Microsoft Visual Basic makes it very easy to write code by providing you skeleton code as we saw above, a very intuitive Code Editor, and other accessories. Indentation is a technique that allows you to write easily readable code. It consists of visually showing the beginning and end of a section of code. Indentation is done at various levels. For the code of an event, indentation consists of moving your code to the right side so that the only line to the extreme left are those of the Private and End Sub lines, unless specified otherwise.
The easiest and most common way to apply indentation consists of pressing Tab before typing your code. By default, one indentation, done when pressing Tab, corresponds to 4 characters. This is controlled by the Editor property page of the Options dialog box. To change it, on the main menu of Microsoft Visual Basic, you would click Tools -> Options and use the Tab Width text box of the Editor property page:
If you don't want the pressing of Tab to be equivalent to 4 characters, change the value of the Tab Width text box to a reasonable value and click OK. Otherwise, it is (strongly) suggested that you keep it to its default of 4 characters.
The assignment operation is used to make a copy of a value, an expression, or the content of a control and give the copy to another field or expression. The assignment operation is performed with the = sign.
For example, suppose you have a field that displays a first name and that field is called FirstName. If you want that first name to display in another field, with this new field named, in the new field you could type:
On the other hand, you can use the assignment operator to give a value to a declared variable. Here is an example:
Private Sub Form_Load() Dim NumberOfTracks As Integer NumberOfTracks = 16 End Sub
When the assignment operator is provided to a variable as a starting value for the variable, this is referred to as initializing the variable.
The comma is used to separate variables used in a group. For example, a comma can be used to delimit the names of variables that are declared on the same line. Here is an example:
Sub Exercise() Dim FirstName As String, LastName As String, FullName As String End Sub
Most of the time, to make various statements easier to read, you write each on its own line. Here are examples:
Sub Exercise() Dim FirstName As String, LastName As String FirstName = "Arsène" LastName = "Nkoulou" End Sub
The Visual Basic language allows you to write as many statements as necessary on the same line. When doing this, the statements must be separated by a colon. Here is an example:
Sub Exercise() Dim FirstName As String, LastName As String FirstName = "Arsène" : LastName = "Nkoulou" End Sub
Double-quotes are used to display a string. First...
A string is an empty space, a character, or a group of characters that you type or provide to a control and you want this character or this group of characters to be considered "as is". In other words, the expression or the control that receives the string should keep it or them the way you supplied it or them.
A string can be an empty space or one character, such as $ or w; a group of characters, like home or Manchester United or Verbally speaking, I mean… Ah forget it. Most of the time, you will want the program to keep this character or group of characters exactly the way you or the user supplied them. In order to let the program know that this is a string, you must enclose it in double quotes. From our examples, our strings would be "$", "w", "home", "Manchester United", and "Verbally speaking, I mean… Ah forget it".
To assign a string to an expression or a field, use the assignment operator as follows:
= "Manchester United"
In the same way, to initialize a variable with a string , use the assignment operator. Here is an example:
Private Sub Form_Load() Dim Address As String Address = "12404 Lockwood Drive Apt D4" End Sub
The & operator is used to append two strings, the contents of two controls, or expressions; this is considered as concatenating them. For example, it could allow you to concatenate a first name and a last name, producing a full name. The general formula of the concatenation operator is expressed as:
Value1 & Value2
To display a concatenated expression, use the assignment operator. To assign a concatenated expression to a variable, use the assignment operator the same way. Here is an example:
Private Sub Form_Load() Dim FirstName, LastName As String Dim FullName As String FirstName = "Francis " LastName = "Pottelson" FullName = FirstName & LastName Text0 = FullName End Sub
To concatenate more than two expressions, you can use as many & operators between any combination of two expressions as necessary. After concatenating the expressions or values, you can assign the result to another value or expression using the assignment operator. The syntax used is:
=Value1 & " " & Value2
In mathematics, an integer such as 120 or a double floating number such as 98.005 is qualified as positive; that is, it is considered greater than 0. If a number is less than 0, to express it, you can add the - sign on the left side of the number. Examples are -5502 or -240.65. The - sign signifies that the number is negative.
A variable or an expression can also be represented as negative by prefixing it with a - sign. Examples are -Distance or -NbrOfPlayers. To initialize a variable with a negative value , use the assignment operator. Here is an example:
Private Sub Form_Load() Dim NumberOfTracks As Byte Dim Temperature As Integer NumberOfTracks = 16 Temperature = -94 End Sub
The addition is used to add one value or expression to another. It is performed using the + symbol and its formula is:
Value1 + Value2
The addition allows you to add two numbers such as 12 + 548 or 5004.25 + 7.63
After performing the addition, you get a result. You can provide such a result to another variable or control. This is done using the assignment operator. The formula used would be:
= Value1 + Value2
The subtraction is performed by retrieving one value from another value. This is done using the - symbol. The syntax used is:
Value1 - Value2
The value of Value1 is subtracted from the value of Value2. After the operation is performed, a new value results. This result can be used in any way you want. For example, you can display it in a control using the assignment operator as follows:
= Value1 - Value2
The multiplication allows adding one value to itself a certain number of times, set by the second value. The multiplication is performed with the * sign which is typed with Shift + 8. Here is an example:
Value1 * Value2
During the operation, Value1 is repeatedly added to itself, Value2 times. The result can be assigned to another value or displayed in a control as follows:
= Value1 * Value2
Dividing an item means cutting it in pieces or fractions of a set value. For example, when you cut an apple in the middle, you are dividing it in 2 pieces. If you cut each one of the resulting pieces, you will get 4 pieces or fractions. This is considered that you have divided the apple in 4 divisions. Therefore, the division is used to get the fraction of one number in terms of another.
Microsoft Visual Basic provides two types of results for the division operation. If you want the result of the operation to be a natural number, called an integer, use the backlash operator "\" as the divisor. Here is an example:
Value1 \ Value2
This operation can be performed on two types of valid numbers, with or without decimal parts. After the operation, the result would be a natural number. The result of the operation can be assigned to another value. It can also be displayed in a control using the assignment operator:
= Value1 \ Value2
The second type of division results in a decimal number. It is performed with the forward slash "/". Its syntax is:
Value1 / Value2
After the operation is performed, the result is a decimal number. The result of either operation can be assigned to another value. It can also be displayed in a control using the assignment operator:
= Value1 / Value2
Exponentiation is the ability to raise a number to the power of another number. This operation is performed using the ^ operator (Shift + 6). It uses the following mathematical formula:
In Microsoft Visual Basic (and Microsoft Access), this formula is written as:
and means the same thing. Either or both y and x can be values or expressions, but they must carry valid values that can be evaluated.
When the operation is performed, the value of y is raised to the power of x. You can display the result of such an operation in a field using the assignment operator as follows:
You can also assign the operation to an expression as follows:
Total = y^x
The division operation gives a result of a number with or without decimal values, which is fine in some circumstances. Sometimes you will want to get the value remaining after a division renders a natural result. Imagine you have 26 kids at a football (soccer) stadium and they are about to start. You know that you need 11 kids for each team to start. If the game starts with the right amount of players, how many will seat and wait?
The remainder operation is performed with keyword Mod. Its syntax is:
Value1 Mod Value2
The result of the operation can be used as you see fit or you can display it in a control using the assignment operator as follows:
= Value1 Mod Value2
A comment is a piece of text in a code section that the database engine would not consider when reading your code. As such, a comment can be written any way you want.
In Visual Basic, the line that contains a comment can start with a single quote. Here is an example:
Private Sub Form_Load() ' This line will not be considered as part of the code End Sub
Alternatively, you can start a comment with the Rem keyword. Anything on the right side of rem, Rem, or REM would not be read. Here is an example:
Private Sub Form_Load() ' This line will not be considered as part of the code Rem I can write anything I want on this line End Sub
Comments are very useful and it is strongly suggested that you use them regularly. They can never hurt your code and they don't increase the size of your database. Comments can help you and other people who read your code to figure out what a particular section of code is used for, which can be helpful when you re-visit your code after months or years of not seeing it.
We saw earlier that you could declare a variable based on a built-in object of VBA. To specify the particular object you are referring to, you can (must) use the Set operator to assign an existing object to your variable. This would be done as follows:
dim ctlFirstName as Control Set ctlFirstName = TextBox
If you are displaying a string but judge it too long, you can segment it in appropriate sections as you see fit. To do this, you can use vbCrLf. Here is an example:
Sub Exercise() Dim FirstName As String, LastName As String, FullName As String Dim Accouncement As String FirstName = "Valère" LastName = "Edou" FullName = FirstName & " " & LastName Accouncement = "Student Registration - Student Full Name: " & _ vbCrLf & FullName End Sub
Parentheses are used in two main circumstances: in an event (or procedures, as we will learn) or in an operation. The parentheses in an operation help to create sections in an operation. This regularly occurs when more than one operators are used in an operation.
Consider the following operation: 8 + 3 * 5
The result of this operation depends on whether you want to add 8 to 3 then multiply the result by 5 or you want to multiply 3 by 5 and then add the result to 8. Parentheses allow you to specify which operation should be performed first in a multi-operator operation. In our example, if you want to add 8 to 3 first and use the result to multiply it by 5, you would write (8 + 3) * 5. This would produce 55. On the other hand, if you want to multiply 3 by 5 first then add the result to 8, you would write 8 + (3 * 5). This would produce 23.
As you can see, results are different when parentheses are used on an operation that involves various operators. This concept is based on a theory called operator precedence. This theory manages which operation would execute before which one; but parentheses allow you to completely control the sequence of these operations.
In Lesson 2, we saw that it was suitable to use one-word names for objects in Microsoft Access. In reality, Microsoft Access, as mentioned already, is particularly flexible with names. We saw that we could use square brackets to enclose a name made of. As seen in Lesson 2, this principle is the same here.
Once again, in Lesson 3, we mentioned that the exclamation point operator "!" was used to access a member of a collection.
It is usually suitable to write lines of code that are not too long. This makes it easy to read code and avoid scrolling left and right. In some cases, you will not have much choice but to create long expressions. Unlike many other languages such as C/C++/C#, Java, etc, Visual Basic doesn't allow to simply continue a line of code from one line to the next without alerting the compiler. Still, if a line of code becomes too long, there is a technique you can use to span on various lines.
To continue a piece of code from one line to the next, type an empty space followed by an underscore symbol, then continue your code on the next line.
So far, we have seen various ways of creating a database, including creating a blank database or using the wizard. Besides these techniques, you can also programmatically create a database. To do this, first declare a variable of type Application and initialize the variable with the version of the Microsoft Access that will be used. To actually create the database, call the NewCurrentDatabase method of the Application class. This method takes as argument the path and the name of the new database. The name should include the .mdb extension but if you omit it, the extension would be added when the database is created. Here is an example that creates a new database named Championship in a folder named Programs on the C: drive:
Private Sub cmdCreateDatabase_Click() Dim strNewDB As String Dim appAccess As Access.Application strNewDB = "C:\Programs\Championship.mdb" Set appAccess = CreateObject("Access.Application.9") appAccess.NewCurrentDatabase strNewDB End Sub
From our introduction to variables, you may remember that the computer stores its data in memory using small locations that look like boxes and each box contains a bit of information. Because a bit can be represented only either as 1 or 0, we can say that each box contains 1 or 0. Bit manipulation consists of changing the value (1 or 0, or 0 or 1) in a box. As we will see in the next few operations, it is not just about changing a value. It can involve reversing a value or kind of "moving" a box from its current position to the next position.
The operations on bits are performed on 1s and 0s only. This means that any number in decimal or hexadecimal format involved in a bit operation must be converted to binary first.
You will almost never perform some of the operations we are going to review. You will hardly perform some other operations. There is only one operation you will perform sometimes: the OR operation.
Remember that, at any time, a box (or chunk) in memory contains either 1 or 0:
Bit reversal consists of reversing the value of a bit. If the box contains 1, you can reverse it to 0. If it contains 0, you can reverse it to 1. To support this operation, the Visual Basic language provides the Not Operator.
As an example, consider the number 286. The decimal number 286 converted to binary is 100011110. You can reverse each bit as follows:
Bitwise conjunction consists of adding the content of one box (a bit) to the content of another box (a bit). To support the bitwise conjunction operation, the Visual Basic language provides the And operator.
To perform the bit addition on two numbers, remember that they must be converted to binary first. Then:
As an example, consider the number 286 bit-added to 475. The decimal number 286 converted to binary is 100011110. The decimal number 4075 converted to binary is 111111101011. Based on the above 4 points, we can add these two numbers as follows:
Therefore, 286 And 4075 produces 100001010 which is equivalent to:
This means that 286 And 4075 = 256 + 16 + 2 = 266
This can also be programmatically calculated as follows:
Sub Exercise() Dim Number1 As Integer Dim Number2 As Integer Dim Result As Integer Number1 = 286 Number2 = 4075 Result = Number1 And Number2 End Sub
Bitwise disjunction consists of disjoining one a bit from another bit. To support this operation, the Visual Basic language provides the Or operator.
To perform a bitwise conjunction on two numbers, remember that they must be converted to binary first. Then:
As an example, consider the number 305 bit-disjoined to 2853. The decimal number 305 converted to binary is 100110001. The decimal number 2853 converted to binary is 101100100101. Based on the above 4 points, we can disjoin these two numbers as follows:
Therefore, 305 Or 2853 produces 101100110101 which is equivalent to:
This means that 286 And 4075 = 2048 + 512 + 256 + 32 + 16 + 4 + 1 = 2869
This can also be programmatically calculated as follows:
Sub Exercise() Dim Number1 As Integer Dim Number2 As Integer Dim Result As Integer Number1 = 286 Number2 = 4075 Result = Number1 Or Number2 End Sub
Bitwise exclusion consists of adding two bits with the following rules. To support bitwise exclusion, the Visual Basic language provides an operator named Xor:
As an example, consider the number 618 bit-excluded from 2548. The decimal number 618 converted to binary is 1001101010. The decimal number 2548 converted to binary is 100111110100. Based on the above 2 points, we can bit-exclude these two numbers as follows:
Therefore, 305 Or 2853 produces 101110011110 which is equivalent to:
This means that 286 And 4075 = 2048 + 512 + 256 + 128 + 16 + 8 + 4 + 2 = 2974
This can also be programmatically calculated as follows:
Sub Exercise() Dim Number1 As Integer Dim Number2 As Integer Dim Result As Integer Number1 = 286 Number2 = 4075 Result = Number1 Xor Number2 End Sub