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

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

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

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.

Introduction Data input PROGRAM Information output

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

DFDs Here is an example of a Data Flow Diagram

DFDs Here is another example

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

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

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

Entities Doctor Cashier Student Patient Teacher User Customer Manager

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

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

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

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

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

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

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

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

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.

Data Name Date Grade Amount Symptom

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

We will stick to the symbols below …

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.

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.

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

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.

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

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.

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.

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.

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

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

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

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

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.

Your Questions ?

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?

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

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