16.35 Midterm Debrief

1. 16.35 Midterm Debrief October 30, 2002 Malia Kilpinen

2. Question 1 • What are process models? Compare and contrast the spiral model of development with another model of your choice. (1+5p)

3. 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.

4. 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.

5. Solution 1 - Spiral Model • Objective Setting • Risk assessment and reduction • Development and validation • Planning • <<There is a picture in Van Vliet pg 62.>>

6. Question 2 • Who are the expected readers of a requirement specification and what properties do they want the specification to have? (5p)

7. 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

8. 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 < C loop i := i+1; if A(i) > max then max := A(i); end if; -- max is the maximum of A(0..i); end loop;

9. 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)

10. 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)>max is true sometimes, and false sometimes. For node coverage, it may always be true. In any case, only one test is necessary. Say C=3, and A=[5,2,3,6]

11. 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.)]

12. Question 4 • Describe the architectures repository model and client-server. Compare their pros and cons. (6p)

13. 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

14. 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

15. Question 20 • Cow-moon Ada code trace problem. (2p)

16. Solution 20 • Output is: • 3 14 • Moo! Moo! Moo! Moo! • 7 6

17. Question 21 • What does it mean to declare a variable as private? When would you declare a variable as private? (1p)

18. Question 22 • Write a program to calculate the cube of a user-inputted integer. Your program should include error checking. (6p)

19. Solution 22 • Should use Skip_Line function in error checking. • Simplified version of problem set 2.

20. 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;

21. Solution 23 • Discussed in Ada Compendium. • BMW := Audi; • BMW points to Audi’s memory location • BMW.all := Audi.all; • BMW holds same values as Audi