Data flow diagram is soft to test one of the more important part of the test sites. Not only multiple-choice examination in the morning and afternoon to test a big problem.
So learning data flow diagram can not be ignored.
For data flow diagram, we do not do too much specific introduction, the Internet has so many rich resources ( Wikipedia ), some understanding of its origin, this is the key to learning.
Below we look at the data flow diagram showing the basic graphic symbols, design principles, application, and summary.
The basic graphic symbols
Design Principles
Three design principles we focus on the data flow graph. These three principles are designed to solve problems of magic.
(1) Parent balance principle subgraph of FIG.
Sub-picture data stream input and output corresponding to the same parent FIG processing input and output streams must be the same, namely the balance of the parent subgraph FIG.
(FIG. 1, the parent does not comply with the balance principle of FIG subgraph) (FIG. 2, in line with the principle of balance of the parent FIG subgraph)
In Figure 1, we can see the output stream Father figure: the bill of lading. But there is no sub-picture stream output corresponding thereto.
(2) Data principle of conservation
A process for whatever it is that all of the output data stream data to be processed from the input data stream is obtained directly. Or that the data processing can produce.
1. The stream does not exist between the external entities and external entities
2. stream does not exist between the external entities and data storage
3. The data stream does not exist between the data storage and data storage
(3) working principle of conservation
For the same process, the names of the input and output must not be the same. Even if they are the same constituent components.
1. For each of the processing must have both the input data stream. Another output data stream.
2. For the data flow and processing, and must be processed.
For processing. Input is A, output or A, also violates the principle of conservation of data, the same input and output processing has no effect. For example, people can not eat anything. It is the same the same.
For processing, only input, no output violation of the principle of conservation of data.
For example. People can not just eat. No toilet.
For processing, only the output. No input data in violation of the principle of conservation. For example. People can not always urine. But do not eat.
After processing, the relationship between the data streams. Figure:
Data Dictionary
数据流图描写叙述了系统的分解。但没有对图中各成分进行说明。数据字典是对数据流图中出现的全部被命名的图形元素在数据字典中作为一个词条加以定义，使每一个图形元素的名称都有一个确切的解释。
在数据字典中有4种类型的条目：
1、数据项条目：通常为数据项的值类型，同意的取值范围等
2、数据流条目：给出某个数据流的定义，列出该数据流的各组成数据项。
3、文件条目：对文件的定义。列出期组成的数据项
4、加工条目：对每一个不能再分解的加工做说明。包含加工的激发条件。加工的逻辑，优先级等等。
应用
以下我看一道选择题，这道题全然能体现上面全部的原则。最具代表性。
我们先分析一下。无非就是那三大原则：依据这些原则。DF2违背了数据守恒原则，外部实体与外部实体之间不存在数据流，DF6违背了数据守恒原则，外部实体与数据存储之间不存在数据流，DF7违背了数据守恒原则。数据存储与数据存储之间不存在数据流；P1和P3缺少数据流。违背了守恒加工原则，P4的输入输出数据流同样。违背了守恒加工原则。因此共同拥有6个错误。
总结
数据流图题，相对而言还是非常easy，仅仅要记得三大原则，相信你没有问题。最后祝大家明天考试顺利。取得优异的成绩。
