System Development Lifecycle

What is the System Development Life Cycle?

• An organised plan of how to create a program — consists of several stages
• Analysis: identify the problem
• Design: details of the solution are set out
• Coding: the program is developed
• Testing: program is tested for errors

Analysis Phase

Why is it a good idea to perform decomposition?

• The phase that involves identifying the problem and the requirements
• Decomposition: break-down of a problem into small manageable parts
• Abstraction: remove unnecessary parts, simplify, filter irrelevant characteristics
• Makes the problem simpler to solve
• Can allow developers to distribute work to different teams

Components after Decomposition

• INPUTS — what is put into the system
• OUTPUTS — what is taken out of the system
• PROCESS — actions taken to achieve the result
• STORAGE — what needs to be kept for future use

Tasks in the Analysis Phase

• Decomposition
• Abstraction
• Identification of a problem
• Identification of requirements

Design Phase

• Construction of a solution using standard methods (e.g. flowcharts)
• Pseudocode — shows what a program does in plain language
• Flowchart — a diagram showing the ordered steps to complete a program
• Structure diagram — hierarchical diagram showing breakdown into sub-programs

Pseudocode — Variables and Constants

• Variables/constants are used to store items of data
• Data is accessed using an identifier
• Value of a variable may change during execution
• Value of a constant remains the same during execution

Calling vs Defining a Function

• A function is defined once and called many times
• Define = setting up a function; Call = using a function

Processes in an Algorithm

• Initialisation of a variable
• INPUT of data
• Storing values in a variable/array
• Sorting values in an array (Bubble sort)
• Displaying (output) values
• Iteration (use of loops)
• Selection (use of IF/CASE OF)

Types of Iteration

• Count Controlled — number of iterations is pre-determined
• Pre-Condition — checks condition at start; loop won't iterate if false
• Post-Condition — checks condition at end; always iterates at least once

Storing Data in a File

• Data is stored permanently
• Data can be moved to another computer
• Another copy can be made and stored/accessed elsewhere

Global vs Local Variables

• Local variables — scope is a defined block of code (procedure/function)
• Global variables — scope is the whole program
• Local — value cannot be changed elsewhere in the program
• Global — value can be changed anywhere in the program

Procedures and Parameters

• Procedures: write a collection of statements under a single identifier
• Procedures: allow modular programs; can be re-used
• Procedures: make programs shorter and easier to maintain
• Parameters: pass values from the main program to a procedure/function
• Parameters: allow the procedure/function to be re-used with different data

Maintainable Programs

• Meaningful identifiers — easily recognise the purpose of variables
• Comments — annotations explaining each section of code
• Procedures & Functions — make programs modular and easier to update
• Indentation — makes the program more readable

Coding Phase

• Program is written
• Iterative testing takes place

Testing Phase

• Program is tested for errors
• Program is tested that it meets requirements

Test Data

• Check that the program works as expected
• Check for logic/runtime errors
• Normal — data the program should accept
• Abnormal/Erroneous — data the program should reject
• Extreme — largest/smallest data that should be accepted
• Boundary — data on the edge of accepted/rejected

Data Validation

• Automated check to make sure data entered is reasonable
• Presence Check — checks field is not blank
• Type Check — checks data type is correct
• Range Check — checks data is within set range
• Length Check — checks data is within character limit
• Format Check — checks data matches required format

Data Verification

• Ensures data has been accurately copied from one source to another
• Double Entry — data entered twice; accepted if both inputs are identical
• Visual Check — data compared against original copy