IMSE Week 7 NassiShneiderman Diagrams

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IMSE Week 7Nassi-Shneiderman Diagrams

• developed by I. Nassi and B. Shneiderman in 1973,
  and extended by N. Chapin in 1974
• they graphically represent the tasks and control
  flow within a function, in terms of sequence,
  selection and iteration
• begin with a large rectangle representing the
  function to be designed
• then the inside of the rectangle is partitioned
  to represent the internal processing (the tasks
  and control flow)
• Nassi-Shneiderman diagrams aid well structured
  design with good control flow - its impossible
  to draw NSDs for badly designed functions

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Nassi-Shneiderman - Sequence

• 3
  tasks, performed
• Do this
  sequentially, e.g.
• then this
  print hello
• a
  10
• then finally this b 20

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Nassi-Shneiderman - Selection

• if then else
• if k gt max
• print k gt max
• else
• print k lt max
• T condition F
• do this do this

• Case (e.g. a menu)
• case condition
• value 1 value 2 value 3
• do this do this do this

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Nassi-Shneiderman - Iteration

• while (or for)
• while (k gt min)
• print k
• k k-1
• while true
• do this
• then this

• do while (or repeat until)
• do
• print k
• k k-1
• while k gt min
• do this
• then this
• while true

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Nassi-Shneiderman abstract example

• do this
• then this
• then this
• while true
• T if this is
  true F
• do this
• while true do
  this
• finally do this

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End of a working day

• Produce a Nassi-Shneiderman diagram
  corresponding to a single function which performs
  the following tasks
• Suppose its the end of a working day.
• After 6pm on Fridays you go to the Manor Quay.
• If youre driving you drink Coke, if not you
  drink Beamish. At 11.30 you go home and go to
  bed.
• If its not a Friday you work late before going
  home and making dinner. After dinner you watch TV
  and then go to bed.

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End of a working day

• End of a Working
  Day
• calculate time and day of week
• T if Friday and after 6.00pm
  F
• go to the Manor Quay
  work late
• T if driving F
  go home
• drink Coke drink Beamish make
  dinner
• until its 11.30 pm
• go home
  watch TV
• go to bed

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Back to the student marks program

• First Cut
• Structure Chart Student
• 
  Marks
• Valid
  Stored
• Marks
  Ranges
• Accept Calculate
  Generate
• Valid Marks Marks Range
  Graph
• We now produce one low level diagram or PDL
  procedure for each of these 4 modules

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NSDs for student marks program

• Remember that the program accepts pre-validated
  student marks and produces a graph showing the
  count for each of the ranges 0-10, 11-20,
  21-30, ..., 81-90, 91-100.
• Stated assumptions the marks are input as whole
  numbers, and the graph is to be a bar chart of
  asterisks.
• Student Marks
• set up data
• call accept valid marks (out valid marks)
• call calculate marks range (in valid marks, out
  stored ranges)
• call generate graph (in stored ranges)

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NSD for Accept Valid Marks

• Accept Valid Marks (out valid marks)
• set up data
• display How many marks?
• accept the number of marks
• for each mark
• accept mark
• display the entered mark
• store mark
• return the marks

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NSD for Calculate Marks Range

• Calculate Marks Range (in valid marks, out
  stored ranges)
• set up data
• for each mark
• subtract 1 from mark
• divide mark by 10 and ignore decimal
  part
• to find the correct range array
  element
• increment the range array element
• return the ranges

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NSD for Generate Graph

• Generate Graph (in stored ranges)
• set up data
• clear the screen
• display graph title
• for each range
• calculate lower and upper boundaries
• T if lower boundary 1
  F
• set lower boundary to 0
• display range boundaries
• for each star
• display

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Notes on PDL

• See Sommerville p126 - 129
• In principle, PDLs may be based on any
  programming language. Ada, a standard high level
  language in the defence industry, was designed
  for readability. It is therefore a good choice as
  a base language for a PDL.
• PDLs, with elements of computer code and
  structured natural language, are a compromise
  between comprehensibility and precision.