Nassi-Schneiderman diagrams
After completing this lesson you should be able to:
There is an exercise for you to do with a sample answer and after you have finished you might like to attempt these questions to assess how much you have learned.
This is the last lesson in the course.
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The Nassi-Schneiderman (NS) diagram is a graphical method of stating algorithms. Many people claim that it is very easy to read, especially for beginners. That is probably true but maintaining NS diagrams can be a problem for the program designer. Here is a simple example of a NS diagram:
The diagram indicates an algorithm with a process (Process 1) followed by a decision. Each outcome of the decision has a process associated with it. If the decision evaluates as true then Proc 2 is carried out, if false then Proc 3 is carried out. In the table that follows you will see the symbols that are usd in NS diagrams. As with flow charts the symbols are few and easily remembered. The different symbols are "fitted" together like pieces in a jigsaw puzzle. |
If you go searching for more information on Nassi-Schneiderman you will find a variety of spellings of the name Schneiderman, for example Shneiderman and Shneidermann. I just thought I'd tell you that. It is not unusual for the spelling of names to change over time. For instance I am descended from Bostocks, Hewitts, Hoares, Roches, SideBothams, Beechs, Proudloves and so on. Over time the spelling of the names Hoare, Roche and Sidebotham have been variously Hoar, Hore, Roach, Roache, SideBottom and SideBothom. I just thought I'd tell you that. The Roche or Roach is a European freshwater fish. The rumour that I am descended from a cockroach is quite unfounded.
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The process symbol looks much the same as the program flow chart symbol. You'll see soon though that NS diagrams look quite different from the flow charts you have used before. |
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The decision symbol is quite different. You will see soon how it is used. |
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The repeat construct is also quite different and like the while construct shown below it gives a good indication of the scope of the repeat. The repeat condition extends up on the left of the processes it controls. |
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You can see that the while construct is the reverse of the repeat. The while condition extends down on the left to border the processes it controls. |
Drawing program designs with NS diagrams is quite simple but the NS method differs from the program flow chart method. Apart from different symbols NS does not use connectors between the symbols. A comparison of an NS chart and a flow chart is shown below:
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As with the flow the direction of sequence in the NS chart is from top to bottom. There are some problems with the NS charts that don't really occur with flow charts:
Sketch NS diagrams for the following pseudocode fragments:
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(1) IF x = = 100
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(2) X = 1
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(3) X = 0
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There is a sample answer here but attempt the exercise for yourself before looking at the sample.
In this lesson you studied Nassi-Schneiderman diagrams, another graphical method of of designing 3GL programs. This was quite a short lesson and you should have learned:
The course is finished now and I hope that you found it challenging and rewarding. Program design is one part of the much bigger discipline of software engineering and perhaps this course has whetted your appetite for software engineering.
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and may not be reproduced by any means without the written permission of
David Beech.
9 Wyndella Street, Tasmania, Australia