Question

In this part of the Visual Basic tutorial, we will talk about the flow control. We will define several keywords that enable us to control the flow of the Visual Basic program.

In Visual Basic language there are several keywords that are used to alter the flow of the program. When the program is run, the statements are executed from the top of the source file to the bottom. One by one. This flow can be altered by specific keywords. Statements can be executed multiple times. Some statements are called conditional statements. They are executed only if a specific condition is met.

The If statement

The If statement has the following general form:

If (expression)
  statement
End If

The If keyword is used to check if an expression is true. If it is true, a statement is then executed. The statement can be a single statement or a compound statement. A compound statement consists of multiple statements enclosed by the If/End If block.

Option Strict On

Module Example

  Dim num As Byte = 31

  Sub Main()

    If (num > 0)
      Console.WriteLine("num variable is positive")
    End If

  End Sub

End Module

We have a num variable. It is assigned 31. The If keyword checks for a boolean expression. The expression is put between square brackets. The 31 > 0 is true, so the statement inside the block is executed.

$ ./ifstatement.exe 
num variable is positive

The condition is met and the message is written to the console.

Option Strict On


Module Example

  Dim num As Byte = 31

  Sub Main()

    If (num > 0)
      Console.WriteLine("num variable is positive")
      Console.WriteLine("num variable equals {0}", num)
    End If

  End Sub

End Module

More statements can be executed inside the block, created by the If , End If keywords.

We can use the Else keyword to create a simple branch. If the expression inside the square brackets following the If keyword evaluates to false, the statement following the Else keyword is automatically executed.

Option Strict On


Module Example

  Dim sex As String 

  Sub Main()

    sex = "female"

    If (sex = "male") 
      Console.WriteLine("It is a boy")
    Else 
      Console.WriteLine("It is a girl")
    End If

  End Sub

End Module

We have a sex variable. It has "female" string. The boolean expression evaluates to false and we get "It is a girl" in the console.

$ ./branch.exe 
It is a girl

We can create multiple branches using the Else If keyword. The Else If keyword tests for another condition if and only if the previous condition was not met. Note that we can use multiple Else If keywords in our tests.

Option Strict On


Module Example

  Dim a As Byte = 0

  Sub Main()

    If (a < 0) 
      Console.WriteLine("a is negative")
    Else If (a = 0) 
      Console.WriteLine("a equals to zero")
    Else
      Console.WriteLine("a is a positive number")
    End If

  End Sub

End Module

We have a numerical variable and we test it if it is a negative number or positive or if it equals to zero. The first expression evaluates to false. The second condition is met. The program prints 'a equals to zero' to the console. The rest of the branch is skipped.

Select statement

The Select statement is a selection control flow statement. It allows the value of a variable or expression to control the flow of program execution via a multi way branch. It creates multiple branches in a simpler way than using the combination of If , Else If statements.

We have a variable or an expression. The Select keyword is used to test a value from the variable or the expression against a list of values. The list of values is presented with the Case keyword. If the values match, the statement following the Case is executed. There is an optional Case Else statement. It is executed if no other match is found.

Option Strict On


Module Example

  Dim domain As String

  Sub Main()

    domain = Console.ReadLine()
  
    Select domain
      Case "us"
        Console.WriteLine("United States")
      Case "de"
        Console.WriteLine("Germany")
      Case "sk"
        Console.WriteLine("Slovakia")
      Case "hu"
        Console.WriteLine("Hungary")
      Case Else
        Console.WriteLine("Unknown")
    End Select

  End Sub

End Module

In our program, we have a domain variable. We read a value for the variable from the command line. We use the Case statement to test for the value of the variable. There are several options. If the value equals for example to "us" the "United States" string is printed to the console.

$ ./selectcase.exe 
hu
Hungary

We have entered "hu" string to the console and the program responded with "Hungary".

The Select keyword enables to validate a range of numerical cases.

Option Strict On


Module Example

  Dim age As Byte

  Sub Main()

    Try
      age = Console.ReadLine()
    Catch
      Console.WriteLine("Invalid value")
      End
    End Try
  
    Select age
      Case 0 To 21
        Console.WriteLine("Junior")
      Case 22 To 60
        Console.WriteLine("Adult")
      Case Else
        Console.WriteLine("Senior")
    End Select

  End Sub

End Module

The preceding program uses range of numerical values to identify an age group of a person.

Try
  age = Console.ReadLine()
Catch
  Console.WriteLine("Invalid value")
  End
End Try

A value is read from the console. We can use only numerical data. The Try , Catch , End Try keywords are used for exception handling. If an exception is thrown, the statements following the Catch keyword are executed. The End statement terminates the program.

Case 0 To 21
  Console.WriteLine("Junior")

Here we specify a range of values. If the value entered by the user is in between 0 and 21, inclusive, then the program prints "Junior" to the console.

$ ./agerange.exe 
43
Adult

We have entered 43 and the program responded with the "Adult" string.

The While statement

The While statement is a control flow statement that allows code to be executed repeatedly based on a given boolean condition.

This is the general form of the While loop:

While (expression):
  statement
End While

The While keyword executes the statements inside the block enclosed by the While , End While keywords. The statements are executed each time the expression is evaluated to true.

Option Strict On

Module Example

  Sub Main()

    Dim i As Integer = 0
    Dim sum As Integer = 0

    While i < 10

      i = i + 1
      sum += i
      
    End While

    Console.WriteLine(sum)

  End Sub

End Module

In the code example, calculate the sum of values from a range of numbers.

The While loop has three parts: initialization, testing, and updating. Each execution of the statement is called a cycle.

Dim i As Integer = 0

We initiate the i variable. It is used as a counter.

While i < 10
...
End While

The expression following the While keyword is the second phase, the testing. The statements in the body are executed, until the expression is evaluated to false.

i = i + 1

The last, third phase of the While loop. The updating. We increment the counter. Note that improper handling of the While loops may lead to endless cycles.

It is possible to run the statement at least once. Even if the condition is not met. For this, we can use the Do , Loop While keywords.

Option Strict On

Module Example

  Sub Main()

    Dim count As Integer = 0
    
    Do 
      Console.WriteLine(count)
    Loop While (count <> 0)

  End Sub

End Module

First the iteration is executed and then the truth expression is evaluated.

The For Next statements

When the number of cycles is know before the loop is initiated, we can use the For Next statements. In this construct we declare a counter variable, which is automatically increased or decreased in value during each repetition of the loop.

Option Strict On


Module Example

  Sub Main()

    For i As Integer = 0 To 9
      Console.WriteLine(i)
    Next

  End Sub

End Module

In this example, we print numbers 0..9 to the console.

For i As Integer = 0 To 9
  Console.WriteLine(i)
Next

We initiate the counter i to zero. The Next statement increases the counter by one until the counter equals to 9.

Visual Basic has an optional Step keyword. It controls how the counter variable is going to be increased or decreased.

Option Strict On


Module Example

  Sub Main()

    For i As Integer = 9 To 0 Step -1
      Console.WriteLine(i) 
    Next

  End Sub

End Module

In the above example, we print numbers 0..9 in the reverse order.

For i As Integer = 9 To 0 Step -1
  Console.WriteLine(i) 
Next

The step may be a negative number too. We initiate the counter to 9. Each iteration the counter is decreased by the step value.

The For Each statement

The For Each construct simplifies traversing over collections of data. It has no explicit counter. The For Each statement goes through the array or collection one by one and the current value is copied to a variable defined in the construct.

Option Strict On

Module Example

  Sub Main()
    
    Dim planets() As String = { "Mercury", "Venus", _
      "Earth", "Mars", "Jupiter", "Saturn", _
      "Uranus", "Neptune" }

    For Each planet As String In planets
      Console.WriteLine(planet)
    Next 
    
  End Sub

End Module

In this example, we use the For Each statement to go through an array of planets.

For Each planet As String In planets
  Console.WriteLine(planet)
Next

The usage of the For Each statement is straightforward. The planets is the array that we iterate through. The planet is a temporary variable that has the current value from the array. The For Each statement goes through all the planets and prints them to the console.

$ ./planets.exe 
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune

Running the above Visual Basic program gives this output.

The Exit, Continue statements

The Exit statement can be used to terminate block defined by While , For or Select statements.

Option Strict On

Module Example

  Dim val As Integer

  Sub Main
    
    While (True)
    
      val = CType((30 * Rnd), Integer) + 1
      Console.Write(val.ToString & " ")
      If (val = 22)
        Exit While
      End If    

    End While

    Console.Write(vbNewLine)

  End Sub

End Module

We define an endless While loop. There is only one way to jump out of a such loop. We must use the Exit While statement. We choose a random value from 1 to 30. We print the value. If the value equals to 22, we finish the endless while loop.

$ ./exitstm.exe 
30 12 13 20 19 4 2 9 6 9 22 

We might get something like this.

The Continue statement is used to skip a part of the loop and continue with the next iteration of the loop. It can be used in combination with Do , For and While statements.

In the following example, we will print a list of numbers that cannot be divided by 2 without a remainder.

Option Strict On

Module Example

  Dim num As Integer = 0

  Sub Main()
    
    While (num < 1000)
    
      num = num + 1

      If ((num Mod 2) = 0)
        Continue While
      End If    

      Console.Write(num.ToString() + " ") 

    End While

    Console.Write(vbNewLine)

  End Sub

End Module

We iterate through numbers 1..999 with the While loop.

If ((num Mod 2) = 0)
  Continue While
End If  

If the expression num Mod 2 returns 0, the number in question can be divided by 2. The Continue statement is executed and the rest of the cycle is skipped. In our case, the last statement of the loop is skipped and the number is not printed to the console. The next iteration is started.

In this part of the Visual Basic tutorial, we were talking about control flow structures.