1. CAPE COMPUTER SCIENCE UNIT 2
DATA FLOW DIAGRAMS
CAPE COMPUTER SCIENCE UNIT 2
T.Batchelor in collaboration with Y.C.LeMard

2. Introduction
Data input
process
Information output
We’ve learnt that a computer takes in data, processes it, and produces information as output

3. We also know that the processing is really done by a program.
Introduction
We also know that the processing is really done by a program.
Therefore a program (or a set of programs) receives data, does something to it, and produces information for the user
process
= program

4. And hopefully this is what your I.A. program will do too.
Introduction
If you think about it carefully you will acknowledge that this is what all the programs you have written up to this point do.
And hopefully this is what your I.A. program will do too.

5. Introduction
Data input
PROGRAM
Information output

6. It is an essential systems analysis and design tool
Data Flow Diagrams
A Data flow Diagram (DFD) is a pictorial representation of this flow of data through a computer system.
It is an essential systems analysis and design tool

7. DFDs
Here is an example of a Data Flow Diagram

8. DFDs
Here is another example

9. Luckily our DFDs will not need to be so complex.
But from these examples we can isolate the components of a DFD

10. Data Flow Diagrams have 4 symbols
DFDs
Data Flow Diagrams have 4 symbols
Sharp cornered, shadowed rectangles
Rounded cornered rectangles
Opened sided rectangles
Arrows

11. 1. External Entities
The sharp cornered rectangles (or simply boxes) in a DFD indicate entities.
Entities are people, things, organizations, etc outside of the system.
Entities are the source of data and the destination of data.
External Entity

12. Entities
Doctor
Cashier
Student
Patient
Teacher
User
Customer
Manager

13. As you will have guessed, all external entities must be labeled.
It must be obvious too that every system must have at least one entity
Ms LeMard

14. 2. Processes
The rounded cornered rectangles in a DFD indicate processes.
Processes are the changes that data go through causing it to be transformed gradually into information.
Process
Every computer program is a process

15. Patient Administration system
Processes
Shipping system
Accounting system
Reservation system
Grading system
Marketing system
Patient Administration system

16. Just like entities all processes must have a name.
Medical Expert System
Also every computer system has at least one process

17. This is a generally accepted sign for a data store (file)
File Name
This is a generally accepted sign for a data store (file)

18. Data Store Employees’ Data Students’ Grades Order Details
Accounts Receivable File

19. A ‘data store’ is a secondary storage device which holds a file.
Most meaningful systems store permanent data in one or more master files. Transactions files are also used in batch processing systems

20. Not every system has a file however, some are totally interactive.
Data Store
Not every system has a file however, some are totally interactive.
In addition very simple DFDs [ Context Level DFDs ] do not include any data stores

21. 4. Data
Data
The arrows in a DFD show the data. Data must come from somewhere and go to somewhere else. The direction of the arrow indicates this.
All arrows must be labelled to show what particular piece of data is being referred to.

22. Data
Name
Date
Grade
Amount
Symptom

23. FYI Some texts use a circle for the process
Some texts also use a different sign for the file depending on the type of media on which it is stored.
Process

24. We will stick to the symbols below …

25. The Top-Down approach is taken to the design of DFDs.
Types of DFD
The Top-Down approach is taken to the design of DFDs.
Data flow diagrams are therefore classified by levels. Each subsequent level indicates greater details.

26. Levels
Level 0 DFD
The Level 0 DFD (also known as the Context Level DFD) is the simplest DFD. This diagram shows a single process and all external entities which interact with this process, and the data flowing to and from the process.

27. Context Level DFD for a Mail Order Business
Order Processing
Customer
Delivery

28. Decomposition
Level 1 DFDs
A Context Level DFD is broken down (exploded) into a set of processes, the data moving between these processes, the entities interacting with these processes and the data stores.

29. Level 1 DFD for a Mail Order Business
Sales processing
Customer
Order
Delivery
Order
Credit Status
Order
Customer Database
Orders
Credit Status
Accounting System
Customer number

30. Decomposition Level 2 and other Level DFDs
Each process from Level 1 is exploded even more into sub processes. This decomposition continues for each subsequent level. The number of levels possible depends on the complexity of the system.

31. The Value of a DFD
With a dataflow diagram, users are able to visualize how the system will operate, what the system will accomplish, and how the system will be implemented.
Dataflow diagrams can be used to provide the end user with a physical idea of how the data they input ultimately has an effect upon the structure of the whole system.

32. The Value of a DFD
The old system's dataflow diagrams can also be drawn up and compared with the new system's dataflow diagrams to draw comparisons in order to help implement a more efficient system.

33. Context Level DFD for a Payroll System
Supervisor
Hours worked
Payroll System
Personal Data
Employee
Pay cheque

34. Level 1 DFD for a Payroll System
Batching system
Personnel
system
Supervisor
Hours
worked
Employee personal data
Hours worked
Personal Data
Employee Database
Accounting data
Employee record
Employee
Accounting details
Accounting details
Salary
Generation
System
Pay cheques

35. How to Create A Level 0 Data Flow Diagram
Identify your main system’s focus
Identify the external people/ organisations who interact with the system
Decide what data these entities will enter into the system
Determine what these entities expect as output from the system

36. How to Create A Level 1 Data Flow Diagram
Focus on your process and break it into 2 or more sub-processes
Identify what data flows between these processes and between the entities
Identify what permanent data files are used in this system.
Note that no new entities can be introduced

37. BEWARE Data can not flow directly from file to file.
Data can not flow directly from entity to entity
No process can have no input data yet have output data, that would be a miracle.
No process can have input data yet have no output data, that would be a black hole.

38. Your Questions ?

39. Questions What is the purpose of a data flow diagram?
What 4 symbols are used in a DFD?
Which symbols must appear on every DFD?
Every process must have data going in and data coming out. Why?

40. Exercise
1. Draw a Level 0 Data Flow Diagram for the following situation:
A travel agency takes calls from clients, checks seat availability on the flights they desire and makes a reservation for them if all is clear.
2. Explode your Context Level diagram into a Level 1 DFD

41. ~~ T h e E N D ~~