Nassi-Shneiderman Charts

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Nassi-Shneiderman Charts

• Andrea Schumacher

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Background

• Nassi-Shneiderman (N-S) charts were initially
  developed in the early 1970s by Ike Nassi and
  Ben Shneiderman
• They are based on and perform a function similar
  to flowcharts
• The initial purpose was to develop a technique to
  chart the logic design of structured programs
• N-S Charts are also know as Program Structure
  Diagrams (PSD) and Chapin Charts.

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Agenda

• 1)      Overview
• 2)      Primary Uses
• 3) Advantages
• 4)      Disadvantages
• 5)      Relationship with Other Techniques
• 6) Symbols
• 7)      Examples
• 8)      Resources/Tools
• 9) Group projects

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Overview

• Nassi-Shneiderman charts have four primary uses
• Creating the logic design
• Programming from the charts
• Writing program documentation
• Maintenance of the program

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Primary Uses1. Creating the Logic Design

• As mentioned previously this was the initial
  purpose of Nassi-Shneiderman charts
• They can be thought of as the building blocks of
  the design
• The completed chart is the blueprint for the
  program code

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Primary Uses2. Programming From the Charts

• The N-S chart can be use to do the actual
  programming
• It can be considered as a type of pictorial
  pseudocode
• Some of the N-S editors convert the diagram
  directly into source code

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Primary Uses3. Writing Program Documentation

• The N-S chart is a pictorial representation of
  the logical design of the program
• Using the N-S chart as an actual part of the
  documentation is a great idea!

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Primary Uses4. Maintenance of the Program

• N-S charts provide a quick reference to the
  structure and design of the program which helps
  during on-going maintenance and upgrades.

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Advantages - General

• One main advantage is that N-S charts adopt the
  structured programming philosophy
• N-S charts use only a few symbols which make them
  easier to learn then traditional flowcharts
• They help to recognize missing policies or
  procedures from business rules
• Completed charts should fit on one page, there
  are no complex arrows to follow

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Advantages General (cont.)

• Have one entry point and one exit point
• They can be nested to show complex procedures
• N-S charts are independent of any specific
  programming language

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Advantages Programming

• The logic is documented in the N-S chart so
  regardless of the coding language used, N-S
  charts make coding much easier then starting from
  scratch
• Clearly displays sequence, selection, and
  repetition (such as nested loops) to make
  debugging easier
• Confusing GOTOs are not represented

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Disadvantages

• They can be difficult to draw
• They can be harder to update or change then
  traditional flow charts because everything stays
  together
• They must be comprehensive to be understood

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Relationship with Other Techniques
Kind of Technique
Example Technique
Relation

• The activities/processes defined
• in the process models can be further
• described in detail in a N-S chart
• A N-S chart can also be used as a
• translation of process model

wwwis.cs.utwente.nl
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Symbols

• Only 3 basic symbols used in N-S charts
• Process (Sequence)
• Decision (Selection)
• Iteration (Repetition)
• Other symbols are used but they are based on
  these three symbols

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Process

• A stackable box is used to represent a sequence
  or process

Step 1
Step 2
Step 3
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Decision (Selection)

• The if-then-else

If
(then)
(else)
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Decision (Selection)

• The case/switch/selection statement

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Iteration

• These symbols show loops

While
Repeat Until
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Graduation Example N-S Chart
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Graduation Example Pseudocode
While FOR ALL SECOND-YEAR STUDENTS do if
ENOUGH CREDITS? then YES if BILLS
PAID? then YES GRANT DIPLOMA
else NO NO DIPLOMA
end if else NO NO DIPLOMA
end if end while
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Newspaper Subscription N-S Chart
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Newspaper Subscription Pseudocode
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Drawing Hints

• The size of the final rectangle has no bearing on
  the program
• Fit one drawing to a page
• They work the best with small or medium-sized
  programs
• Draw the major loops first, complete the inner
  loops later

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Drawing Hints

• Nested loops are indented, of course as the
  number of nested loops increase, the width of the
  chart changes
• Sub-procedures can be drawn using a different N-S
  chart
• If the N-S diagram is cluttered it may be the
  policy you are diagramming is unclear

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User Comments

• The University of Liverpool develop a N-S editor
  using a VAX based Pascal complier called NSEDIT
• User responses
• 60 agreed that it helped to ensure the correct
  syntax
• 21 said it enforced structured programming
• 12 felt it made the transition from design to
  code easier

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Editor Downloads/Tools

• Nassi-Shneiderman Diagrams Editor for Unix
• http//www.kfa-juelich.de/zam/nassi/
• free download for a demo or about 30 dollars for
  a user license
• Microsoft Visio 2000 supports N-S charts

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Visio Screen Shot
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MS Visio Example
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Editor Downloads (cont.)

• StruktoGraaf 3.0 Windows 95/98/ME/NT/2000
• http//www.sichemsoft.nl/producuk.html
• A free download for a demo, about 135 (or
  student version for 57) for a user license

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StruktoGraaf Screen Shot
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StruktoGraaf Screen Shot (Cont.)
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Summary

• We have seen some of the advantages and
  disadvantages of using Nassi-Shneiderman charts
• I think you can clearly see the benefits to using
  N-S charts for system design as well as
  documentation and maintenance

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References

• Beek, Marcel, Koolen, Hans, Van Setten, Mark
  (1996), Program Structure Diagrams, (visited
  3/18/01), wwwis.cs.utwente.nl8080/dmrg/mee/misop.
  htm
• Cary, David (2000) Nassi-Shneiderman on the
  Web, (visited 3/18/01), www.rdrop.com/cary/html/
  psd.html
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• Coenen, Frans Comp101 Design and Style
  Review, (visited 3/6/01), www.csc.liv.ac.uk/fran
  s/comp101/week9/designAndStyle.htm.
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• Halewood, K. and Woodward M.R. (1988), NSEDIT
  A Syntax-directed Editor and Testing Tool based
  on Nassi-Shneiderman Charts, Software-Practice
  and Experience, Vol. 18, No.10, pp. 987-998
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• Juch, Forschungszentrum (1997) Nassi
  Tutorial,(visited 3/12/01), www.kfa-juelich.de/za
  m/nassi/tutorial.html
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• Kendall, Kenneth E. and Kendall, Julie E. (1999)
  System Analysis and Design, Prentice Hall, New
  Jersey
•  

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References (cont.)

• Seesing, Paul R. (1993), Basic System Analysis
  Tools for Computer Users, (visited 4/9/01),
  www.open.org/prslkg/sy_chap.htm
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• Tiemann, Peter Nassi-Shneiderman Charts,
  (visited 4/9/01), www.preview.org/software/sc_help
  /nassishneiderman
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• Why Should I Use Structograms? (visited
  4/9/01), www.blue-river-software.com/products/whys
  tructs.htm
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• Weiss, Edmond H. (1990). Visualizing a
  Procedure with Nassi-Schneiderman Charts,
  Journal of Technical Writing and Communication,
  Vol. 20, No.3, pp. 237-254
• Woodward, M.R. (1987) The Use of
  Nassi-Shneiderman Charts and Supporting Tools in
  Software Engineering Education, Computer
  Education, Vol. 11, No. 4, pp. 267-279

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Class Exercises