Visual Basic

Rosalind Archer

Spring 2002

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Running Visual Basic

• Visual Basic is included with Microsoft Excel. My examples use Excel ‘97.

• When you open a file containing Visual Basic code make sure you choose “Enable Macros”.

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Your message may look a little different to this one.

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To access the Visual Basic code used in any spreadsheet choose Tools->Macro->Visual Basic Editor (or hit Alt+F11)

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The Visual Basic Editor

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Modules

• Visual Basic code can be divided into a series of “modules”. To start new program you need to insert a new module:

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VB as an Extension of Excel

• Excel has many built in functions sin(), cos(), sum(), sqrt() etc.– what if you wanted to add your own

functions?– Visual Basic allows you to do that.

Let’s add a function which computes relative permeability.

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Krw Function

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Using the Function in Excel

Now that the function is defined we can use it like any other Excel function.

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The Visual Basic Language

• Comments … statements that allow the programmer to describe what a piece of code does.

• Begin with an ‘

• Shown in green automatically

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Comments• Comments are important!

• They provide English language statements to describe what the computer code is doing.

• Make liberal use of comments - it will help you and others understand your program.

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Variables

• Variables are labels assigned to numeric values (we’ll ignore text for now). You can manipulate them in algebraic expressions.

• It’s a good idea tell the program what variables you’re going to use ahead of time. This can be enforced using:

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Variable Types

• Variables all have a type associated with them. The types will be using are integers and doubles.– Integers ..,-1,0,1,2,3 …– doubles -1.89475, 2.0, 3.498 etc

• Variables are declared using the statement Dim.

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Declaring Lots of Variables

Dim a,b,c,d As Integer• This might look like a good idea to save

some typing (and in some languages stuff like this works.)

• In Visual Basic only d would be declared correctly. a,b and c would exist but may not have the correct type.

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Combing Integers and Doubles

• Be careful with the difference between integers and doublesDim n As Integer

n = 4/3

The value that n will actually take is 1 because that is the nearest integer to 4/3 = 1.3333

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• Visual Basic adds a # sign to doubles that have fractional part e.g.:

Dim dx As Double• If you type dx = 1.0

Visual Basic will change this to

dx = 1#

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Converting Variable Types

• Sometimes we want to convert an integer (such as a counter in a For loop) to a double.

• The CDbl function does this.

Dim x As Double

Dim I As Integer

x = CDbl(I)

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Constants

• If we want to define a constant we can use the keyword Const

Const g As Double = 9.81

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Arrays

• We can store matrices and vectors in array variables. Declaring them is a two step process.‘This code solves Ax=bDim A() As Double, x() As Double, b() As DoubleReDim A(10,10), x(10), b(10)

• Do NOT assume that the elements in matrix are set to zero after it is declared. This can get you into trouble!

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Array Elements

• The elements in the array can be accessed using ( , ) e.g.:A(2,3) = 4.4

• Note that the indices start from 1 i.e. A(1,1) is the first element in the matrix (some other languages have indices which start from 0)

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Intrinsic Functions• Visual Basic has many common functions built in e.g.:

Dim f As Double, x As Double

f = Log(x)

Note that in Log(x) is the natural log

not log base 10.

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Input and Output to/from Excel

• We’ll use a worksheet to supply the input to the program and to display the output.

• Note the a single workbook (.xls file) can have several worksheets inside it e.g.:

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• Before we do any input/output we have to choose which worksheet to work with, e.g. to choose the “Results” sheet:With Worksheets(“Results”)

…

End With

• Within the With statement we can read or write values from/to the worksheet.

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• We can read or write text and numbers in the same way:

.Cells(1, 1) = "Time” ‘ Writes to A1

.Cells(1,2) = delx ‘ Writes to cell B1

x = .Cells(1,3) ‘ Reads from cell C1

• If you a certain sheet to come to the frontof the workbook you can activate it.This is just like clicking on its tab. The statement involved is:

.Activate

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• Another useful function is clearing all the cells in a worksheet:.Cells.ClearContents

• All statements beginning with . must be inside With/End With

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Conditional Statements

• If, then, else allows us to test a condition and do something as a result:

If x<10 Then

y = 10.3

Else

y = 0.5

End If

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Looping

• Loops are used to execute a piece of code repetitively. There are two main kinds For and While.– For executes a set number of times– While executes until a condition

remains true.

• Both kinds of loops are very useful.

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Example - For Loop

Dim i As Integer

For i=1 to nx

x = x + dx

...

Next i

• Loops can be nested

inside each other e.g.:

For i=1 to N

For j=1 to N

A(i,j) = 0

Next j

Next i

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Example - While Loop

While T < Tend

T = T + dt

…

Wend

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User-defined Functions

• Visual Basic has plenty of common functions but often it’s a good idea to define your own if there is somethng you need to compute often (such as relative permeability):

Function krw(Sw As Double) As Double

krw = Sw^2.0

End Function

(A function could have several arguments)

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Function Arguments

• The parameters passed to a function are known as arguments.

• You don’t have pass arguments with the same names as the ones in the function declaration e.g.

‘This function uses interpolation to evaluate y(xTarget)‘ given known x and y vectors

Function Interpolate(x() As Double, y() As Double, _xTarget As Double) As Double

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Function ArgumentsDim y1 As Double

Dim x1 As Double

Dim x2 As Integer

Dim x() As Double

Dim y() As Double

y1 = Interpolate(x,y,x1) ‘ OK

y2 = Interpolate(x,y,x2) ‘ Not OK

This doesn’t work because x2 is a different type to xTarget.

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Subroutines

• Subroutines are a way to divide a program into pieces where each piece performs a well defined task.

• Using subroutines makes your program much easier to understand!

• Your whole program can be written as a subroutine called when you push a button on a worksheet.

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Example - Subroutines

• In a reservoir simulator we need to form a matrix (A) which depends on dx and dt (among other things).

Sub BuildA(A() as Double, dx as double, dt As Double)

A(1,1) = 1.0/dt

…

End Sub

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Example - Subroutines

• When we are ready to use a subroutine use the statement “Call”

Call BuildA(A,dx,dt)

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The Visual Basic Editor

• If you type something VB doesn’t like it will tell you!

• Visual Basic Editor automatically colors text according to the following scheme:Green = comments

Blue = VB keyword (For, While etc)

Black = everything else

Red = errors

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Indenting

• It’s a good habit to indent code so you can see the connection between For-Next, If-End If etc.

For i = 1 to 10

For j = 1 to 10

A(i,j) = 0.0

Next j

Next i

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• Here’s an example of a statement Visual Basic didn’t like.

– What’s wrong with it?

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Long lines of code

• Lines of code can be as long as you like. But to make code easier to read it is wise to break long lines up using a continuation character ( _ ).

Product = a*b*c*d*e*f*g*h*i*l*j*k*l*m

orProduct = a*b*c*d*e*f*g*h _

*i*j*k*l*m

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Running the Program

• If everything goes to plan running the program is as simple as clicking the button for it on the worksheet!

• There is no need to “compile” or “link” the program after you edit it.

• If something goes wrong when you run your program you’ll get a message.

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Example

error message ->

The problem

will be highlighted

in yellow. You can edit the code and

rerun your program. But first you must

“reset” it using Tools->Reset in VB.

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If you don’t reset

your code you’ll get

this message.

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Debugging

• When a program

crashes VB

will show you the

value of a

variable if you hold your cursor over it.

Here I was dividing by zero - always a bad thing to do!

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Breakpoints

• By right clicking in

the grey bar by the

code you can set a

breakpoint.

• When the code gets

to this point it will

stop and let step through examining

variable values.

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Watches

• The red dot and highlight show this is a breakpoint.

• Debug->Add Watch adds variable to the bottom window to watch. If you click on the + sign by “b” you can see the values of all the elements in the vector.

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Stepping

• To step through code line by line from a breakpoint (watching variables) use:– Debug->Step IntoDebugs into functions and subroutines as

they are called.

– Debug->Step OverDoes not debug into functions and subroutines as

they are called.

– Debug->Step OutQuits debugging a subroutine or function.

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Adding buttons to worksheets

• If you want to add a new button to a worksheet, choose View->Toolbar->Forms.

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• Choose the button icon

from the toolbar and

drag an area in the

worksheet to create the

button.

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• Your new button now needs to be assigned to a subroutine. If that subroutine doesn’t exist you’ll get an error.

• Once you’ve assigned a subroutine you can edit the name of the button also.