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IGCSE Computer Science CIE Your notes
7.1 Development Life Cycle
Contents
Program Development Life Cycle - Analysis
Program Development Life Cycle - Design
Program Development Life Cycle - Coding
Program Development Life Cycle - Testing
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Program Development Life Cycle - Analysis
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Abstraction
This is the act of removing unimportant details of the problem to focus on important elements. An
example of abstraction would be the London underground train route map; travellers do not need to
know the geographical layout of the routes, only that getting on at stop A will eventually transport you
to stop B
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Figure 1: London Underground train route map
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Figure 2: The geographical London underground train map
Requirements
Identification of the problem: Before tackling a problem, it needs to be clearly understood by
everyone working on it. The overall goal of the solution needs to be agreed as well as any
constraints such as limited resources or requiring a platform specific solution
Requirements: To create a solution, a requirements document is created to define the problem
and break it down into clear, manageable, understandable parts by using abstraction and
decomposition. A requirements document labels each requirement, gives it a description as well
as success criteria which state how we know when the requirement has been achieved
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Program Development Life Cycle - Design
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Decomposition
This is the act of breaking down a large problem into smaller, clear, manageable and
understandable sub-parts. Sub-parts can be divided until they are easily solvable and cannot be
broken down any further. An example of decomposition could be getting ready in the morning to
go to school
Step 1: Wake up
Step 2: Get breakfast
Step 3: Brush teeth
Step 4: Put clothes on
Step 5: Make sure the bag and school supplies are ready
Step 6: Find transport to school e.g. walk, bus, car, bike, etc
These steps could be further subdivided, for example, “Step 2: Get breakfast” would entail:
Step 2.1 Get a bowl
Step 2.2 Get cereal
Step 2.3 Get milk
Step 2.4 Get a spoon
Step 2.5 Put cereal in a bowl
And so on…
Once the requirements document has been created, developers need to design the structure
and algorithms to solve the problem:
Structure charts are created to show the breakdown of tasks in a hierarchy
Flowcharts may be created to visually show how tasks should be carried out
Pseudocode is created, sometimes from flowcharts, to allow programmers to easily
translate task instructions into programming code
The design of a solution identifies what tasks need completing, how to complete the tasks and
how each task works together with other tasks
A computer system includes several components that work together: software, hardware, data,
networking and people
Systems can be broken down into sub-systems that can be further broken down into more sub-
systems, until each sub-system has a single purpose. This decomposition is known as top-down
design
Decomposing a system
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To create an overall system and solve a problem, it must first be broken down into subsystems
that are easier to solve and create. The act of breaking down the problem is known as stepwise
refinement Your notes
Decomposing the problem this way creates smaller, more manageable and more easily
understandable sub-parts
Each sub-system can be assigned to a developer or group of developers who create
subroutines from these sub-systems. Each sub-system can then be created at the same time,
reducing development and testing time, especially on large projects
Decomposing the system using stepwise refinement requires developers to think about four key
areas:
Inputs: data entered into the system
Processes: subroutines and algorithms that turn inputs and stored data into outputs
Outputs: data that is produced by the system, such as information on a screen or printed
information
Storage: data that is stored on a physical device, such as on a hard drive
To solve a problem all aspects must be thoroughly understood by the developers
There are many methods used to design and construct solutions. Three such methods are
illustrated below:
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Structure Diagrams
Structure diagrams show hierarchical top-down design in a visual form. Each problem is divided Your notes
into sub-problems and each sub-problem divided into further sub-problems. At each level the
problem is broken down into more detailed tasks that can be implemented using a single
subroutine
Figure 1: A structure diagram
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Figure 2: A structure diagram for a mobile application
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Worked example
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A satellite navigation system is an example of a computer system that is made up of subsystems.
Part of a satellite navigation system: allows the user to enter details for a new destination or
select a previously saved destination and displays directions in the form of a visual map or as a
list. Draw a structure diagram for this part of the satellite navigation system.
[4]
[1] for a hierarchical structure
[1] for suitable names for the sub-systems
[1] for identifiable inputs
[1] for identifiable outputs
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Flowcharts
Flowcharts show how algorithms can be represented visually in a diagrammatic format Your notes
Each flowchart has a start and an end with arrows showing the order each task or instruction
needs to be carried out in
Flowcharts are made of several symbols:
Terminator symbols: Also known as Begin/End symbols. These indicate where a flowchart
starts and stops
Process symbols: These show simple actions being performed such as assigning values or
performing arithmetic operations on values
Processes can also represent other flowcharts or summarised actions. For example,
searching or sorting a list is a complex process which would require its own flowchart. A
process symbol could be used to represent sorting or searching in a separate flowchart.
This is represented by a process with an additional bar on either side of the symbol
Input/Output symbols: These show the input of data and output of data
Decision symbols: These symbols are used to decide whether to take one of two routes by
answering a true/false or yes/no question. They can be used for selection and iteration
Flow lines: Flow lines use arrows to show the direction of flow and what task to perform next.
Usually these are top to bottom and left to right
Figure 3: Flowchart symbols: terminator, input/output, process, decision (left to right)
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Figure 4: Flowchart for finding the largest of ten numbers
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Pseudocode
Pseudocode is a programming-like language that does not have syntax. It can be considered
“fake” code.
It uses english words and phrases to represent instructions and is very similar to programming
code but does not and cannot run on any computer
The purpose of pseudocode is to allow developers to understand how to create a program
regardless of the programming language used to implement the solution
While pseudocode has no specific syntax, it is important to stick to a consistent style. This will
make it easier and quicker for programmers to read and create programs from the pseudocode
Examples of a consistent style can include:
Keywords are written in capital letters e.g. INPUT, OUTPUT, IF, THEN, ELSE
Variable and subroutine names start with capital letters e.g. Age, Name, Date,
CalculateArea, Sortlist
Indentation can be used for iteration and selection
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Program Development Life Cycle - Coding
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Writing Code
Developers begin programming modules in a suitable programming language that works
together to provide an overall solution to the problem
As each developer programs, they perform iterative testing. Iterative testing is where each
module is tested and debugged thoroughly to make sure it interacts correctly with other
modules and accepts data without crashing or causing any errors. Developers may need to
retest modules as new modules are created and changed to make sure they continue to interact
correctly and do not cause errors
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Program Development Life Cycle - Testing
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Testing
Once the overall program or set of programs is created, they are run many times using varying sets
of test data. This ensures the program or programs work as intended as outlined in the initial
requirements specification and design and rejects any invalid data that is input
Examples of test data include alphanumeric sequences to test password validation routines.
Such validation routines could be:
A password must be between 8-20 characters
Test data would be passwords of less than 8 characters or greater than 20 characters
A password must include only alphanumeric characters
Test data would be passwords including non-alphanumeric symbols such as @, ?, #, !, $ or
%, etc
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