Unit 12: Software Development

Learning aim A Unit 12 · A · Why is software used?

Why is software used?

By the end you can: explain what software is, the main categories of software, and why organisations and people develop programs.

A software program is a set of instructions that tells a computer’s processor (CPU) what to do, step by step. Hardware cannot do anything useful without software. Programs turn inputs into useful outputs, quickly, accurately and without getting tired.

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Learning aim A Unit 12 · A · Programming languages

Programming languages and compiling

By the end you can: describe high-level and low-level languages, procedural and event-driven languages, and why programs are compiled.

The only language a CPU understands is machine code: binary 1s and 0s. Every other language has to be translated first.

Program design methods include top-down design (break the problem into smaller parts), flowcharts, pseudocode and screen designs. You will use all of these in learning aim B.

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Learning aim A Unit 12 · A · Characteristics of software programs

Constructs and techniques: the building blocks of programs

By the end you can: identify command words, subroutines, string handling, file handling, data structures and event handling in a program.

All programming languages are built from the same kinds of building blocks. When you look at someone else’s program (as in Assignment 1), find these and say what each one does.

Reading a program: an annotated example

' Program: cinema ticket price (event-driven)
Private Sub btnPrice_Click(sender As Object, e As EventArgs) Handles btnPrice.Click   ' event handling
    Const ADULT_PRICE As Decimal = 9.5D                ' a constant
    Dim age As Integer = CInt(txtAge.Text)              ' input + data type
    Dim price As Decimal
    If age < 16 Then                                    ' selection
        price = ADULT_PRICE / 2
    Else
        price = ADULT_PRICE
    End If
    lblPrice.Text = "Price: " & price.ToString("C")   ' output + string handling
End Sub

Comments (after the apostrophe) are ignored by the computer but explain the code to people.

Learning aim A Unit 12 · A · Quality of software programs

The quality of software programs

By the end you can: explain how design and coding techniques affect efficiency, maintainability, portability, reliability, robustness and usability.

Spot the quality problems

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim a As Integer = TextBox1.Text
    Dim b As Integer = TextBox2.Text
    Label1.Text = a * 1.2 + b * 1.2
End Sub

No comments, meaningless names, the 1.2 is repeated and unexplained, and typing a letter crashes it.

Improved: name the controls (txtPrice, btnTotal), use a constant VAT_RATE, add comments, and validate the inputs with Integer.TryParse so a wrong entry shows a friendly message instead of crashing.

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Learning aim B Unit 12 · B · Software development life cycle

The software development life cycle

By the end you can: describe the stages of the software development life cycle and why it is used.

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Learning aim B Unit 12 · B · Designing software

Requirements, problem definition and design specification

By the end you can: describe the purpose and user requirements from a brief, write a problem definition statement and outline a design specification.

Your assignment brief describes a client and a problem. Before designing anything, show that you understand what the program is for and who will use it.

Learning aim B Unit 12 · B · Proposed solution

Designing the solution: tasks, screens, navigation and alternatives

By the end you can: describe the main program tasks, design screen layouts and navigation, and compare alternative solutions.

Your proposed solution shows the client what the program will look like and how it will work, before any code is written.

Alternative solutions

Good designers consider more than one idea, then choose and justify the best. You can offer alternatives for the screen layout, navigation, algorithms, data structures or control structures.

Learning aim B Unit 12 · B · Algorithms

Algorithms: pseudocode, flowcharts and trace tables

By the end you can: write algorithms as pseudocode and flowcharts, and check them with a trace table.

An algorithm is a set of step-by-step instructions to solve a problem. Designing the algorithm first means you solve the problem before worrying about the exact code.

The same algorithm in pseudocode

INPUT age
IF age < 16 THEN
    price ← 4.75
ELSE IF age >= 65 THEN
    price ← 6.00
ELSE
    price ← 9.50
END IF
OUTPUT price

Pseudocode is structured English: no exact syntax, but clear enough to turn into code.

Checking with a trace table

total ← 0
FOR count ← 1 TO 4
    total ← total + count
NEXT count
OUTPUT total

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Learning aim B Unit 12 · B · Proposed solution · test plan · constraints

Data, validation, error handling, predefined code and test plans

By the end you can: plan data structures and storage, validation, error handling, predefined code and a test plan with test data.

Predefined code

Reusing code (from the language’s libraries, your teacher, or the internet) saves time, but you must have permission and list the source in your design and in a comment in the code.

Test plan with test data

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Learning aim C Unit 12 · C · Developing software · Event handling

The development environment, forms and events

By the end you can: use an IDE to create a form, set properties of controls, and write event-handling code for inputs and outputs.

You will develop programs in Visual Basic .NET using Visual Studio (Community edition is free). Choose Windows Forms App (.NET Framework) – Visual Basic when you create a project.

Public Class Form1
    ' Event handler: runs when the user clicks btnHello
    Private Sub btnHello_Click(sender As Object, e As EventArgs) Handles btnHello.Click
        Dim name As String = txtName.Text          ' INPUT from a text box
        lblMessage.Text = "Hello, " & name & "!"   ' OUTPUT to a label
        MessageBox.Show("Welcome to programming")    ' OUTPUT in a message box
    End Sub
End Class

Press F5 to build and run. Stop with the red square, or by closing the form.

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Learning aim C Unit 12 · C · Constructs and techniques · data types

Variables, constants, data types and operators

By the end you can: declare variables and constants with suitable data types, use assignment, arithmetic and logical operators, and explain local and global scope.

Public Class Form1
    Const VAT_RATE As Decimal = 0.2D          ' constant: never changes
    Dim basketTotal As Decimal = 0             ' GLOBAL: usable by every subroutine in the form

    Private Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click
        Dim itemPrice As Decimal = CDec(txtPrice.Text)   ' LOCAL: only exists inside this sub
        basketTotal = basketTotal + itemPrice              ' assignment
        lblTotal.Text = (basketTotal * (1 + VAT_RATE)).ToString("C")
    End Sub
End Class

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Learning aim C Unit 12 · C · Constructs: sequence, selection

Sequence and selection

By the end you can: write programs that use sequence, If…Then…Else, ElseIf, nested If, logical operators and Select Case.

Sequence means instructions run one after another, in order. Selection means the program chooses which instructions to run, depending on a condition.

' Login check: sequence then selection
Dim attempt As String = txtPassword.Text
If attempt = "Lantern42" Then
    MessageBox.Show("Access granted")
Else
    MessageBox.Show("Access denied")
End If

Set the text box’s PasswordChar property to * to hide what is typed.

' Grade bands with ElseIf
Dim mark As Integer = CInt(txtMark.Text)
If mark >= 70 Then
    lblGrade.Text = "Distinction"
ElseIf mark >= 55 Then
    lblGrade.Text = "Merit"
ElseIf mark >= 40 Then
    lblGrade.Text = "Pass"
Else
    lblGrade.Text = "Not yet achieved"
End If

Conditions are checked from the top; the first true one runs and the rest are skipped.

' Select Case: neater than many ElseIfs when checking one value
Select Case cboDay.Text
    Case "Saturday", "Sunday"
        lblRate.Text = "Weekend rate"
    Case "Monday", "Tuesday", "Wednesday", "Thursday", "Friday"
        lblRate.Text = "Weekday rate"
    Case Else
        lblRate.Text = "Choose a day"
End Select

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Learning aim C Unit 12 · C · Constructs: iteration, recursion

Iteration: counter-controlled and conditional loops, and recursion

By the end you can: use For…Next, Do While, Do…Loop Until and nested loops, and explain recursion.

' Counter-controlled: a times table
lstTable.Items.Clear()
For i As Integer = 1 To 12
    lstTable.Items.Add(i & " x 7 = " & (i * 7))
Next

' Conditional: keep asking until the PIN is right (max 3 tries)
Dim tries As Integer = 0
Dim pin As String
Do
    pin = InputBox("Enter your PIN")
    tries = tries + 1
Loop Until pin = "2468" Or tries = 3
If pin = "2468" Then MessageBox.Show("Unlocked") Else MessageBox.Show("Locked out")

Do … Loop Until always runs at least once; Do While … Loop may not run at all.

Recursion

A recursive function calls itself on a smaller version of the problem, and must have a base case that stops it.

Function Factorial(n As Integer) As Integer
    If n <= 1 Then Return 1            ' base case: stop
    Return n * Factorial(n - 1)         ' calls itself with a smaller n
End Function
' Factorial(4) = 4 × 3 × 2 × 1 = 24

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Learning aim C Unit 12 · C · Constructs: subroutines/functions/procedures

Subroutines, functions and procedures

By the end you can: write procedures (Sub) and functions (Function) with parameters and return values, and explain why they improve programs.

A subroutine is a named block of code that does one job. A procedure (Sub) does a task; a function does a task and returns a value.

Public Class Form1
    ' Function: returns the price after discount
    Private Function DiscountedPrice(price As Decimal, memberType As String) As Decimal
        Select Case memberType
            Case "Junior" : Return price * 0.5D
            Case "Senior" : Return price * 0.7D
            Case Else : Return price
        End Select
    End Function

    ' Procedure: clears the form ready for the next booking
    Private Sub ClearForm()
        txtName.Clear()
        cboClass.SelectedIndex = -1
        lblPrice.Text = ""
    End Sub

    Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
        Dim total As Decimal = DiscountedPrice(6D * CInt(txtSessions.Text), cboMember.Text)
        lblPrice.Text = total.ToString("C")
    End Sub
End Class

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Learning aim C Unit 12 · C · Data structures · string handling

Data structures and string handling

By the end you can: use one-dimensional arrays, records (user-defined types) and string-handling functions.

Dim classes() As String = {"Yoga", "Spin", "Pilates", "HIIT", "Swim"}
For i As Integer = 0 To classes.Length - 1        ' loop through every element
    cboClass.Items.Add(classes(i))
Next

Structure Booking                                 ' a record / user-defined type
    Dim memberName As String
    Dim className As String
    Dim sessions As Integer
    Dim price As Decimal
End Structure
Dim todaysBookings As New List(Of Booking)        ' a list of records

Arrays in VB start at index 0.

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Learning aim C Unit 12 · C · File handling · annotation · compiling

File handling, validation, error handling and compiling

By the end you can: open, read, write and close text files, validate input, handle errors so the program does not crash, comment the code and compile an executable.

Imports System.IO

' WRITE: add one booking to the end of the file (creates it if needed)
Private Sub SaveBooking(line As String)
    Using writer As New StreamWriter("bookings.txt", True)   ' open, True = append
        writer.WriteLine(line)                                  ' write
    End Using                                                   ' close (automatically)
End Sub

' READ: show every booking in a ListBox
Private Sub LoadBookings()
    lstBookings.Items.Clear()
    If Not File.Exists("bookings.txt") Then Exit Sub          ' error handling: no file yet
    Using reader As New StreamReader("bookings.txt")           ' open
        Do While Not reader.EndOfStream
            lstBookings.Items.Add(reader.ReadLine())            ' read
        Loop
    End Using                                                   ' close
End Sub

Validation and error handling

Dim sessions As Integer
If txtName.Text.Trim() = "" Then                                  ' presence check
    MessageBox.Show("Please enter the member's name.") : Exit Sub
End If
If Not Integer.TryParse(txtSessions.Text, sessions) Then         ' type check
    MessageBox.Show("Sessions must be a whole number.") : Exit Sub
End If
If sessions < 1 Or sessions > 10 Then                            ' range check
    MessageBox.Show("Enter between 1 and 10 sessions.") : Exit Sub
End If
Try
    SaveBooking(txtName.Text & "," & cboClass.Text & "," & sessions)
Catch ex As IOException                                         ' error handling
    MessageBox.Show("The booking could not be saved. Check the file is not open.")
End Try

Validation stops bad data getting in; Try…Catch stops unexpected errors from crashing the program.

Annotate your code with comments explaining what each part does and why. It shows the assessor your understanding (2C.P4 asks for commentary throughout) and makes the code maintainable. Finally, compile the program into an executable (Build → Build Solution) so the client can run it without Visual Studio.

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Learning aim C Unit 12 · C · Testing and refining the software

Testing, debugging, feedback and refining

By the end you can: test a program against the test plan and requirements, find and fix errors with the debugger, gather feedback and refine the program.

Refine the program using test results and feedback, then re-test, because one change can break something else. Record every change, update your design documents, and add any new predefined code to your sources table.

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Learning aim D Unit 12 · D · Reviewing software

Reviewing the finished program

By the end you can: review a finished program against user requirements, fitness for purpose, user experience, constraints and quality, and recommend improvements.