Visual Basic lexical structure
Computer languages, like human languages, have a lexical structure. A source code of a Visual Basic program consists of tokens. Tokens are atomic code elements. In Visual Basic, we have comments, variables, literals, operators, delimiters and keywords.Comments
Comments are used by humans to clarify the source code. All comments in
Visual Basic follow either the ' character or the
Rem keyword.
Option Strict On
' This is comments.vb
' Author: Jan Bodnar
' ZetCode 2010
Module Example
Rem program starts here
Sub Main()
Console.WriteLine("This is comments.vb")
End Sub
End Module
Comments are ignored by the Visual Basic compiler.
White space
White space in Visual Basic is used to separate tokens in the source file. It is used to improve readability of the source code.Dim i As Integer
White spaces are required in some places. For example between the
Dim
keyword and the variable name. In other places, it is forbidden. It cannot be present in
variable identifiers or language keywords.
a=1 b = 2 c = 3The amount of space put between tokens is irrelevant for the Visual Basic compiler.
Line continuation character
We must use a line continuation character, if a statement spans more than one line. This differs from C and C based languages.Console.WriteLine("The length of the first string is " _
+ str1.Length.ToString() + " characters")
Mainly for readability reasons, we don't want to have too many characters on one
line. We break the line and continue on the following line. In Visual Basic, we
must use the line continuation character, otherwise the compilation would fail.
Variables
A variable is an identifier, which holds a value. In programming we say, that we assign a value to a variable. Technically speaking, a variable is a reference to a computer memory, where the value is stored. Variable names can have alphanumerical characters and underscores. An identifier may begin with a character or an underscore. It may not begin with a number. Variable names are not case sensitive. This means, that Name, name or NAME refer to the same variable. Variable names also cannot match language keywords.Dim name23 As String Dim _col As Integer Dim birth_date As DateThese are valid Visual Basic identifiers.
Option Strict On
Module Example
Sub Main()
Dim name As String = "Robert"
Dim Name As String = "Julia"
Console.WriteLine(name)
Console.WriteLine(Name)
End Sub
End Module
This code will not compile. Identifiers are not case sensitive.
Literals
A literal is a textual representation of a particular value of a type. Literal types include Boolean, Integer, floating point, string, character, and date. Technically, a literal will be assigned a value at compile time, while a variable will be assigned at runtime.Dim age As Byte = 29 Dim nationality As String = "Hungarian"Here we assign two literals to variables. Number 29 and string "Hungarian" are literals.
Option Strict On
Module Example
Sub Main()
Dim sng As Boolean = True
Dim name As String = "James"
Dim job As String = Nothing
Dim age As Byte = 23
Dim weight As Single = 68.5
Dim born As DateTime = DateValue("November 12, 1987")
Console.WriteLine("His name is {0}", name)
If sng Then
Console.WriteLine("He is single")
Else
Console.WriteLine("He is in a relationship")
End If
Console.WriteLine("His job is {0}", job)
Console.WriteLine("He weighs {0} kilograms", weight)
Console.WriteLine("He was born in {0}", _
Format(born, "yyyy"))
End Sub
End Module
In the above example, we have other literals. The Boolean literal
may have value True or False.
James is a string literal.
The Nothing represents the default value
of any data type. 23 is an Integer literal.
68.5 is a floating point literal.
Finally, the November 12, 1987 is a date literal.
$ ./literals.exe His name is James He is single His job is He weighs 68.5 kilograms He was born in 1987This is the output of the program.
Operators
An operator is a symbol used to perform an action on some value.+ - * / \ ^ & = += -= *= /= \= ^= < > &= >>= <<= >= <= >> <> <<These are Visual Basic operators. We will talk about operators later in the tutorial.
Separators
Aseparator is a sequence of one or more characters used
to specify the boundary between separate, independent regions in plain text or other data stream.
( ) { } ! # , . : := ?
These are Visual Basic separators.
Dim language As String = "Visual Basic"The double characters are used to mark the beginning and the end of a string.
Console.WriteLine("Today is {0}", GetToday)
Parentheses (square brackets) are used to mark the method signature. The signature consists
of method parameters. Curly brackets are used to denote the evaluated value.
Dim array() As Integer = { _
2, 4, 5, 6, 7, 3, 2 }
The curly brackets are also used to create arrays.
Keywords
A keyword is a reserved word in the Visual Basic language. Keywords are used to perform a specific task in the computer program. For example, print a value, do repetitive tasks or perform logical operations. A programmer cannot use a keyword as an ordinary variable.Visual Basic is rich in keywords. Many of them will be explained in this tutorial. The keywords include If, Else, Dim, For, Date, Double, Or, Exit and many others.
Option Strict On
Module Example
Sub Main()
Dim i As Integer
For i = 0 To 35 Step 5
Console.WriteLine(i)
Next
End Sub
End Module
In the above example, we use several keywords. Option, On, Module, Sub,
Dim, As, Integer, For, To, Step, Next, End are Visual Basic keywords.
In this part of the Visual Basic tutorial, we covered the basic lexis for the Visual Basic language.
source : http://zetcode.com/lang/visualbasic/
0 comments:
Post a Comment