Learning Visual Basic .Net

2017-07-07 02:10:07

6.2 Conditional Branching Statements

While methods branch unconditionally, often you will want to branch within a method depending on a condition that you evaluate while the program is running. This is known as conditional branching. Conditional branching statements allow you to write logic such as "If you are over 25 years old, then you may rent a car."

VB.NET provides a number of constructs that allow you to write conditional branches into your programs; these constructs are described in the following sections.

6.2.1 If Statements

The simplest branching statement is If . An If statement says, "if a particular condition is true, then execute the statement; otherwise skip it." (The condition is a Boolean expression. An expression is a statement that evaluates to a value. A Boolean expression evaluates to either true or false.)

The formal description of an If statement is:

If expression Then statements End If

You are likely to find this kind of description of the If statement in your compiler documentation. It shows you that the If statement takes an expression (a statement that returns a value) and Then executes the statements until the End If , but only if the expression evaluates true.

An alternative one-line version is:

If expression Then statement

Many VB.NET developers avoid the single-line If statement because it can be confusing and thus difficult to maintain.

Example 6-2 illustrates the use of an If statement.

Example 6-2. Using the If statement

Option Strict On Imports System Module Module1 Sub Main( ) Dim valueOne As Integer = 10 Dim valueTwo As Integer = 20 Dim valueThree As Integer = 30 Console.WriteLine("Testing valueOne against valueTwo...") If valueOne > valueTwo Then Console.WriteLine( _ "ValueOne: {0} larger than ValueTwo: {1}", _ valueOne, valueTwo) End If Console.WriteLine("Testing valueThree against valueTwo...") If valueThree > valueTwo Then Console.WriteLine( _ "ValueThree: {0} larger than ValueTwo: {1}", _ valueThree, valueTwo) End If Console.WriteLine("Testing is valueTwo > 15 (one line)...") If valueTwo > 15 Then Console.WriteLine("Yes it is") End Sub 'Main End Module

Output : Testing valueOne against valueTwo... Testing valueThree against valueTwo... ValueThree: 30 larger than ValueTwo: 20 Testing is valueTwo > 15 (one line)... Yes it is

In this simple program, you declare three variables , valueOne, valueTwo, and valueThree, with the values 10, 20, and 30, respectively. In the first If statement, you test whether valueOne is greater than valueTwo.

If valueOne > valueTwo Then Console.WriteLine( _ "ValueOne: {0} larger than ValueTwo: {1}", valueOne, valueTwo) End If

Because valueOne (10) is less than valueTwo (20), this If statement fails (the condition returns false), and thus the body of the If statement (the statements between the If and the End If ) doesn't execute.

The test for greater than uses the greater than operator (>), which is discussed in detail in Chapter 7.

You then test whether valueThree is greater than valueTwo:

If valueThree > valueTwo Then Console.WriteLine( _ "ValueThree: {0} larger than ValueTwo: {1}", valueThree, valueTwo) End If

Since valueThree (30) is greater than valueTwo (20), the test returns true, and thus the statement executes. The statement in this case is the call to the WriteLine( ) method, shown in bold.

Finally, you use a one-line if statement to test whether valueTwo is greater than 15. Since this evaluates true, the statement that follows executes, and the words "Yes it is" are displayed.

If valueTwo > 15 Then Console.WriteLine("Yes it is")

The output reflects that the first If statement fails, but the second and third succeed:

Testing valueOne against valueTwo... Testing valueThree against valueTwo... ValueThree: 30 larger than ValueTwo: 20 Testing is valueTwo > 15 (one line)... Yes it is

6.2.2 If . . . Else Statements

Often, you will find that you want to take one set of actions when the condition tests true and a different set of actions when the condition tests false. This allows you to write logic such as "If you are over 25 years old, then you may rent a car; otherwise , you must take the train."

The otherwise portion of the logic is executed in the Else statement. For example, you can modify Example 6-2 to print an appropriate message whether or not valueOne is greater than valueTwo, as shown in Example 6-3.

Example 6-3. The Else statement

Output : Testing valueOne against valueTwo... Nope, ValueOne: 10 is NOT larger than valueTwo: 20

Because the test in the If statement fails (valueOne is not larger than valueTwo), the body of the If statement is skipped and the body of the Else statement is executed. Had the test succeeded, the If statement body would execute and the Else statement would be skipped.

6.2.3 Nested If Statements

It is possible, and not uncommon, to nest If statements to handle complex conditions. For example, suppose you need to write a program to evaluate the temperature and specifically to return the following types of information:

If the temperature is 32 degrees or lower, the program should warn you about ice on the road.

If the temperature is exactly 32 degrees, the program should tell you that there may be ice patches.

If the temperature is higher than 32 degrees, the program should assure you that there is no ice.

There are many good ways to write this program. Example 6-4 illustrates one approach, using nested If statements.

Example 6-4. Nested If statements

Option Strict On Imports System Module Module1 Sub Main( ) Dim temp As Integer = 32 If temp <= 32 Then Console.WriteLine("Warning! Ice on road!") If temp = 32 Then Console.WriteLine("Temp exactly freezing, beware of water.") Else Console.WriteLine("Watch for black ice! Temp: {0}", temp) End If 'temp = 32 End If 'temp <= 32 End Sub 'Main End Module

Output : Warning! Ice on road! Temp exactly freezing, beware of water.

The logic of Example 6-4 is that it tests whether the temperature is less than or equal to 32. If so, it prints a warning:

If temp <= 32 Then Console.WriteLine("Warning! Ice on road!")

The program then uses a second If statement, nested within the first, to check whether the temp is equal to 32 degrees. If so, it prints one message ("Temp exactly freezing, beware of water."); if not, the temp must be less than 32 and an Else is executed, causing the program to print the next message ("Watch for black ice . . ."). Because the second If statement is nested within the first If , the logic of the Else statement is: "since it has been established that the temp is less than or equal to 32, and it isn't equal to 32, it must be less than 32."

The less-than -or-equal-to operator is <= , as described in Chapter 7.

6.2.4 ElseIf

The ElseIf statement allows you to perform a related sequence of If s without nesting per se. The logic of ElseIf is that if the first If evaluates false, then evaluate the first ElseIf . The first If / ElseIf combination to evaluate true will have its statements executed (and no others will even be evaluated). If none of the statements evaluates true, the final Else clause is executed. Example 6-5 uses ElseIf to perform the same actions as Example 6-4 used nested If s for.

Example 6-5. ElseIf

Option Strict On Imports System Module Module1 Sub Main( ) Dim temp As Integer = -32 If temp > 32 Then Console.WriteLine("Safe driving...") ElseIf temp = 32 Then Console.WriteLine("Warning, 32 degrees, watch for ice and water") ElseIf temp > 0 Then Console.WriteLine("Watch for ice...") ElseIf temp = 0 Then Console.WriteLine("Temperature = 0") Else Console.WriteLine("Temperatures below zero, Wicked Cold!") End If End Sub 'Main End Module

Output : Temperatures below zero, Wicked Cold!

6.2.5 Select Case Statements

Nested If statements and long sequences of ElseIf statements are hard to read, hard to get right, and hard to debug. When you have a complex set of choices to make, the Select Case statement is a more powerful alternative. The logic of a Select Case statement is this: "pick a matching value and act accordingly ." The syntax is as follows:

Select [ Case ] testExpression [ Case expressionList [ statements ] ] [ Case Else [ else-statements ] ] End Select

It is easiest to understand this construct in the context of a sample program. In Example 6-6 a value of 15 is assigned to the variable targetInteger. The Select Case statement tests for the values 5, 10, and 15. If one matches, the associated statement is executed.

Example 6-6. Using Select Case

Option Strict On Imports System Module Module1 Sub Main( ) Dim targetInteger As Integer = 15 Select Case targetInteger Case 5 Console.WriteLine("5") Case 10 Console.WriteLine("10") Case 15 Console.WriteLine("15!") Case Else Console.WriteLine("Value not found") End Select End Sub 'Main End Module

Output : 15!

The output shows that 15 matched, and the associated statement was executed, displaying the value 15. If none of the values matched, any statements following Case Else would be executed.

Note that Case also allows you to check a variable against a range of values. You can combine Case with the keywords Is and To to specify the ranges, as illustrated in Example 6-7. Note that the target value (targetInteger) has been changed to 7.

Example 6-7. Testing for a range of values

Option Strict On Imports System Module Module1 Sub Main( ) Dim targetInteger As Integer = 7 Select Case targetInteger Case Is < 10 Console.WriteLine("Less than 10") Case 10 To 14 Console.WriteLine("10-14") Case 15 Console.WriteLine("15!") Case Else Console.WriteLine("Value not found") End Select End Sub 'Main End Module

Output : Less than 10

In Example 6-7, the first test examines whether targetInteger is less than 10. You specify this by combining Case with the Is keyword followed by the less-than operator and the number 10 to specify the range:

Case Is < 10

You then use Case with the To keyword to specify a range of 10 through 14:

Case 10 To 14

The preceding Case will match any value of 10 through 14 inclusive. You are not restricted to just testing for a numeric value. You can also test for string values. In fact, you can test ranges of string values, examining whether a target value fits alphabetically within the range, as shown in Example 6-8.

Example 6-8. Testing alphabetic ranges

Option Strict On Imports System Module Module1 Sub Main( ) Dim target As String = "Milo" Select Case target Case "Alpha" To "Lambda " Console.WriteLine("Alpha To Lambda executed") Case "Lamda" To "Zeta" Console.WriteLine("Lambda To Zeta executed") Case Else Console.WriteLine("Else executed") End Select End Sub 'Main End Module

Output : Lambda To Zeta executed

Example 6-8 tests whether the string "Milo" fits within the alphabetic range between the strings "Alpha" and "Lambda"; then it tests whether "Milo" fits within the range between the strings "Lambda" and "Zeta". Both ranges are inclusive. Clearly the second range encompasses the string "Milo" and the output bears that out.

You can also simply test whether one string matches another. The following case tests whether the string "Milo" is the same as the string "Fred":

Dim target As String = "Milo" Select Case target Case "Fred" Console.WriteLine("Fred")

But clearly "Milo" does not equal "Fred."

You can also combine a series of tests in a single Case statement, separating them by commas. Thus you could test whether "Milo" matches either of the strings "Fred" or "Joe" and also whether it falls within the (admittedly small) alphabetic range that comes before "Alpha" using the following code:

Dim target As String = "Milo" Select Case target Case "Fred", "Joe", Is < "Alpha" Console.WriteLine("Joe or Fred or < Alpha")

Clearly "Milo" would not match any of these cases, but changing the target string to "Aardvark" would get you somewhere.