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Hierarchical diagram and HIPO diagram

A hierarchical diagram is used to describe the hierarchical structure of the software. Although the form of the hierarchical diagram is the same as the hierarchical block diagram that depicts the data structure, the content is completely different. A rectangle in the hierarchical diagram represents a module, and the lines between the boxes represent the invocation relationship rather than the composition relationship like the hierarchical block diagram. The following figure is an example of a hierarchical diagram. the top box represents the main module of the text processing system, which calls the lower module to complete all the functions of text processing; each module of the second layer controls one of the main functions of text processing. For example, the "edit" module can complete any of the six editing functions by calling its subordinate modules.

Hierarchical diagrams are suitable for top-down software design.

HIPO diagram is the abbreviation of "hierarchical graph plus input / processing / output graph" invented by IBM Company in the United States. In order to make the HIPO diagram traceable, each box is numbered in the H graph (hierarchical diagram) except for the top box. As shown below:

Corresponding to each box in the H diagram, there should be an IPO diagram depicting the processing of the module represented by this box. The IPO diagram is described in more detail below. However, it should be pointed out that each IPO diagram in the HIPO diagram should clearly mark the number of the module it describes in the H diagram in order to track the location of the module in the software structure.

IPO diagram is the abbreviation of input, processing and output graph. It is a kind of graphic tool developed and perfected by American IBM company. It can easily describe the relationship between input data, data processing and output data.

IPO diagrams use few and simple basic symbols, so it's easy to learn to use this graphical tool. Its basic form is to list the relevant input data in the left box, the main processing in the middle box, and the resulting output data in the right box. The order of processing listed in the processing box implies the order of execution, but these basic symbols are not enough to accurately describe the details of the execution of the processing. Large arrows similar to vector symbols are also used in the IPO diagram to clearly indicate the situation of data communication. The following figure is an example of a master file update, through which it is not difficult to understand the use of IPO diagrams.

The editor suggests using an improved IPO diagram (also known as the IPO table), which contains some additional information, which will be more useful in the software design process than the original IPO diagram. The following figure shows the additional information contained in the improved IPO diagram, which mainly includes the system name, the author of the diagram, the date of completion, the name of the module described in this diagram, the number of the module in the hierarchical diagram, the list of modules that call this module, the list of modules called by this module, comments and local data elements used by this module, etc. In the requirements analysis phase, the IPO diagram can be used to briefly describe the main algorithms of the system (that is, the basic algorithms of each processing in the data flow diagram).

Of course, in the requirements analysis stage, a lot of additional information in IPO diagrams is not available yet, but in the software design stage, these diagrams can be further modified as documents in the design phase. This is the important advantage of using IPO diagram as a tool to describe the algorithm in the requirement analysis phase.

Excerpt from Software Engineering edited by Zhang Haifan

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