Com S 362 Lecture Outline 
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  ABSTRACTION, DECOMPOSITION, LOCALITY, 
  SEPARATION OF CONCERNS and MODULARITY

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READING:

Dijkstra, E. W. (1982), On the role of scientific 
thought, in `Selected Writings on Computing: A 
Personal Perspective', Springer-Verlag, pp. 60--66.

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Overview of the lecture:

- Programming in the large vs programming in the 
  small 

- Abstraction: [Hide the unnecessary details]

- Locality: [More precisely source locality]

- Modularity: property of a module that enables 
  us to understand it by looking at its implementation
  and the specification of other module that it 
  uses. We [SHALL NOT] need to look at the 
  implementation of other modules.

- Decomposition

- Separation of concerns 

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The role of decomposition

 - Small programs can be implemented by one person as 
   monolithic units

 - As size increases monolithic structure is no longer 
   reasonable

 - Code becomes difficult to understand

 - Often a team is working on the program 
   (not a single person)

 - Solution: Partition the program into a number of 
   independent small programs (often called 
   decomposition)

 - Desired property: People must be able to work on 
   pieces separately

How do we decompose a problem?

 - Each subproblem is at the same level of detail
 - Each subproblem can be solved independently; and 
 - the solutions to the subproblem can be combined 
   to solve the original problem

Example: Merge sort is a sorting algorithm for 
rearranging lists (or any other data structure 
that can only be accessed sequentially, e.g. file 
streams) into a specified order. It is a particularly 
good example of the divide and conquer algorithmic 
paradigm. It is a comparison sort.

We can decompose the problem of merge sort into the 
following subproblems.

1. Divide the unsorted list into two sublists of 
   about half the size 
2. Sort each of the two sublists 
3. Merge the two sorted sublists back into one 
   sorted list.
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Role of abstraction is software development

 - A method to change the level of details to be 
  considered

 - Many to one mapping (simplifies analysis)

Abstraction by parameterization 

 - Through the introduction of program parameters 
 - represent possibly infinite set of computation 
   by a single program

Abstraction by specification

 - Abstact from the the computation, replacing it 
   with the specification