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Example research essay topic: Second Step Data Flow - 1,057 words

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... n informational cluster) will be drawn as follows: The boxes at the top are labeled with the names of each of the interface functions that can be used by the rest of the system to access (or change) the data area that is now shown below them. In a ''draft'' or ''first cut''s structured chart, you would draw arrows from each one of the modules (in the rest of the system) that could access the data area, down to the top horizontal line in the representation of the data area. In the ''completed''s structured chart, each of these modules (from the rest of the system) must call one or more of the ''interface'' functions - so you'd draw the arrows down to the box (or boxes) for the interface function (or interface functions) that the module calls. It doesn't ever make sense to show an arrow going up from a data area or informational cluster to another module in the system, because (it's generally the case) that the ''data area'' is passive, and can't call (or, therefore, ''control'') anything else. You can also use this ''informational cluster'' notation to group together a set of ''interface functions'' that collectively provide access to an I/O device, so that the part of the system that communicates directly with that device - and would need to be modified if that device was replaced - are grouped together, and easy to find.

Unfortunately, you won't find any reference to these ''informational clusters'' in Pressman's books; they " re included (in Chapter 7) in Page-Jones' book on Structured Design. I believe they " re also a recent addition to the notation and the method, representing a good idea that's been borrowed from object-oriented development - namely, a good extension of this older design technique to facilitate ''information hiding'' (and combine it better with ''data design''). The structure chart also shows data that can be passed between modules (or between modules and interface functions for data areas or I/O devices). Two symbols are used for data items - one for ''regular'' data items, and another for ''control signals'' - which you can think of as corresponding to someone pushing a button (and which would be drawn on augmented data flow diagrams using dashed arrows, as previously described. A ''regular'' item is drawn using a short arrow, with a hollow circle at the one end of the arrow (and the arrow head at the other).

This is labeled by the name of the data item. A ''control signal'' is drawn using almost the same symbol: The circle at the end of the arrow is filled in, rather than hollow. It is labeled by the name of the control signal. The data flows are drawn close to ("'flowing along'') the connections between modules, or between modules and interface functions, and each point from the module or function sending the data to the module or function receiving it. Each of these connections between modules corresponds to a (possible) procedure call - so you'd generally have data flows down to the called module corresponding to each of the inputs provided by the module calling it, and you'd have data flows back up from the called modules corresponding to each of the outputs that the called module might return. For example, the following diagram shows that the module ''Register Student'' can call the module ''Get ISBN Number, '' and that ''Get ISBN Number'' can return a control signal, ''cancel, '' and a ''regular'' data item, ''ID number, '' as input. (Yes, this example is nonsensical; the diagram will be fixed as soon as time permits! ) The structure chart does not generally include anything that corresponds directly to the ''Terminators'' on data flow diagrams - that is, to the people or other systems that the system being developed interacts with.

It also does not generally include any of the data flows between the system and these terminators. There is one more symbol that isn't ''standard'' - you might not even find it in Page-Jones' book - but that does seem necessary (and that is, in fact, a standard flow chart symbol) - the Off Page Connector. " If you " ve run out of room at the bottom of page then, instead of having an arrow down to another module that's to be called, draw the arrow as going down to the top of one of these connectors. Label the connector ''To (name of module you " re calling) '' by writing this message inside it, and put the number of the page that will contain the module being called, beside the connector. Show the data flowing to and from the called module beside the arrow to the connector, just as you would if it pointed directly to the module instead. On the page that contains the called module, show a matching off page connector at the top. Label this with ''From (module that did the calling) '' and put the page number of the module that called this one beside the connector.

Draw an arrow from the bottom point of the connector down to the called module and reproduce the data flowing from and back to the calling module along this connection. With the possible exception of this addition of an ''off page connector, '' the notation described here matches the notation used by Page-Jones exactly, in his book on Structured Design. You can find quite a few examples of structure charts in that book, if you'd like to see a complete one now. A few of these will be appearing in the next pages of online notes. Structured Design consists of two major steps: 1. Produce a ''First Cut''s structured Chart 2.

Evaluate and improve the structure chart until a usable structure chart has been obtained The first step starts with the data flow diagrams and consists of lots of small decisions and tests. The second step provides a way to improve the result of first step (which we don't generally expect to be very good). Since the second step does exist, it isn't necessary to worry too much about how to make some of the decisions in the first step if they aren't clear - you will have an opportunity to improve the structure chart that you create by making these decisions later on. Bibliography:


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Research essay sample on Second Step Data Flow

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