Visual Basic 2008 9.0 .NET Examples and Ebook

In both If and Do statements we need to work with conditions. These conditions are formed by conditional expressions.

In a statically types programming language like Visual Basic .NET all expressions are of a - by the compiler know - datatype. Conditional expressions are of type Boolean, therefore they are also called Boolean expressions.

Boolean expressions can only evaluated to two possible values, True or False.

Just like any other datatype variables can be declared of type Boolean.

Module Example1
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        Dim value1Higher As Boolean = value1 > value2
        '
        If value1Higher Then
            Console.WriteLine(value1 & " > " & value2)
        Else
            Console.WriteLine(value1 & " <= " & value2)
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>10</i>
 Value 2 ?
 <i>5</i>
 10 > 5

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 5 <= 10

Variable value1Higher is declared of type Boolean, and is assigned a Boolean value.

A iteration ( Do ) or decision ( If ) can use this variables of type Boolean to form the condition.

In the above example variable value1Higher represents a value ( True or False ) whether ( True ) or not ( False ) value1 is more than value2.

When we want to check equality ( is value1 equal to value2 ) the equality operator = can be used ( value1 = value2 ).

Module Example2
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        Dim equal As Boolean = value1 = value2                             ' (1)
        '
        If equal Then
            Console.WriteLine(value1 & " = " & value2)
        Else
            Console.WriteLine(value1 & " <> " & value2)
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>5</i>
 5 = 5

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 5 <> 10

Don't confuse the equality operator '=' with the assignment operator '='.
The same symbol is used, but for a different purpose, depending on the syntactical use.

A more readable version of line (1) can be formed when using parentheses :

 Dim equal As Boolean = (value1 = value2)

Module Example3
    Sub Main()
        Dim integerVariable As Integer
        Console.WriteLine("Integer default value : " & integerVariable)
        integerVariable = 5
        Console.WriteLine("Integer changed value : " & integerVariable)
        '
        Dim stringVariable As String
        Dim x As String = ""
        Console.WriteLine("String default value  : " & stringVariable)
        stringVariable = "text"
        Console.WriteLine("String changed value  : " & stringVariable)
        '
        Dim booleanVariable As Boolean
        Console.WriteLine("Boolean default value : " & booleanVariable)
        booleanVariable = True
        Console.WriteLine("Boolean changed value : " & booleanVariable)
        booleanVariable = False
        Console.WriteLine("Boolean changed value : " & booleanVariable)
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Integer default value : 0
 Integer changed value : 5
 String default value  :
 String changed value  : text
 Boolean default value : False
 Boolean changed value : True
 Boolean changed value : False

The above example illustrates the default values of the different datatypes.
A default value is a value a variable hold after declaration ( without initialization ).

- 0 for Integer variables ( and all other numeric datatypes )
- Nothing for String variables ( printed out as "" ( no characters ) )
- False for Boolean variables

You can also see how literals ( constant expressions ) are formed with these datatypes :

- Integer literals are formed by an integral value ( between -2147483648 and +2147483647 )
- String literals are formed with Nothing or surrounding double quotes ( " )
- Boolean literals are formed with True or False

Suppose we need to give the sum of two entered values if both values are positive ( above zero ).

Module Example4
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 Then
            If value2 > 0 Then
                Console.WriteLine("Sum : " & (value1 + value2))
            End If
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

The goal is achieved by nesting two Ifs. Only if value1 is more than 0,
and value2 is more than 0, the sum will be given.

An alternative would be to use the logical operator And.
This operator combines two Boolean expressions to one Boolean expression.

Module Example5
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 And value2 > 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

Only if both ( combined ) Boolean expressions are correct ( True ) the result is True :

 condition-1      condition-2         condition-1 And condition-2
 True             True                True
 True             False               False
 False            True                False
 False            False               False

When two nested Ifs are used, each failing condition can have its alternate instructions, for instance to give specific errors like "Value 1 not positive." and "Value 2 not positive.".

Module Example6
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 Then
            If value2 > 0 Then
                Console.WriteLine("Sum : " & _
                                  (value1 + value2))
            Else
                Console.WriteLine("Value 2 not positive.")
            End If
        Else
            Console.WriteLine("Value 1 not positive.")
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>
 Value 1 not positive.

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>
 Value 2 not positive.

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>
 Value 1 not positive.

It the And operator is used, only one general error "Value 1 and/or 2 not positive." can be given.

Module Example7
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 And value2 > 0 Then
            Console.WriteLine("Sum : " & _
                              (value1 + value2))
        Else
            Console.WriteLine("Value 1 and/or 2 not positive.")
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>
 Value 1 and/or 2 not positive.

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>
 Value 1 and/or 2 not positive.

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>
 Value 1 and/or 2 not positive.

Therefore one could prefer the solution with the nested Ifs above the solution with the And operator.

The solution using the And operator is less efficient than the solution with the two nested Ifs.

The expression with the And operator will always evaluate both ( combined ) expressions.
The version with the two nested Ifs will only evaluate value2 > 0 in worst case scenario ( if value1 > 0 is correct ).

Later on we'll see how the 'AndAlso' operator can be used to avoid unnecessary checks.

Suppose we need to give the sum of two entered values if at least one of the values is positive ( above zero ).

Module Example8
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        Else
            If value2 > 0 Then
                Console.WriteLine("Sum : " & (value1 + value2))
            End If
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>
 Sum : 5

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>
 Sum : -5

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

Again by using two nested Ifs this result can be achieved.

ElseIf can also be used.

Module Example9
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        ElseIf value2 > 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>
 Sum : 5

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>
 Sum : -5

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

Another solution could use the logical operator Or.
This operator combines two Boolean expressions and results in one Boolean expression.

Module Example10
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 Or value2 > 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>
 Sum : 5

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>
 Sum : -5

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

When at least one of the combined conditions is True the result will be True :

 condition-1      condition-2         condition-1 Or condition-2
 True             True                True
 True             False               True
 False            True                True
 False            False               False

Following example prints out all values from 1 to 10.

Module Example11
    Sub Main()
        Dim value As Integer
        '
        Do While value < 10
            value = value + 1
            Console.WriteLine(value)
        Loop
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 1
 2
 3
 4
 5
 6
 7
 8
 9
 10

When we want to convert the Do While to a Do Loop, we need to invert the condition.

A value is not less than 10 only when it is more than 10 or equal to 10.

Module Example12
    Sub Main()
        Dim value As Integer
        '
        Do Until value >= 10
            value = value + 1
            Console.WriteLine(value)
        Loop
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 1
 2
 3
 4
 5
 6
 7
 8
 9
 10

An easier way to invert a condition is by using the negation operator Not.
This operator inverts one Boolean expression.

 condition-1       Not condition-1
 True              False
 False             True

Module Example13
    Sub Main()
        Dim value As Integer
        '
        Do Until Not value < 10
            value = value + 1
            Console.WriteLine(value)
        Loop
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 1
 2
 3
 4
 5
 6
 7
 8
 9
 10

Suppose we need to give the sum of two entered values if the first value is not more than 10 and the second value is not less than or equal to 100.

The condition Not number1 > 10 And Not number2 <= 100 can be used.

Module Example14
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If Not value1 > 10 And Not value2 <= 100 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>11</i>
 Value 2 ?
 <i>200</i>

Output :

 Value 1 ?
 <i>9</i>
 Value 2 ?
 <i>200</i>
 Sum : 209

Equal resulting conditional expressions are value1 <= 10 And value2 > 100 and Not (value1 > 10 Or value2 <= 100) ( parentheses are required ).

Module Example15
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If Not (value1 > 10 Or value2 <= 100) Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>11</i>
 Value 2 ?
 <i>200</i>

Output :

 Value 1 ?
 <i>9</i>
 Value 2 ?
 <i>200</i>
 Sum : 209

Lets examine some situations to see if both solutions will result in identical behaviour.

Suppose the first value is 11 and the second value is 200. The first value exceeds 10, so no sum may be printed out.

Output :

 Value 1 ?
 <i>11</i>
 Value 2 ?
 <i>200</i>

Not value1 > 10 And Not value2 <= 100 with value1 11 and value2 200 :
= Not 11 > 10 And Not 200 <= 100
= Not True And Not False
= False And True
= False
No sum will be printed out.

Not (value1 > 10 Or value2 <= 100) with value1 11 and value2 200 :
= Not (11 > 10 Or 200 <= 100)
= Not (True Or False)
= Not True
= False
No sum will be printed out.

Suppose the first value is 9 and the second value is 200. In this case the sum may be printed out.

Output :

 Value 1 ?
 <i>9</i>
 Value 2 ?
 <i>200</i>
 Sum : 209

Not value1 > 10 And Not value2 <= 100 with value1 9 and value2 200 :
= Not 9 > 10 And Not 200 <= 100
= Not False And Not False
= True And True
= True
The sum will be printed out.

Not (value1 > 10 Or value2 <= 100) with value1 9 and value2 200 :
= Not (9 > 10 Or 200 <= 100)
= Not (False Or False)
= Not False
= True
The sum will be printed out.

Generally :

 Not X And Not Y =  Not (X Or Y)

And :

 Not X Or Not Y =  Not (X And Y)

These are called the rules of "De Morgan".

These rules can also be proven by following truth table :

 Not X And Not Y =  Not (X Or Y) :

 X     | Y     | Not X | Not Y | X Or Y | Not X And Not Y | Not (X Or Y)
 True  | True  | False | False | True   | False           | False
 True  | False | False | True  | True   | False           | False
 False | True  | True  | False | True   | False           | False
 False | False | True  | True  | False  | True            | True

Last two columns have identical result, so :

 Not X And Not Y =  Not (X Or Y)

For :

 Not X Or Not Y =  Not (X And Y) :

 X     | Y     | Not X | Not Y | X And Y | Not X Or Not Y | Not (X And Y)
 True  | True  | False | False | True    | False          | False
 True  | False | False | True  | False   | True           | True
 False | True  | True  | False | False   | True           | True
 False | False | True  | True  | False   | True           | True

Last two columns have identical result, so :

 Not X Or Not Y =  Not (X And Y)

Beside the rules of De Morgan other rules exist to simplify conditional expressions.

 p And False = False
 p Or False = p
 p And True = p
 p Or True = True
 p And p = p
 p Or p = p
 p And (Not p) = False
 p Or (Not p) = True
 Not (Not p) = p

Commutativity :

 (p And q) = (q And p) -> And is commutative
 (p Or q) = (q Or p)   -> Or is commutative

Distributivity :

 p And (q Or r) = (p And q) Or (p And r) -> And distributes over Or
 p Or (q And r) = (p Or q) And (p Or r)  -> Or distributes over And

Associativity :

 p And (q And r) = (p And q) And r -> And is associative
 p Or (q Or r) = (p Or q) Or r     -> Or is associative

An "exclusive or" operator Xor exists. A combination of two Boolean expressions with this operator will only result in True when exactly one of the two combined expressions is True :

 condition-1   condition-2     condition-1 Xor condition-2
 False         False           False
 False         True            True
 True          False           True
 True          True            False

Suppose we need to give the sum of two entered values only if exactly one of the two values is positive ( above zero ).

Module Example16
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 Xor value2 > 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>
 Sum : -5

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>
 Sum : 5

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

Suppose we need to give the sum of two entered values only if both values are positive ( above zero ).

Module Example17
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 And value2 > 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

A more efficient solution would use the short-circuit conjunction operator AndAlso.

Module Example18
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 > 0 AndAlso value2 > 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>
 Sum : 15

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

The result will always be False if the first evaluated condition is False :

 condition-1   condition-2     condition-1 And(Also) condition-2
 True          True            True
 True          False           False
 False         True            False
 False         False           False

AndAlso will only evaluate the second condition if the first condition is True.

The short-circuit disjunction operator OrElse will only evaluate the second condition if the first condition is False. The result will always be True if the first condition is True.

 condition-1   condition-2     condition-1 Or(Else) condition-2
 True          True            True
 True          False           True
 False         True            True
 False         False           False

To improve performance short-circuit operators ( AndAlso and OrElse ) should be used when available.
Only in a few rare circumstances the normal operators ( And and Or ) are needed. Later more about this circumstances.

Task :

Give the sum of two entered values only if exactly one value is positive ( above zero ).

Do this without using the Xor operator.

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>-10</i>
 Sum : -5

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>10</i>
 Sum : 5

Output :

 Value 1 ?
 <i>5</i>
 Value 2 ?
 <i>10</i>

Output :

 Value 1 ?
 <i>-5</i>
 Value 2 ?
 <i>-10</i>

Solution :

Module Exercise1Solution
    Sub Main()
        Console.WriteLine("Value 1 ?")
        Dim value1 As Integer = Console.ReadLine()
        '
        Console.WriteLine("Value 2 ?")
        Dim value2 As Integer = Console.ReadLine()
        '
        If value1 <> 0 AndAlso Not value2 <> 0 OrElse _
           Not value1 <> 0 AndAlso value2 <> 0 Then
            Console.WriteLine("Sum : " & (value1 + value2))
        End If
        '
        Console.ReadLine()
    End Sub
End Module

Task :

Make a program that calculates the sum of all entered values, while values are entered that are between 10 and 20, more than 100 or smaller than or equal to 0.

Do this with a Do While and a Do Loop.

Output :

 Value ?
 <i>15</i>
 Value ?
 <i>150</i>
 Value ?
 <i>0</i>
 Value ?
 <i>-5</i>
 Value ?
 <i>5</i>
 Sum : 160

Solution :

Module Exercise2Solution
    Sub Main()
        Dim value, sum As Integer
        '
        ' 'Do While ... Loop' :
        Do While (value > 10 AndAlso value < 20) OrElse _
                 (value > 100) OrElse (value <= 0)
            sum = sum + value
            Console.WriteLine("Value ?")
            value = Console.ReadLine()
        Loop
        '
        '' 'Do Until ... Loop' : version 1 :
        'Do Until Not ((value > 10 AndAlso value < 20) OrElse _
        '              (value > 100) OrElse (value <= 0))
        '    sum = sum + value
        '    Console.WriteLine("Value ?")
        '    value = Console.ReadLine()
        'Loop
        '
        '' 'Do Until ... Loop' : version 2 :
        'Do Until (value <= 10 OrElse value >= 20) AndAlso _
        '         (value <= 100) AndAlso (value > 0)
        '    sum = sum + value
        '    Console.WriteLine("Value ?")
        '    value = Console.ReadLine()
        'Loop
        '
        Console.WriteLine("Sum : " & sum)
        '
        Console.ReadLine()
    End Sub
End Module