or manually and if manually who does them ? A logical DFD of any information system is one that models what occurs without showing how it occurs.

A physical DFD shows, how the various functions are performed? Who does them? Consider the following figure:

The figure 30.5 is opposite, it shows the actual devices that perform the functions. Thus there is an "order processing clerk", an "entry into computer file" process and a "run locate program" process to locate the parts ordered. DFD(s) that shows how things happen, or the physical components are called physical DFD(s).

Typical processes that appear in physical DFDs are methods of data entry, specific data transfer or processing methods.

The program flowchart describes boxes that describe computations, decisions, interactions & loops. It is an important to keep in mind that data flow diagrams are not program flowcharts and should not include control elements . A good DFD should

• have no data flows that split up into a number of other data flows
• have no crossing lines
• not include flowchart loops of control elements
• not include data flows that act as signals to activate processes.

30.4 DECISION TABLES AND DECISION TREES

Decision tables and trees were developed long before the widespread use of computers. They not only isolate many conditions and possible actions but they help ensure that nothing has been overlooked.

The decision table is a chart with four sections listing all the logical conditions and actions. In addition the top section allows space for title, date, author, system and comment as shown in the fig.30.6

Five sections of a decision table:

The condition stub contains a list of all the necessary tests in a decision table. In the lower left-hand corner of the decision table we find the action stub where one may note all the processes desired in a given module. Thus Action Stub contains a list of all the processes involved in a decision table.

The upper right corner provides the space for the condition entry - all possible permutations of yes and no responses related to the condition stub. The yes and no possibilities are arranged as a vertical column called rules. Rules are numbered 1,2,3 and so on. We can determine the rules in a decision table by the formula:

Number of rules = 2^N = 2^N where N represents the number of condition and ^ means exponentiate. Thus a decision table with four conditions has 16 (2⁴ = 2 x 2 x 2 x 2 = 16) rules one with six conditions has 64 rules and eight conditions yield 256 rules.

The Condition entry contains a list of all the yes/no permutations in a decision table. The lower right corner holds the action entry. X’s or dots indicate whether an action should occur as a consequence of the yes/no entries under condition entry. X’s indicate action; dots indicate no action.

Thus Action entry indicates via dot or X whether something should happen in a decision table. Let us consider the following example of book order illustrated by figure 30.7

If order is from book store

And if order is for 6 copies

Then discount is 25%

Else (if order is for less then 6 copies)

No discount is allowed

Else (if order is from libraries)

If order is for 50 copies or more

Then discount is 15%

Else if order is for 20 to 49 copies

Then discount is 10%

Else if order is for 6 to 19 copies

Then discount is 5%

Else (order is for less then 6 copies)

No discount is allowed

A decision table for the above process is illustrated below

Figure 30.7: Decision Table

The decision tree defines the conditions as a sequence of left to right tests. A decision tree helps to show the paths that are possible in a design following an action or decision by the user. Figure 30.8 illustrates the concept of decision tree.

Figure 30.8: Decision Tree

Decision tree turns a decision table into a diagram. This tool is read from left to right, decision results in a fork, and all branches end with an outcome. Figure 6 illustrates the decision tree for the book order decision table we saw earlier.

Figure 30.9: Decision Tree for Book Order

Various specification tools and techniques of system analysis and designing were discussed in detailed. The various tools such as flowcharts, data flow diagrams, decision tables & decision trees were explained. These tools and techniques are used when the system under study involves the development of computer based information system.

1. What is flow chart ?
2. Explain different types of flow charts.
3. Define DFD. Explain different components of DFD.
4. Explain decision table and decision tree.

Top