Wednesday, December 12, 2012

Control structure in visual basic


Visual Basic provides three decision control flow structures:
  If...Then
  If...Then...Else
  Select Case
If...Then: The If...Then structure tests the condition specified, and if it’s True, executes the statement(s) that follow. The If structure can have a single-line or a multiple-line syntax. To execute one statement conditionally, use the single-line syntax as follows:
                               If condition Then statement
You can also execute multiple statements by separating them with a colon:
                               If condition Then statement: statement: statement

If...Then...Else: A variation of the If...Then statement is the If... Then... Else statement, which executes one block of statements if the condition is True and another if the condition is False. The syntax of the If...Then...Else statement is as follows:

               If condition1 Then
                  statementblock-1
               ElseIf condition2 Then
                  statementblock-2
               ElseIf condition3 Then
                  statementblock-3
               Else
                  statementblock-4
               End If

Select Case: The Select Case structure compares one expression to different values. The advantage of the Select Case statement over multiple If...Then...Else statements is that it makes the code easier to read and maintain.

The Select Case structure tests a single expression, which is evaluated once at the top of the structure. The result of the test is then compared with several values, and if it matches one of them, the corresponding block of statements is executed. Here’s the syntax of the Select Case statement:
               
               Select Case expression
               Case value1
                  statementblock-1
               Case value2
                  statementblock-2
                 .
                 .
                 .
               Case Else
                  statementblock
               End Select

Loop Statements: Loop statements allow you to execute one or more lines of code repetitively. Many tasks consist of trivial operations that must be repeated over and over again, and looping structures are an important part of any programming language. Visual Basic supports the following loop statements:

  Do...Loop
  For...Next
  While...Wend

Do...Loop: The Do...Loop executes a block of statements for as long as a condition is True. Visual Basic evaluates an expression, and if it’s True, the statements are executed. If the expression is False, the program continues and the statement following the loop is executed.


There are two variations of the Do...Loop statement and both use the same basic model. A loop can be executed either while the condition is True or until the condition becomes True. These two variations use the keywords While and Until to specify how long the statements are executed. To execute a block of statements while a condition is True, use the following syntax:
Do While condition
   statement-block
Loop
To execute a block of statements until the condition becomes True, use the following syntax:
Do Until condition
   statement-block
Loop
Another variation of the Do loop executes the statements first and evaluates the condition after each execution. This Do loop has the following syntax:
Do
statements
Loop While condition
or
Do
statements
Loop Until condition

For...Next: The For...Next loop is one of the oldest loop structures in programming languages. Unlike the Do loop, the For...Next loop requires that you know how many times the statements in the loop will be executed. The For...Next loop uses a variable (it’s called the loop’s counter) that increases or decreases in value during each repetition of the loop. The For...Next loop has the following syntax:

For counter = start To end [Step increment]
  statements
Next [counter]
The keywords in the square brackets are optional.

While...Wend: The While...Wend loop executes a block of statements while a condition is True. The While...Wend loop has the following syntax:

While condition
  statement-block
Wend