Lecture 7: Software Design (Part II)

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Lecture 7 Software Design (Part II)

• Dr Valentina Plekhanova
• University of Sunderland, UK

http//www.cet.sunderland.ac.uk/cs0vpl/SE-Com185.
htm
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Low-level Design

• Also known as procedural or functional design
• the design of the internal workings of a module
• the fine details of the system
• adds to the high level design details kept
  separate from the high level design, for clarity.

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Low-level Design

• If our designs are good, each procedure or
  function will only be required to carry out a
  fairly small and specific subtask.
• We now need to consider the design of the code
  that will carry out each of the identified
  subtasks.
• Once that is done, it should be fairly easy to
  translate each low-level design into program code.

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Low-level Design

• The actual choice of design representation to use
  is not important, designers have their own
  favourite methods.
• It is a matter of individual choice (unless you
  work for an organisation that imposes a
  particular design representation) as to the
  method of design representation you use to design
  low-level code.
• What all low-level design representation methods
  do is to portray in a way that is not specific to
  any one programming language.

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Low-level Design

• define input data
• assign data to a variable
• define output data from the module
• define another (sub-) modules that can be 'called
  up' from this module.

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Low-level Design

• 4 main methods
• pseudo code
• flow charts
• JSP
• Nassi-Shneiderman diagrams

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Low-level DesignBasic Constructs

• All these 4 main methods of producing low-level
  design documents are based on 3 basic constructs
• sequence
• selection
• iteration (repetition) .

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Low-level Design

• Sequence is a linear progression where one task
  is performed sequentially after another.
• Iteration WHILE is a loop with a simple
  conditional test at its beginning.
• Selection IF-THEN-ELSE is a decision in which a
  choice is made between two alternative courses of
  action.

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Low Level Design

• Although these constructs are sufficient, it is
  often useful to include three more constructs
• REPEAT-UNTIL is a loop with a simple conditional
  test at the bottom.
• FOR is a special loop in which an index variable
  is automatically initialised, incremented, and
  tested.
• CASE is a multiway branch (decision) based on the
  value of an expression. CASE is a generalisation
  of IF-THEN-ELSE.

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Basic Sections

• There are usually 3 sections to all Low-level
  designs initialisation, processing, and
  termination.

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Low Level Design Initialisation

• Initialisation section includes opening files,
  reading the first record, and, if necessary,
  printing page and report headings.

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Low Level Design Processing

• Processing section includes DO statements to show
  repetitive tasks that are performed on each
  record in the file there are two ways to show
  this loop
• DO while data remains
• Processing steps
• Read next record
• END DO

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Low Level Design Processing

• DO until end-of-file
• Processing steps
• Read next record
• END DO

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Low Level Design Termination

• Termination section includes what happens at the
  very end of the program, such as closing the
  files and stopping the program.

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What is Pseudocode?

• Pseudocode is simply a way of describing the
  steps to the solution of a programming task,
  using a few English words in a structured way,
  though not in any particular programming
  language.
• Instead, or in combination, we can use program
  flowcharts.

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Pseudocode Important Notes

• Each textbook and each individual designer may
  have their own personal style of pseudocode.
• Pseudocode is not a rigorous notation, since it
  is read by other people, not by the computer.
• There is no universal "standard" for the
  industry, but for instructional purposes it is
  helpful if we all follow a similar style.

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Examples of Pseudocode Example 1 - The problem
is to find the roots of a quadratic equation.

• Step 1 Prompt for and read the coefficients a,
  b, and c
• Step 2 If a 0, do steps 3,4
• Step 3 If b 0 also, Error message "no roots"
  and exit
• Step 4 Write single root -c/b and exit
• Step 5 Set d b2 - 4ac and if d lt 0 Error
  message "not real" and exit
• Step 6 Set s -(b sqrt(d)sign(b))/2
• Step 7 Set x1 s/a. Write this first (robust)
  root
• Step 8 Set x2 c/s. Write this second
  (robust) root.
• Step 9 Set x3 -(b - sqrt(d)sign(b))/(2a).
• Write this second (risky) root.

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Example 2 Pseudocode

• prompt the user to input the number of days
• user inputs the number of days
• multiply the number of days by 7 hours
• display the total hours with an output prompt.

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Low-level Design Flow charts

• A flowchart is a pictorial representation of the
  logic in a computer program
• Based on the 3 basic constructs, sequence,
  selection and iteration.
• In flow chart, certain shapes have special
  meaning, and arrows are used to connect pieces of
  the flowchart.

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Low-level Design Flow charts

• Used to indicate the beginning (start) or end
  (stop) of a computer program or subroutine.
• Used to indicate some type of input or output,
  such as opening or closing Files.
• Also known as a decision symbol, this is used to
  indicate a decision to be made, choices based on
  logic an if statement
• Arrows are used to show the flow of the program
  to show where the next item is located.

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Some Examples

• Pseudocodes Flow Charts

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Flow chart Sequence

• Sequential control is indicated by writing one
  action/task after another, each action/task on a
  line by itself, and all actions/tasks aligned
  with the same indent.
• The actions/tasks are performed in the sequence
  (top to bottom) that they are written.

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Flow chart Selection IF-THEN-ELSE

• Binary choice on a given Boolean condition is
  indicated by the use of four keywords IF, THEN,
  ELSE, and ENDIF.
• The general form is
• IF condition THEN
• sequence 1
• ELSE
• sequence 2
• END IF
• The ELSE keyword and "sequence 2" are optional.
  If the condition is true, sequence 1 is
  performed, otherwise sequence 2 is performed.

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Flow chart Selection IF-THEN-ELSE Example

• IF hours gt 35 THEN
• Display overtime message
• ELSE
• Display regular time message
• END IF

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Flow chart - Iteration (e.g. for, while)

• This loop is a specialised construct in which an
  index variable is automatically initialised,
  incremented and tested.
• Two keywords, FOR and END FOR are used.
• Example The general form is
• FOR index start to finish
• sequence
• END FOR
• At the initiation of the loop, the index variable
  is set to the starting value.
• At the end of each iteration, the index variable
  is automatically incremented.
• The loop repeats until the index value reaches
  the finish value.

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Flow chart - Iteration WHILE

• The WHILE construct is used to specify a loop
  with a test at the top.
• The beginning and ending of the loop are
  indicated by two keywords WHILE and END WHILE.
• The general form is
• WHILE condition
• sequence
• END WHILE
• Example
• WHILE number of days lt 6
• Compute hours as number of days hours
• END WHILE

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Flow chart - Iteration WHILE

• The loop is entered only if the condition is
  true.
• The "sequence" is performed for each iteration.
• At the conclusion of each iteration, the
  condition is evaluated and the loop continues as
  long as the condition is true.

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Low-level Design Important Notes

• Time spent on the design stage of the development
  cycle will lead to less time spent on testing,
  debugging and re-writing your programs to make
  them work properly.
• Lecture Notes are based on materials taken from
  Books Pfleeger Sommerville Vliet (see
  References Site).

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Week 8 24.04.2003-28.04.2003Project Control
Session

• Tutorial Time 10 minutes for each Team
• Students will present project file, particularly
  Schedule, plus any project documentation.
• Students will describe where they are in the
  project and any problems encountered.
• During the discussion reviewers will ask to see
  evidence of deliverables for any tasks that are
  complete to determine whether they have in fact
  been done.