16.35 Midterm Debrief

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16.35 Midterm Debrief

• October 30, 2002
• Malia Kilpinen

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Question 1

• What are process models? Compare and contrast
  the spiral model of development with another
  model of your choice. (15p)

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Solution 1 - Process Model

• A software process model is an abstract
  representation of a process. It presents a
  description of a process from some particular
  perspective.

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Solution 1 - Spiral Model

• Process is represented as a spiral rather than as
  a sequence of activities with backtracking.
• Each loop in the spiral represents a phase in the
  process.
• No fixed phases such as specification or design
  -loops in the spiral are chosen depending on what
  is required.
• Risks are explicitly assessed and resolved
  throughout the process.

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Solution 1 - Spiral Model

• Objective Setting
• Risk assessment and reduction
• Development and validation
• Planning
• ltltThere is a picture in Van Vliet pg 62.gtgt

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Question 2

• Who are the expected readers of a requirement
  specification and what properties do they want
  the specification to have? (5p)

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Solution 2

• Users and the customer (in case of custom-made
  product)
• want readability, completeness, consistency,
  correctness, may serve as a contract
  (testability)
• Designers
• readability, consistency, non-ambiguity,
  traceability
• Testers
• testability
• Maintenance staff
• completeness, traceability

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Question 3

• Consider the following piece of program, which
  assumes a large integer C and an array A(0..C).
  It is intended to assign the maximum of A to the
  variable max.
• max A(0)
• i 1
• while i lt C loop
• i i1
• if A(i) gt max
• then max A(i)
• end if
• -- max is the maximum of A(0..i)
• end loop

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Question 3 (cont.)

• a) Show what steps must be taken to find test
  cases for coverage testing, using this program as
  an example. Show the distinction between
  node-coverage and branch-coverage. (5p)
• b) What method(s) of verification are both
  practical and would likely reveal the error? Note
  that C is large. Motivate your answer. (3p)

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Solution 3 - Part a

• 1) draw flow-chart, identify nodes and branches
• 2) find a collection of paths that visits all
  nodes/branches
• 3) find input data that makes the program follow
  each path (if it turns out that there is no such
  data, the path is infeasible, go back to 2 and
  replace it by other paths)
• in this case, the difference is that branch
  coverage requires that the test A(i)gtmax is true
  sometimes, and false sometimes. For node
  coverage, it may always be true. In any case,
  only one test is necessary. Say C3, and
  A5,2,3,6

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Solution 3 - Part b

• The error is that A(1) is never checked. A
  statistical test has probability 1/C to detect
  this.
• From a) we can see that it is easy to achieve
  branch coverage and yet miss the error.
• A boundary value test might detect it, if you
  have a rather broad definition of boundary.
  Such a definition will soon be unworkable.
  Inspection has a good chance.
• (Formal verification, that can be done mostly
  automatically when loop invariants are provided,
  is another candidate.)

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Question 4

• Describe the architectures repository model and
  client-server. Compare their pros and cons. (6p)

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Solution 4 - Repository

• Advantages
• Efficient way to share large amounts of data
• Sub-systems need not be concerned with how data
  is produced
• Centralized management e.g. backup, security,
  etc.
• Disadvantages
• Sub-systems must agree on a repository data
  model. Inevitably a compromise
• Data evolution is difficult and expensive
• No scope for specific management policies
• Difficult to distribute efficiently

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Solution 4 - Client Server

• Advantages
• Distribution of data is straightforward
• Makes effective use of networked systems. May
  require cheaper hardware
• Easy to add new servers or upgrade existing
  servers
• Disadvantages
• No shared data model so sub-systems use different
  data organisation. data interchange may be
  inefficient
• Redundant management in each server
• No central register of names and services - it
  may be hard to find out what servers and services
  are available

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Question 20

• Cow-moon Ada code trace problem. (2p)

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Solution 20

• Output is
• 3 14
• Moo! Moo! Moo! Moo!
• 7 6

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Question 21

• What does it mean to declare a variable as
  private? When would you declare a variable as
  private? (1p)

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Question 22

• Write a program to calculate the cube of a
  user-inputted integer. Your program should
  include error checking. (6p)

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Solution 22

• Should use Skip_Line function in error checking.
• Simplified version of problem set 2.

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Question 23

• Assume that a record type Beep has been defined.
  The following pointers are then declared (1p)
• type BeepPointer is access Beep
• Audi, BMW BeepPointer
• Audi new Beep
• BMW new Beep
• What is the difference between the following
  commands?
• BMW Audi
• BMW.all Audi.all

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Solution 23

• Discussed in Ada Compendium.
• BMW Audi
• BMW points to Audis memory location
• BMW.all Audi.all
• BMW holds same values as Audi