COMP6015 - An Introduction to Computer Programming : University of the West Indies COMP6015 An...

COMP6015 - An Introduction to Computer Programming : University of the West Indies

COMP6015

An

Introduction

to

Computer Programming

Lecture 08


Software Engineering

Customer sees an Opportunity!!!



How the customer explained it!



How the project leader understood it!!!



How the analyst designed it!!!



How the programmer coded it!!!



How Marketing described it!!!



How the project was documented!!!



What operations installed!!!



What the customer was billed!!!



How it was supported!!!



What the customer actually wanted!!!



A structured set of activities required to develop a software system.

Activities include:

●Software specification●Software design and implementation●Software integration and validation●Software maintenance and evolution



Software specification:

The process of establishing what services are required and the constraints on the system’s operation and development.

Requirements engineering process Feasibility study; Requirements elicitation and analysis; Requirements specification; Requirements validation.



Software specification: Specification documents

Must be:

Clear Complete Correct Understood by both the client as well as the

future design teams.




Must also clearly outline the constraints of the proposed system including:

Restrictions on hardware Portability Reliability Timing other resource usage issues.




Can be expressed:● Informally● Semi-formally● Formally




Informal Specification

Takes the form of natural language to express the specifications in statements such as:

"If the total revenue generated from sales for the previous quarter are higher then the total sales from the current quarter then raise prices by 0.5%"

Despite best attempts at grammatical and linguistic exactitude, informal specifications tend to be ambiguous, incomplete and imprecise. Nevertheless this method is still very popular and potential misunderstandings may be caught by working closely with the client.




Semi-formal Specification

A variety of semi-formal techniques exist for writing specifications, including :-

Data Flow Diagrams (DFDs) Entity-Relationship Modeling

A Data Flow Diagram is a pictorial representation of all aspects of the logical data flows.

DFDs are developed using stepwise refinement techniques and will go through a set of revisions/clarifications while they are being developed.




Semi-formal Specification

Entity-Relationship Modeling Technique used to specify databases, relations and

processes. ERMs allow a range from very simple to very

complex relationships to be expressed.




Formal Specification

This includes the use of finite state machines, Petri nets, formal specifications language languages.

Formal techniques require a higher level of mathematical rigor than semi-formal techniques.



Software specification: Pros & Cons of Techniques

Informal techniques are relatively easy to learn but does not have the power, expressiveness and clarity of semi-formal or formal techniques.

Formal techniques are powerful but are hard to learn and use. The client may have difficulty in understanding formal specifications.

Many times circumstances will dictate the technique that is used. If no one is familiar with formal techniques then an informal or semi-formal approach will have to be taken.



Software design and implementation

Software design is a meaningful engineering representation of some software product that is to be built.

Design should be traced to the customer's requirements and could be assessed for quality against predefined criteria.




In the software engineering context, design focuses on four major areas of concern:

●Data●Architecture●Interfaces●Components.




Data Created by transforming the analysis information

model (data dictionary and ERD) into data structures required to implement the software.

Part of the data design may occur in conjunction with the design of software architecture.

More detailed data design occurs as each software component is designed.




ArchitectureDefines the relationships among the major structural

elements of the software, the “design patterns” and can be used to achieve the requirements that have been defined for the system, and the constraints that affect the way in which the architectural patterns can be applied.

It is derived from the system specification, the analysis model, and the subsystem interactions defined in the analysis model (DFD).




Interfaces

Describes how the software elements communicate with each other, with other systems, and with human users.

The data flow and control flow diagrams provide much of the necessary information required.




Components

Created by transforming the structural elements defined by the software architecture into procedural descriptions of software components using information obtained from the process specification (PSPEC), control specification (CSPEC), and state transition diagram (STD).



Software design and implementationThe fundamental design concepts are: Abstraction - focus on solving a problem without being

concerned with lower level details. Refinement - provides successively more detail for each

design component. Modularity – understanding components independently of

one another. Software architecture - structure and conceptual integrity of

the software components. Control hierarchy or program structure - module

organization (control hierarchy; not concerned with procedural aspects of software).




The fundamental design concepts are:

Structural partitioning – (Horizontal) define input, data transformations, and output); (vertical) top-down refinement

Data structure - logical relationship among individual data elements

Software procedure - precise specification of processing (event sequences, decision points, repetitive operations, data organization/structure)

Information hiding – Modules’ need-to-know basis




Implementation The process of translating the detailed design into code.

Generally done by teams all working at the same time on different components of the product.

Major documentation provided in this stage is the source code; however, the programmer should provide additional documentation to assist in maintenance. This additional documentation includes all test cases against which the code should be tested, the expected results, and the actual output. These documents will be used in regression testing.



Software design and implementation: Implementation

Issues involved:

Choice of programming language In many cases, there is no opportunity for choice

because either only one choice is available or management stipulates that the product should be implemented in a given language.

A cost-benefit analysis may be employed to decide on the language. The language with the largest expected gain would be the one selected.

A risk analysis may also be used to aid in the selection of the language. The language with the smallest overall risk is selected.



Software design and implementation: ImplementationIssues involved:

The use of 4th generation languages (4GLs) The ease of programming The program is shorter, quicker to develop and easier to

maintain There is a potential productivity gain 4GLs such as FoxPro and PowerHouse have case tool

features (e.g. automatic documentation) which aid in the implementation process.



Software design and implementation: ImplementationIssues involved:

Use of good programming practices ● use of consistent and meaningful variable names● provision of proper documentation; both prologue and inline

comments● use of parameters (as opposed to constants)● well formatted source code to increase readability; good use of

blanks and indentation to highlight control structures.● Structured programming (i.e. modular programming) is also

advised to improve code quality, maintainability and decrease the effort required for testing.



Software Development

Waterfall Model



Waterfall Model Advantages Testing is inherent to every phase of the waterfall model It is an enforced disciplined approach It is documentation driven, that is, documentation is

produced at every stage

Disadvantages Changes may cause considerable confusion as the project progresses. As The client usually only has a vague idea of exactly what is required

from the software product, this WM has difficulty accommodating the natural uncertainty that exists at the beginning of the project.

The customer only sees a working version of the product after it has been coded. This may result in disaster if any undetected problems are precipitated to this stage.



Software Development

Incremental Model



Incremental ModelModel combines the elements of the waterfall model with the

iterative philosophy of prototyping. However, unlike prototyping the IM focuses on the delivery of an operational product at the end of each increment.

Example of incremental approach:Develop a Word Processor

1. Basic file management, editing and document production functions.

2. Advanced editing and document production functions3. Spell and grammar checking4. Advance page layout

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