IMSE Lecture 4

1
IMSE Lecture 4

• so far all our structure charts have been
  produced from an English description, e.g.
• - night on the town
• - meal with caviar
• - looking for a placement
• more often well be asked to produce a structure
  chart from a DFD which was drawn up in the
  requirements analysis stage
• this is the aim of this weeks lecture and
  tutorial
• the method of transforming a DFD into a structure
  chart is known as transform analysis

2
Revision notes on Data Flow Diagrams

• Data Flow diagrams show how data passes through
  the system and is transformed by each system
  function.
• Arrows show the direction of data flow. Their
  name describes the data flowing along that path.
• Rectangles show functions which transform inputs
  to outputs. Their names indicate their functions.
• Detailed description of DFDs on p278-280 of
  Sommerville - though slightly different notation
  used.

3
Transform Analysis

• a strategy for deriving high quality structure
  charts
• appropriate for any system of the form
  input-process-output
• 4 steps, producing 3 new diagrams
• 1) annotate the DFD (without redrawing it)
• 2) draw intermediate diagram
• 3) draw first-cut structure chart
• 4) refine the structure chart
• today well cover the first 3 steps, the fourth
  step must wait a few weeks

4
Specification

• Write a program which accepts a set of
  pre-validated examination marks in the range
  0-100. A count of the number of marks entered in
  the following ranges is to be maintained
• 0-10, 11-20, 21-30, 31-40, ... 81-90, 91-100.
• When all the marks have been entered the program
  will generate a graph displaying the count for
  each range.

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Step 1 - Annotate the DFD

• In step 1 we draw on the DFD itself, making 2
  changes
• a) identify the central transform
• b) add a control function

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Step 1a - Identify the Central Transform

• the Central Transform is the function (or
  functions) on a DFD which lie between input and
  output
• the Central Transform starts work on the data
  when the input preliminaries have been completed
  and before the output activities begin

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Step 1a (contd)

• input preliminaries are known as the afferent
  (coming in) processes and include
• input of data into the system
• validation of the data
• any necessary refinement before it can be
  processed
• the Central Transform includes
• main processing of the refined data
• any necessary calculations, etc.
• output activities are known as the efferent
  (going out) processes and include
• formatting the output
• producing the output (displaying, printing,
  writing to file...)

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Step 1a (contd)

• identify the central transform either by
  examining the DFD or by pruning away the afferent
  and efferent processes
• - trace each input flow from the outside of the
  DFD towards the middle, stopping at the point
  where the data has not yet been used for
  processing
• - trace each output flow backwards from the
  outside of the DFD to the middle, stopping at the
  point where the output data has just been
  produced
• - mark the region of the DFD between these 2
  points (the region must contain at least 1
  process)

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Step 1b - Control Function

• step 1b is to add a control function
• - we simply draw a new upper rectangle (the
  control function) above the central transform

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Step 2 - Intermediate Diagram

• step 2 is to draw the first new diagram (known as
  the intermediate diagram) making 3 changes to the
  annotated DFD
• a) redraw the process boxes as rectangles
• b) re-route the data flows
• any data flows which crossed the boundary of
  the central transform are re-routed via the new
  control function
• c) empty the central transform
• any processes within the central transform
  slide down below it and are attached to it

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Step 3 - First Cut Structure Chart

• step 3 is to redraw the intermediate diagram to
  produce the 2nd new diagram (known as the
  first-cut structure chart) making another 3
  changes
• a) give the control function a meaningful name
  describing the overall program (in C this control
  function will represent main, but it should NOT
  be given this name on the diagram)
• b) remove the central transform if it only has
  one process hanging from it (i.e. in VB or C it
  only invokes a single procedure or function),
  otherwise redraw it as a rectangle

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Step 3c - Altering all the Lines

• c) alter all the lines on the intermediate
  diagram
• (i) remove any dangling lines
• (ii) draw straight lines with arrowheads from
  the bottom of upper modules to the top of lower
  ones
• (iii) change the curved lines into small arrows
  with names, representing data passed between
  modules
• your DFD is now transformed into a first-cut
  structure chart.