H SDD Design and Testing Notes

Designing & Testing Software
Development Notes
Software Design & Development:
Design Notations, Development Methodologies &
Testing and Documenting Solutions
The Software Development Process
 Questions
Development Methodologies
 Questions
Design Notations
 Questions
Errors and Bugs
 Questions
Testing Techniques
 Questions
Software Development Process
The software development process is a seven step process that
programmers usually follow when creating software.
Some development methodologies don’t follow all of these steps
The process is sometimes referred to as the waterfall model – as
once you have completed one step, you move onto the next
The steps are
The basic waterfall model is simplistic. In practice, iteration is
used to improve the quality of the created software
In iterative software development, earlier phases of the
development process can be redone based on what has been
learned later.
In other words, you can go back up the waterfall!
The Analysis stage is where you identify what the program is going to do.
The analysis stage usually requires the participation of two parties – the
systems analyst and the client
The client is a person or group who the software is being made for. The
client usually has a good idea of what they want the software to do, but may
not be able to express it well
The systems analyst is a person or group whose job it is to identify what
the clients requirements are.
Systems analysts require both a strong understanding of computing and
interpersonal skills to elicit information from the client
Methods used by the systems analyst might include:
Interviews – with both managers and workers for the client
Observations of existing working practice
Software Specification
At the end of the analysis stage, the systems
analyst and the client will have agreed upon a
software specification document
The software specification is a legally binding
document. It is a contract which states what the
software is going to do, and how much the software
developers will be paid for creating it for the client.
In the design stage, the software specification is used to
plan out how to create the software, and how it will do the
tasks required of it.
Different design notations will be used to create different
parts of the design:
 The user interface can be designed using wireframing
 Individual subprograms can be designed using pseudocode
Depending on the development methodology used, a
design document may be produced at this stage.
While the software specification states what the software has
to do, the design documentation will state how it does it
In the implementation stage, the software is created
The project manager in charge of creating the software has
to manage the implementation stage. They have to lead a team
or teams of programmers to create the software.
Many factors have to be considered when implementing
 What programming language will be used
 Who is going to program which part of the program
 Do additional programmers need to be hired? Will currently hired
programmers require additional training?
 What is the timescale for producing the software, and is the
programming team keeping to the timescale
 What is the budget for creating the software, and is it being kept to
In the testing stage, the software is tested to make sure it is
working correctly. Software needs to be reliable, robust and
fit for purpose
Reliable software processes data correctly.
Robust software is can cope with errors when it runs – it
does not crash unexpectedly
Fit for purpose software fulfils the requirements set out in
the software specification
Testing will be examined in more detail in a later section
Once software is “complete”, it can be documented. A number of
different documents might be created
The user guide or manual tells the user how to complete certain
tasks with the software.
The user guide is written for typical users and does not require
significant computer expertise to understand
Technical documentation covers technical information about the
software, such as configuration, system requirements and application
programming interface (API) documents for the software
Technical documents are written for expert users and other
programmers so they can modify the software
In the evaluation stage, the software is judged in terms of how
good it is.
The most important consideration is whether or not the
software is fit for purpose – does it do what it is supposed to,
as set out in the software specification
There are a number of factors that should be considered as
 Reliability – does the software process data correctly?
 Robustness – can the software handle errors without crashing?
 Efficiency – can the program process data without using to much
memory / processing time?
 Portability – can the software be used on different computers?
 Maintainability – can the software be changed easily
 Readability – is the program code easy to read, using meaningful variable
and subprogram names, and internal commentary
The maintenance stage happens after the software has been
released or passed to the client. It covers changes made to the
software during its lifetime.
Corrective maintenance fixes bugs and other errors
discovered in the software after release
Perfective maintenance adds new features to software
Adaptive maintenance changes software to allow it
continue working despite changes to its environment – such as
running on a updated operating system for example
Questions – The software
Development Process
Answer questions in full sentences!
List and briefly describe the seven steps of the software development
Explain how making the development process iterative can improve
the quality of software produced
Explain why a systems analyst needs both expert computer
knowledge and interpersonal skills
What is a software specification and why is it important to agree on
one before creating software
What is meant by the term “fit for purpose” when testing and
evaluating software. Why is this the most important consideration
when evaluating software?
List and describe other factors to consider when evaluating software
Describe two types of documentation that can be produced during
the documentation stage
List and describe three types of maintenance
Development Methodologies
A development methodology is the way in which software
is created
The waterfall model is an example of a top-down
development methodology.
Top-down development methodologies set out to complete
the entire specification of an item of software in one go. The
project is split into progressively smaller parts, with each part
representing an easier subsection of the task.
Once a task is split into sufficiently
easy chunks, individual programmers
can complete each part of it
This top-down approach represents the traditional
method of developing software. It holds a number
of advantages:
 Thorough, careful and considered – clear goals at each phase
of development
 Clearly defined roles and specialisations – each individual
knows what they are responsible for
 Easy to manage, easy to keep track of where things do go
There are also disadvantages with this approach:
 Unwieldy and prescriptive – flaws in the design are difficult to
correct and may require an extensive redesign and throwing
away completed code to accommodate design changes
 Inefficient – can result in a lot of extra work, especially
documentation, which in turn can result in software being
over budget
 Client sees little progress until the software is complete
Rapid Application Development
Rapid Application Development (RAD) is an
alternative development methodology intended to
overcome the weaknesses of the waterfall model.
In RAD, the specification and planning are less
important. Prototyping is used to produce and
demonstrate incrementally better versions of the
software. Feedback from the clients is used to alter
the requirements and improve the software
User Design and Construction are
iterative stages – each new software build
informs the design of the next build
Using RAD as a development methodology has a
number of advantages over the waterfall model:
 The client gets to see the software very early on and has
input into its progress
 Client feedback results in a very good user interface
 Risk factors can be identified and solved without requiring
complete redesigns of the software – this makes it more likely
that the software will be produced on time and on budget
But reducing the importance of planning has
consequences that are disadvantageous for RAD
 Client has to give up more resources in terms of time and
man-hours to provide the feedback
 Feedback tends to focus on UI – back end code can end up
poorly designed and less efficient as a result
 Less control over the process – in particular since the
requirements change over time, it becomes more difficult to
judge if the developers have fulfilled their contract with the
Agile Methodologies
Agile methodologies are software development
methodologies that favour adaptability and flexibility
over planning and following processes.
Rapid Application Development could be considered an
agile methodology. There are a number of different
agile methodologies such as Extreme Programming and
Scrumm. They follow a number of principles:
Individuals and interactions over Processes and tools
Working software over Comprehensive documentation
Customer collaboration over Contract negotiation
Responding to change over Following a plan
Pair Programming is an example
of an agile technique
Top-Down vs Agile
Top-down and agile methodologies have different
strengths and weaknesses. This makes them
suitable for different types of projects.
Smaller to medium scale projects work well with
agile methodologies. Agile techniques that rely on
close collaboration amongst the development team
can help make better quality software.
Larger projects require more people to complete.
Top down models allow much better control over
larger teams because of the clearly defined roles
and structures
Questions – Development
Answer questions in full sentences!
Explain what is meant by top-down software development
Describe the advantages of top-down development
Describe the disadvantages of top down development
What is Rapid Application Development?
Explain how RAD improves the quality of developed software
Explain how RAD can reduce the risk of cost and time overruns
Why might it be a problem that agile methodologies result in less
control over a project than top down methodologies?
Why would an agile methodology be less suitable for creating an
large information system for a multinational bank with offices in
several countries?
Design Notations
A design notation is a way of describing the
design of a program, or part of a program
There are many different design notations.
Different design notations are more useful for
different parts of the software
 The user interface can be designed using wireframing
 Individual subprograms can be designed using
It is important to select the right design
notation when designing software
Pseudocode is a design notation that sets out what an algorithm will do. It
uses the structure of a programming language, but uses English words instead
of program code
This pseudocode example shows the algorithm for iterating through an array:
1) FOR EACH name FROM names
It uses the for loop programming construct, but because the words used are
closer to English, it is easier for people to understand.
Pseudocode is easy to translate into a programming language – each line of
pseudocode should be equivalent to a line of code in the programming
The equivalent python code
Stepwise Refinement
In stepwise refinement, complicated pseudocode steps that would take more than one line
of code are refined into further pseudocode
Stepwise refinement allows bigger programs to be designed with pseudocode. It helps
developers to create modular software. Refined steps are often made into subprograms
This example shows how steps can be refined:
1) Set up variables
2) Get input
3) Calculate answer
4) Display answer
The refinement as a subprogram
Wireframing is a design notation for creating
user interfaces. A wireframe is sketch that shows
the placement of elements on the UI.
The advantage of wireframing is it can be used to
show to and gain feedback from the client and
users very easily. It does not design the underlying
program code though
A wireframe and corresponding Visual Basic interface
Structure Diagram
A structure diagram shows the relationship between a
program and the subprograms that make it up, as well as the
data flow between them.
Volume of a
width, length, height
width, length, height
Get Inputs
Display answer
Structure diagrams are useful for showing the shape of a
program to people who are not computer experts. The bigger
a program gets, the more difficult it is to draw the diagram
A flowchart is a graphical representation of the steps
taken by a program
Display error
This partial flowchart shows
input validation
Note the use of the diamond
shape for decisions
Flow charts are useful for showing the flow of
information in a program to non-experts, but they
become increasingly difficult to draw as the program
gets bigger
Universal Modelling Language is a design notations
that shows objects – classes, subprograms and database
tables – as well as their attributes and relationships to
each other.
UML is particularly useful when designing software that
includes databases, or when using object orientated
programming languages
Questions – Design Notations
Answer questions in full sentences!
What is pseudocode and stepwise refinement?
Why is stepwise refinement a useful design notation to use when
using a top-down development methodology?
Describe an advantage of pseudocode
What is meant by wireframing?
Give one advantage and one disadvantage of wireframing
Explain why structure diagrams are inappropriate for designing
larger programs
Why is a flow chart better for explaining how input validation
works to a non-expert than the corresponding pseudocode
When might you use UML as a design notation? Why?
Write the pseudocode for a program that calculates the
perimeter of a rectangle. Use stepwise refinement to break the
pseudocode into subprograms
Errors and bugs
During the implementation stage of creating
software, it is common for the programmers to
make mistakes and errors known as bugs.
You are required to be able to identify such bugs
where they occur, the nature of the error and
correct it
Bugs can be identified by testing. Software must
be tested comprehensively in order to find and
correct as many bugs as possible before release
Syntax Errors
A syntax error is when the programmer has made
an error in their use of the programming language.
Syntax errors happen when the computer cannot
understand what a line of code means.
Programs will not compile or run at all if they
include a syntax error. This makes it easy to know if
your program has a syntax error, though it can still
be difficult to find and fix
Most programming environments will highlight
where they have encountered the error. This might
not be where you have to fix a mistake though
Typical causes of syntax errors include
 Spelling errors – programming keywords and
variable names misspelt
 Missing symbols – “, &, : or other symbols have
been missed out, meaning the computer can’t
understand what the program says
 Closure error – blocks of code haven’t been closed
off properly with brackets or end statements
where appropriate
The missing : from the for
line will cause a syntax
Many compilers will
highlight the next line as
being the source of the
error though!
Execution Errors
Execution Errors are bugs that will not be
highlighted when the program is compiled, but
will cause the program to crash whilst running
Some execution errors only happen when
using specific sets of input data, which can
make them very hard to track down.
Most programming environments will report
at which line an execution error occurred
Typical causes of execution errors include
 Spelling and naming errors – trying to access a variable
or object using the wrong name may cause the program
to crash.
 Calculation errors – trying to perform calculations that
are impossible such as dividing by 0 will cause the
program to crash.
The misspelt variable poeple above is
assigned the value 0 instead of what the user
entered. This causes a divide by 0 error which
crashed the program
Logic Error
Logic errors cause a program to process information incorrectly and
give the wrong answer. Logic errors do not cause a program to crash
or fail to run.
Logic errors can only be spotted by testing, and may only occur with
specific inputs. This makes them very hard to detect and fix.
Logic errors are usually caused by bad design and lack of knowledge
This logic error is caused by the programmer not understanding
operator precedence
Questions – Errors
Answer questions in full sentences!
Describe in detail the three types of errors in a program?
Why is it important to thoroughly test a program in order to catch
execution and logic errors?
Execution and logic errors are sometimes also known as runtime
errors. Why?
A program has been written to count the number of A grade passes
in a test. It is shown below
What is the output of the program?
What type of error has occurred?
Identify the line with the error in it and correct it
A program has been written to calculate tax codes. It is
shown below
What type of error does this program include?
Identify the lines with the error in it and correct them
Testing Techniques
In the testing stage, software is checked to make sure
that it works correctly.
In order to test software, the testers must create and
follow a test plan.
Test data is a specific input or inputs for the program.
If the program fails to process a single item of test data
correctly, there is a bug that needs fixed.
Once a program is able to correctly process all the test
data from the test plan, it can be said to be reliable
Component Testing
To simplify the process of testing software, individual components of the
software can be tested.
If a component works correctly it can integrated with the rest of the program
To test an individual component of a program, the testers have to identify
suitable sets of test data to use with it. A test table should be created and
Expected result
Actual result
A test table for checking the input validation of a mark between 0 and 10.
Note the normal, extreme and exceptional values
Comprehensive testing
Test plans should be comprehensive – they
should test each appropriate component with a
range of appropriate values. They should also check
that the software as a whole produces correct
To make a comprehensive test plan:
 Identify each component to be tested
 Identify test data to use with each component – normal,
extreme and exceptional values
 Create a set of test data to check that the program as a whole
gives correct results
 Work out what the expected result for each bit of test data
should be
To follow the plan
 Record the actual results for each bit of test data
 Compare the expected and actual results
Testing should be iterative – if the expected and
actual results do not match, the program has a bug.
The developers should go back to the
implementation phase, fix the bug and then retest the program
In practice, component testing is carried out
continuously during implementation. Programmers
usually check that each module of code is working
as they create it
Dry Runs
In dry run testing, a programmer reads through
pseudocode or program code, keeping track of the
value or state of a variable or other object
Dry run testing is useful as it can be carried out
before programming has even begun. It can be used
to check if an algorithm design is correct.
Dry run testing is not very scalable – most people
find it hard to keep track of several different values
in their head at once!
Trace Tables
A trace table is a dry run where the changes in the values of
variables are recorded in a table.
This makes it easier to track the progress of multiple variables
1) SET total TO 0
2) FOR counter FROM 1 TO 10
SET total TO total + 10
Note how the line numbers repeat
as the loop progresses
A breakpoint is an intentional stopping point placed in a program in
order to help debug the program.
When run, the program will pause when it reaches the breakpoint.
This allows the programmer to study the program’s environment –
including information such as the values stored in variables. This lets
the programmer check if the program has run correctly up to the
Once the programmer is satisfied, the program can then be continued.
Some software development environments include the ability to place
breakpoints in programs
A breakpoint in Visual Basic
Questions – Testing
Answer questions in full sentences!
1. What is meant by component testing?
2. Describe in detail how each individual component in a
program should be tested
3. What is meant by comprehensive testing? Why is it
important to test a program comprehensively?
4. The testing process is usually iterative. Why is it a
good idea to carry out testing iteratively?
5. Explain what is meant by dry run testing
6. Give one advantage and one disadvantage to dry run
7. What is meant by a breakpoint?
Copy and complete the trace table for the following