week 9 dialogues and forms numerical integrationen1811/13s1/lects/week09-forms.pdf · engg1811 ©...

ENGG1811 © UNSW, CRICOS Provider No: 00098G W9 slide 1

ENGG1811 Computing for Engineers

Week 9

Dialogues and Forms

Numerical Integration


References & Info

Chapra (Part 2 of ENGG1811 Text)

• Topic 21 (chapter 15) Custom Dialogue Boxes

This week’s schedule

Tue, Wed am: new material

Wed pm: worked example


Forms and Controls

VBA has access to the full window library

1. Buttons or similar Controls (window elements) can be placed on sheet.

2. Dialogues are provided for standard interaction such as opening files, choosing colours. Can be called from VBA.

3. Forms (windows with controls placed by the designer) can be embedded in document.

Each active element can trigger actions through VBA event procedures


1. Command Buttons

Simplest scheme: command buttons

• Show control tool box

Developer tab – Insert

2003: View – Toolbars – Control Toolbox

• Select command button from bottom group (ActiveX Controls, arrowed)

• Click and drag on sheet

• Right-click*, select Properties

• Change name (cmdDemo), caption, font, other

properties

• Right-click again, select View Code

• VBE allows changes to properties and event handler: cmdDemo_Click( )

• Exit design mode, button becomes active * or pick from Controls on the ribbon

Exit Design Mode

CommandButton


Example: Colour Swatch

• Lay out a new sheet with named ranges

ColourRegion (4 cells) and IndexBox (1 cell)

• Show the control box, add a button called cmdChangeColour labelled Next Colour

ColourRegion

IndexBox

Name box


Colour Swatch Event Procedure

• In Design Mode, double click button. Opens up VBA associated with this worksheet, including events for the command button.

• Sheet needs one variable, changed by the event procedure:

Const NUM_COLOURS = 56 ' Excel VBA standard (2003+ compatible)

Dim colLast As Integer ' last colour index shown (or 0)

Private Sub cmdChangeColour_Click()

colLast = 1 + colLast Mod NUM_COLOURS

With ActiveSheet

.Range("ColourRegion").Interior.ColorIndex = colLast

.Range("IndexBox").Value = colLast

End With

End Sub


Colour Swatch Demo

See sheet ColourSwatch on the week 9 Lecture demo spreadsheet.

This example is simple enough for all VBA code to be included with the sheet itself

Often cmdbuttonXYZ_Click() just calls a

subprogram in a module.


Properties Window

• Edit characteristics of any static object

• View by name or grouped by category

• Accessible from VBE or from control via Properties on shortcut menu or ribbon

• Units are points (1/72 inch, approx 28 pts per cm), 1 decimal place


2. Built-in Dialogues

• You’ve seen MsgBox before, now use its value:

reply = MsgBox("Root found, look for more?", _

vbYesNo, "Newton")

If reply = vbYes Then …

MsgBox "Out of jelly beans!", vbCritical ' OK only

Select Case MsgBox("I'm feeling tired, can I quit?", _

vbAbortRetryIgnore, "Ennui")

Case vbAbort: Application.Exit

Case vbIgnore: ' do nothing (keep going)

Case vbRetry: ' keep going anyway

End Select

Type msgbox to VBA Help for codes


Input box

• To issue a prompt and obtain a single string from the user you can use the InputBox function

Dim strName As String

strName = InputBox("What is Your name?")

If strName = "" Then

MsgBox "Silent type, eh?"

Else

MsgBox "Hello, " & strName

End If

• Occasionally useful, but large data sets should be on the active sheet, and smaller sets could have customised forms (soon)


Other Builtin Dialogues

• Getting the user to select a file name is a common requirement:

GetOpenFileName("Text Files (*.txt), *.txt")

• We would use this if we covered reading and writing to text files in ENGG1811 (but we don’t)


3. User Forms

A user form is an independent window containing controls placed by the designer

In the VBE, select

Insert - UserForm

Resize. From the toolbox, select and place controls.

Each is given a default name.

For our first example, add three labels, a text box and a command button


Toolbox Controls

Control Pfx Tool Graphic Main Properties*

Text box txt Value

Label lbl Caption

Command

button cmd Caption

List box lst Value, ControlSource

Combo box cbo Value , ControlSource

Option button opt Value (Boolean)

Check box chk Value

Spin button spn Value, Min, Max

Image img Picture

*All have Name, Width, Height, Left, Top, Visible, TabIndex etc


Control Properties

Can change most

attributes of each control (select with circled drop-down) in the Properties window. Important ones arrowed. See also

Categorized view.

Content

Identity

Text attributes

Position

Behaviour


Initiating Action

After designing the form, what then?

• All controls detect and manage events such as

frmGame_Activate() rdoGender_Change()

cmdGuess_Click() cmbAction_Change()

txtInput_AfterUpdate()

Private Sub lbl1_MouseMove(ByVal Button As Integer, _ ByVal Shift As Integer, ByVal X As Single)

• Each control’s properties are exposed through the object model, so VBA can use and modify them (especially displayed values)

• Must consider what should happen for each relevant event: event-driven programming

• Within event handlers normal algorithms apply


An Example

Let’s continue with the form we’re building, it’s for an implementation of the children’s High-Low guessing game

– program picks a number between 1 and n (say 100)

– user guesses, is told high/low/correct

– user wins if guessed within limited number of tries

• Can you identify the optimal strategy for the user (it’s certainly not random guessing!) ?


High-Low Form

Rename and reposition elements, add captions in 10pt font, adjust widths:

Added sample text in 14pt to confirm suitability of size.

lblResult is blank, content will be

assigned by VBA


Events

What (meaningful) events can occur?

1. Form is activated

2. Form is closed (via the window bar)

3. Reset button is pressed

4. User enters text

1 and 3 are the same; 2 isn’t handled

Hence only two need to be managed.

Event procedures:

cmdReset_Click()

txtGuess_AfterUpdate() why not txtGuess_Change()?

UserForm_Activate() – calls cmdReset_Click


Document Structure

Designs and VBA code are stored in three places in the document:

Module1 (rename it HiLo_module) has state variables and all algorithm processing code

Worksheet has VBA code to initiate form

frmHiLo has form layout, plus VBA code to handle all events


Startup

We want the form to appear when the sheet is selected (activated)

Use the worksheet activation event to position the form and activate it (Show method):

' Display the HiLo form, offset into Excel's window

Private Sub Worksheet_Activate()

frmHiLo.Left = Application.Top + 100

frmHiLo.Top = Application.Left + 100

frmHiLo.Show

End Sub


Event Responses

Pseudocode for Reset:

think of a number

set guess count to 0

clear lblResult.Caption and txtGuess.Value

Pseudocode for User entry:

fetch typed string (from txtGuess.Value)

if not a sensible guess then

issue error message (assign to lblResult.Caption)

elseif correctly guessed then

issue win message

elseif guess limit reached then

issue loss message

else

increment guess count

issue keep-trying message

dubious logic, allowing the software to be fooled much as viruses exploit logical errors in production software, such as passing a 100,000-character URL to a program that assumes nobody would ever do that!


State Variables

State is maintained in the visible elements and in two variables:

– target number

– number of guesses

These variables and additional procedures are stored in a new module (HiLo_module)

Event procedures should contain minimal code: they call procedures in HiLo_module to update

state

Must use fully qualified names: HiLo_module.PickRandomTarget

HiLo_module.ResetGuessCount

HiLo_module.Guess(strGuess)


Observation

Most of program consists of validating what the user typed

Exercise: add a checkbox labelled Show limits

If checked (chkShowLimits.Checked), display the current range of possible values based on what the player has guessed

– makes the game easier, especially if there were to be a time limit on each guess

– adds two controls (labels for the limits) and more VBA in return for richer functionality

– even fancier: two rectangular frames, one filled to show progress


Debugging

• Aim is to produce VBA procedures that perform exactly as intended

• Errors can be introduced at any stage

– design (poor understanding of problem)

– pseudocode (poor understanding of solution)

– implementation (poor understanding of VBA, or typos)

• VB Editor picks up gross typo-type errors only

• On execution, could get

– run-time error such as divide by zero

– run-time error such as invalid object method

– normal termination and correct result

– normal termination and incorrect result

• Different tests may produce different outcomes


Debugging

• VB interpreter has full control over the execution of your program

• If we get a run-time error, VBE shows current execution position

– Can view simple variable values to help infer error

• If result is wrong, it may be possible to work out the error, but often you need to know things like:

– What was the value of this variable at this point in the execution?

– How many times did this loop execute?

– When was this variable changed?

– Did this part of the procedure get executed?

• VBE can help you get answers to these questions


Error Handling

Programs can have errors of several kinds

1. Syntax and semantic errors due to typos or poor understanding of the language

2. Logical errors that produce a different outcome from the expected one

3. Run-time errors

The VB Editor can catch most of (1), and the programmer will use various techniques including debugging to deal with (2)

(3) normally stops the interpreter


Types of Run-time Error

a) Impossible calculations such as divide by zero

b) Exceeding data limits (integer overflow etc)

c) Array usage errors (index out of bounds)

d) Improper object usage (misspelled method or property, semantic error)

e) Missing or malformed data (no collection membership test, so this can happen easily)

(a) and (c) indicate logical errors or insufficient data validation

(b) may be a logical error, or wrong type used

(d) is a semantic error

(e) is a condition that can be detected and managed


Trapping Errors (Advanced topic)

On Error statement controls error handling

On Error GoTo label

On Error GoTo 0

On Error Resume Next

Error handlers are labelled statements local to a subprogram, often at the end, and preceded by Exit Sub statements

If an error occurs, transfer control to the labelled statement

Restore default handler (terminate execution on error)

Ignore error and continue with statement following the error


Worked Example: Numeric Integration

Numerical integration approximates the solution to a

definite integral that may not have a closed form by using

a series of shapes to model the area under the curve

Simplest of these is the Trapezoidal Rule, which uses thin

vertical slices with a straight line at the top (forming

adjacent trapeziums)

a

f (x)

b

Integral of f (x)

from a to b

is approximated by

the sum of the areas

inside the

red trapeziums


Trapezoidal Rule, theory

References: Wikipedia, many other web refs

Animation: http://math.fullerton.edu/mathews/a2001/Animations/Quadrature/Trape

zoidal/Trapezoidalaa.html

After doing the maths for n equally spaced panels, the

formula reduces to

*/)( ianabiaxwhere i

)()(2)(2)(2)(2

)( 1210 nn

b

a

xfxfxfxfxfdxxf

Twice the middle values

Panel width

n

ab

End-points counted

once only

http://math.fullerton.edu/mathews/a2001/Animations/Quadrature/Trapezoidal/Trapezoidalaa.html

http://math.fullerton.edu/mathews/a2001/Animations/Quadrature/Trapezoidal/Trapezoidalaa.html


Trapezoidal Rule, pseudocode

We can easily convert this to pseudocode:

set sum to f (a) + f (b)

set delta to the panel width __________

For p = _____ To _____

add __________________ to sum

Next p

area = ______________


Trapezoidal Rule, implementation

Function TrapArea(a As Double, b As Double, _

n As Integer) As Double

• Limitations: for this quick example, we will represent the

function to be integrated as a VBA function called directly

from TrapArea. It is possible to specify it as a formula on

the sheet, which is easier for end users


Summary

• Controls can be placed directly on the sheet to add interactivity between sheet and VBA

• Built-in dialogues are powerful but not universally available

• Forms provide more control over user interface

• Form programming focuses on events

• Debugging techniques include

– state display using Debug.Print

– state inspection after error or breakpoint

– single stepping, watch expressions

– careful code reviews

• Error handling regains control after unexpected program state transitions

week 9 dialogues and forms numerical integrationen1811/13s1/lects/week09-forms.pdf · engg1811 ©...

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