Data flow diagram, data dictionary

Data flow diagram

One 、 Basic graphic symbols

 

Two 、 Design principles ： We focus on three design principles of data flow graph . These three design principles are the magic weapon to solve problems .

1、 The balance principle of parent graph and child graph

The input and output data flow of the sub graph must be consistent with that of the corresponding processing of the parent graph , This is the balance between parent graph and child graph .

                                                                        chart 1, It does not conform to the balance principle of parent graph and child graph

                                                                            chart 2, Conform to the balance principle of parent graph and child graph

In the figure 1 in , We can see that there is an output stream in the parent graph ： Bill of lading . But there is no corresponding output stream in the subgraph .

2、 The principle of data conservation

       For any processing , The data in all output data streams must be directly obtained from the input data stream of the processing . Or the data that can be generated through the processing .
（1） There is no data flow between external entities

（2） There is no data flow between the external entity and the data store

（3） There is no data flow between the data store and the data store

3、 Conservative processing principle  

For the same process , The input and output names must be different . Even if their components are the same .
1. For each machining , There must be an existing input data stream . There is also an output data stream .

2. Data flow is related to processing , And must be processed .

For processing . Input is A, Output or A, It also violates the principle of data conservation , Input is the same as output , Processing has no effect .

           

For processing , Just input , No output , It violates the principle of data conservation .

            

For processing , Only output . No input , It violates the principle of data conservation . example . People cannot urinate and urinate all the time . But don't eat .
           

3、 ... and 、 After data flow processing , The relationship of data flow . Pictured ：

The data dictionary

        The data flow diagram describes the decomposition of the system . However, there is no description of the components in the figure . The data dictionary defines all named graphic elements appearing in the data flow graph as an entry in the data dictionary , Make the name of each graphic element have an exact explanation .

In the data dictionary 4 Types of entries ：
1、 Data item entry ： Usually the value type of the data item , Agreed value range, etc
2、 data flow ： Give the definition of a data flow , List the constituent data items of the data flow .
3、 File entry ： Definition of file . List the data items composed of periods
4、 Processing items ： Explain every process that cannot be decomposed . Including the excitation conditions of processing . The logic of processing , Priority, etc .

application

Let me look at a multiple-choice question , This question can fully reflect all the above principles . be the leading example of .   

      Let's analyze it . It's nothing more than those three principles ： According to these principles .DF2 It violates the principle of data conservation , There is no data flow between external entities ,DF6 It violates the principle of data conservation , There is no data flow between the external entity and the data store ,DF7 It violates the principle of data conservation . There is no data flow between the data store and the data store ;P1 and P3 Lack of data flow . It violates the principle of conservative processing ,P4 The input and output data flow is the same . It violates the principle of conservative processing . So jointly owned 6 A mistake .