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As you can see from Table 7.1, the comparison operators compare either variables, literals, control values, or combinations of all those data sources. The comparison operators work on both numeric and alphabetic values. You can compare any kind of number against another number, or any kind of string against another string. The Comparisons NatureWhen you compare strings, Visual Basic uses the ASCII table, included in Appendix B, "ASCII Table," to determine how to compare the characters. For example, the ASCII table says that the uppercase letter A--whose ASCII numeric value is 65--is less than the uppercase letter B--whose ASCII numeric value is 66. Notice that all uppercase letters are less than lowercase letters. Therefore, the abbreviation ST is less than St. New Term: An ASCII table contains a list of characters with corresponding unique numeric representations. To understand how comparison operators work, you must understand how to use their true or false results. The If statement, introduced in the next section, explains how you can use true and false results to make decisions in your program. Before you read the next section, make sure that you understand how these operators compare values. Make sure that you understand the Result column of Table 7.2 before you go any further. Table 7.2. Relationship results.
Keep Each Sides Data Type ConsistentTake extra care that the expressions on both sides of a comparison operator conform to the same data type or at least compatible data types. In other words, you cannot compare a string to a numeric data type. If you try, you will get a type mismatch error because the data types don't match. You can compare any numeric data type against any other numeric data type most of the time. In other words, you can test whether a single-precision value is less than or greater than an integer value.
The comparison operators compare values against one another. You can compare for equality, inequality, and size differences. The comparison operators work for both string data and numeric data. By themselves, the comparison operators would not be worth much. However, you can use them to compare data by using the If statement, which you'll learn about in the next section. The If StatementPerhaps the most important statement in a program is the If statement and its cousin statements. With logic that If provides, your application can begin to analyze data and make decisions based on that analysis. For example, your program can display a three-button message box and determine, with the If statement, which command button the user clicked to close the message box. If uses the comparison operators you learned earlier in this lesson to test data values. If performs one of two possible code actions, depending on the result of the comparison. Visual Basic can test whether to execute complete blocks of code. In other words, If uses comparison operator results to test data. If might execute one or more lines of subsequent code, depending on the results of the comparison's test. Before If, the code you wrote executed sequentially, one statement after another. If lets your program be more decisive and execute only parts of the program if the data warrants partial execution. For example, suppose you were writing an invoicing system. In such a system, no sales tax should be computed for tax-exempt organizations, so your program would skip over the tax computation code when processing such organizations. If makes decisions. If a comparison test is true, the body of the If statement executes. (In fact, the previous sentence is almost identical to Visual Basic's If statement!) Here is one format of If: If comparisonTest Then One or more Visual Basic statements End If End If lets Visual Basic know where the body of the If statement ends. Suppose that the user enters a sales figure into a Text Box control named txtSales. The following If computes a bonus amount based on the sales: If (txtSales.Text > 5000.00) Then sngBonus = txtSales.Text * .12 End If
Remember that Visual Basic stores 0 in all numeric variables that you don't first initialize. Therefore, sngBonus has a 0 before the If executes. Once the If executes, the code changes the sngBonus variable only if the value of the txtSales.Text property is more than 5000.00. In a way, the If reads like this:
Visual Basic stores a null zero in string variables that you have not yet initialized. If you use an uninitialized Variant data type variable, the variable holds a null value that becomes zero if you assign the variable to a numeric variable. The body of an If can have more than one statement. The body is often called a block. The following If calculates a bonus, the cost of sales, and a reorder amount based on the value of the txtSales text box entry: If (txtSales.Text > 5000.00) Then sngBonus = txtSales.Text * .12 curCostOfSales = txtSales.Text * .41 curReorderCost = txtSales.Text * .24 End If The three statements that make up the body of the If execute only if the condition txtSales.Text > 5000.00 is true. Suppose that this code contains another assignment statement immediately after End If. That assignment statement is outside the body of the If, so the true or false result of the condition affects only the body of the If. Therefore, the tax computation in the following routine executes regardless of whether the sales are more than or less than $5,000.00: If (txtSales.Text > 5000.00) Then sngBonus = txtSales.Text * .12 curCostOfSales = txtSales.Text * .41 curReorderCost = txtSales.Text * .24 End If sngTax = .12 * txtSales.Text
Can you see how the program makes decisions using If? The body of the If executes only if the comparison test is true. Otherwise, the rest of the program continues as usual. There is a shortcut form of If that you might run across. The single-line If statement has a format that looks like this: If comparisonTest Then VBStatement The single-line If does not require an End If statement because the comparison test and the body of the If reside on the same line. Single-line If statements do not provide for easy program maintenance. If you decide that you want to add to the body of the If, you must convert the single-line If to a multiple-line If, and you might forget to then add End If. Therefore, even if the body of an If statement takes only one line, code the If as a multiple-line If-End If statement to make the program more maintainable. The If Statements Else BranchWhereas If executes code based on the comparison test's true condition, the Else statement executes code based on the comparison test's false condition. Else is an optional part of the If statement. Else specifies the code that executes if the comparison test is false. Here is the complete format of the If statement with Else: If comparisonTest Then One or more Visual Basic statements Else One or more Visual Basic statements End If Typically, programmers call this full-blown If statement the If-Else statement. The If-Else statement is sometimes called a mutually exclusive statement. The term mutually exclusive simply means that one set of code or the other executes, but not both. The If-Else statement contains two sets of code--that is, two bodies of one or more Visual Basic statements--and only one set executes, depending on the result of the If. An If statement is either true or false because the If's comparison produces either a true or false result. Therefore, either the first or the second body of code in an If-Else executes. Suppose that a salesperson receives a bonus if sales are high (over $5,000.00) or suffers a pay cut if sales are low (below $5,000.00). The If-Else shown next contains the code necessary to reward or punish the salesperson. The code body of the If computes the bonus as done in the previous section. The code body of the Else subtracts $25 from the salesperson's pay, which is stored in the variable named curPayAmt, if the sales quota is not met. The following code computes such a payment amount based on the quota: If (txtSales.Text > 5000.00) Then sngBonus = .05 * txtSales.Text Else curPayAmt = curPayAmt - 25.00 End If curTaxes = curPayAmt * .42 The fourth line in these code lines may surprise you at first. The assignment appears to make the statement that the pay is equal to the pay minus 25. You know that nothing can be equal to itself minus 25. In math, the equal sign acts as a balance for the two sides of the equation. In Visual Basic, however, when the equal sign is not used inside an If's comparison test, it is an assignment that takes everything to the right of the equal sign and stores that value in the variable to the left of the equal sign. Therefore, the fourth line subtracts the 25 from the value stored in curPayAmt and then assigns that result back to curPayAmt. In effect, it lowers the value of curPayAmt by 25.
To further your understanding of the If-Else statement and to demonstrate testing for an input box's return value, study how Listing 7.1 uses If-Else to respond to an input box. The code asks the user for a company name and then accepts the name or recognizes that the user clicked Cancel to get rid of the input box without answering it. (When a user clicks Cancel in response to an input box, the input box returns a null string, "".) Listing 7.1. Checking an input boxs return value.
Dim strCompName As String
Dim intPress As Integer ` MsgBox return value
` Ask the user for a name
` Use XYZ, Inc. for the default name
strCompName = InputBox("What is the company name?", _
"Company Request", "XYZ, Inc.")
` Check the return value
If (strCompName = "") Then
` The user pressed Cancel
intPress = MsgBox("Thanks anyway")
Else
` The user entered a company name
intPress = MsgBox("You entered " & strCompName)
End If
Compound Comparisons with the Logical OperatorsVisual Basic supports three additional operators--And, Or, and Not--that look more like commands than operators. And, Or, and Not are called logical operators. Logical operators let you combine two or more comparison tests into a single compound comparison. Table 7.3 describes the logical operators, which work just like their spoken counterparts. Table 7.3. The logical operators.
As you can see from Table 7.3, the And and Or logical operators let you combine more than one comparison test in a single If statement. The Not negates a comparison test. You can often turn a Not condition around. Not can produce difficult comparison tests, and you should use it cautiously. The last If in Table 7.3, for instance, could easily be changed to If (strAns <> "Yes") to eliminate the Not. Your code often must perform an assignment, print a message, or display a label if two or more conditions are true. The logical operators make the combined condition easy to code. Suppose that you want to reward the salesperson if sales total more than $5,000 and if the salesperson sells more than 10,000 units of a particular product. Without And, you have to embed an If statement in the body of another If statement like this: If (sngSales > 5000.00) Then
If (intUnitsSold > 10000) Then
sngBonus = 50.00
End I
End If
Here is the same code rewritten as a single If. It is easier to read and to change later if you need to update the program: If (sngSales > 5000.00) And (intUnitsSold > 10000) Then sngBonus = 50.00 End If How can you rewrite this If to pay the bonus if the salesperson sells either more than $5,000 in sales or if the salesperson sells more than 10,000 units? Here is the code: If (sngSales > 5000.00) Or (intUnitsSold > 10000) Then sngBonus = 50.00 End If Listing 7.2 contains an If-Else that tests data from two divisions of a company and calculates values from the data. Listing 7.2. Calculating sales figures for a companys divisions.If (intDivNum = 3) Or (intDivNum = 4) Then curDivTotal = curDivSales3 + curDivSales4 curGrandDivCosts = (curDivCost3 * 1.2) + (curDivCost4 * 1.4) Else curDivTotal = curDivSales1 + curDivSales2 curGrandDivCosts = (curDivCost1 * 1.1) + (curDivCost5 * 1.9) End If
Multiple Choice with Select CaseIf is great for data comparisons in cases where one or two comparison tests must be made. When you must test against more than two conditions, however, If becomes difficult to maintain. The logical operators help in only certain kinds of conditions. At other times, you must nest several If-Else statements inside one other. Consider the If statement shown in Listing 7.3. Although the logic of the If statement is simple, the coding is extremely difficult to follow. Listing 7.3. Nested If-Else statements get complex quickly.
If (intAge = 5) Then
lblTitle.Caption = "Kindergarten"
Else
If (intAge = 6) Then
lblTitle.Caption = "1st Grade"
Else
If (intAge = 7) Then
lblTitle.Caption = "2nd Grade"
Else
If (intAge = 8) Then
lblTitle.Caption = "3rd Grade"
Else
If (intAge = 9) Then
lblTitle.Caption = "4th Grade"
Else
If (intAge = 10) Then
lblTitle.Caption = "5th Grade"
Else
If (intAge = 11) Then
lblTitle.Caption = "6th Grade"
Else
lblTitle.Caption = "Advanced"
End If
End If
End If
End If
End If
End If
End If
Select Case Expression
Case value
One or more Visual Basic statements
Case value
One or more Visual Basic statements
[Case value
One or more Visual Basic statements]
[Case Else
One or more Visual Basic statements]
End Select
Select Case is a good substitute for long, nested If-Else conditions when several choices are possible. You set up your Visual Basic program to execute one set of Visual Basic statements from a list of statements inside Select Case. The format of Select Case makes the statement look as difficult as a complex nested If-Else, but you will soon see that Select Case statements are actually easier to code and to maintain than their If-Else counterparts. Expression can be any Visual Basic expression--such as a calculation, a string value, or a numeric value--provided that it results in an integer or a string value. Each value must be an integer or a string value that matches Expression's data type. The Select Case statement is useful when you must make several choices based on data values. Select Case can have two or more Case value sections. The code that executes depends on which value matches Expression. If none of the values matches Expression, the Case Else body of code executes if you code the Case Else. Otherwise, nothing happens and control continues with the statement that follows End Select.
The fastest way to learn Select Case is to see an example of it. Listing 7.4 contains a Select Case version of the child grade assignments shown in Listing 7.3. Select Case organizes the multiple-choice selections into a more manageable format. Listing 7.4. Using Select Case to simplify complex nested If-Else statements.Select Case intAge Case 5: lblTitle.Caption = "Kindergarten" Case 6: lblTitle.Caption = "1st Grade" Case 7: lblTitle.Caption = "2nd Grade" Case 8: lblTitle.Caption = "3rd Grade" Case 9: lblTitle.Caption = "4th Grade" Case 10: lblTitle.Caption = "5th Grade" Case 11: lblTitle.Caption = "6th Grade" Case Else: lblTitle.Caption = "Advanced" End Select
Here's how the Select Case works: If the intAge variable holds the value 5, the label is assigned "Kindergarten" in the second line. If the intAge variable holds the value 6, the label is assigned "1st Grade" in the third line. The logic continues through the Case 11: statement. If intAge holds a value that does not fall within the range of 5 through 11, the final Case Else assigns "Advanced" to the label. The body of each Case can consist of more than one statement, just as the body of an If or If-Else can consist of more than one statement. Visual Basic executes all the statements for any given Case match until the next Case is reached. Once Visual Basic executes a matching Case value, it skips the remaining Case statements and continues with the code that follows the End Select statement. Notice the colons after each Case value statement in Listing 7.4. The colons are optional, but they do help separate the case being tested from its code that executes.
Two Additional Select Case FormatsThe two additional formats differ only slightly from the standard Select Case that you learned about in the previous section. They enable you to extend the power of Select Case so that Visual Basic can make Case matches on both comparison tests and on ranges of values. Here is the first additional format: Select Case Expression
Case Is Relation:
One or more Visual Basic statements
Case Is Relation:
One or more Visual Basic statements
[Case Is Relation:
One or more Visual Basic statements]
[Case Else:
One or more Visual Basic statements]
End Select
Relation can be whatever comparison test you want to perform against Expression at the top of the Select Case. The standard Select Case statement, discussed in the previous section, compared the Expression value against an exact Case match. When you use the comparison Is Select Case option, each Case can be matched on a comparison test. Here is the format of the second extra Select Case format: Select Case Expression
Case expr1 To expr2:
One or more Visual Basic statements
Case expr1 To expr2:
One or more Visual Basic statements
[Case expr1 To expr2:
One or more Visual Basic statements]
[Case Else:
One or more Visual Basic statements]
End Select
The Case lines require a range, such as 4 To 6. The To Select Case option enables you to match against a range instead of a relation or an exact match.
Study Listing 7.5 to learn how to combine different Select Case statements to test for various values. Listing 7.5. Using Select Case to simplify complex nested If-Else statements.
Rem The following Select Case to End Select code
Rem assigns a student's grade and school name
Rem to the label on the form. The code checks
Rem to make sure that the student is not too
Rem young to be going to school.
Select Case intAge
` Check for too young...
Case Is <5: lblTitle.Text = "Too young"
` Five-year olds are next assigned
Case 5: lblTitle.Text = "Kindergarten"
` Six to eleven...
Case 6 To 11: lblTitle.Text = "Elementary"
lblSchool.Text = "Lincoln"
` Twelve to fifteen...
Case 12 To 15: lblTitle.Text = "Intermediate"
lblSchool.Text = "Washington"
` Sixteen to eighteen
Case 16 To 18: lblTitle.Text = "High School"
lblSchool.Text = "Betsy Ross"
` Everyone else must go to college
Case Else: lblTitle.Text = "College"
lblSchool.Text = "University"
End Select
If you were to rewrite this code using embedded If-Else logic, the code would become a nightmare. The Select Case's range testing, such as Case 16 to 18, saves a tremendous amount of If-Else logic. SummaryIn this hour you have learned how to write Visual Basic programs that make decisions. Once your programs are able to test data values against certain conditions, your programs can begin to make execution decisions and perform smarter calculations. Visual Basic offers several forms of the If and the Select Case statements to make comparisons. The comparison operators, especially when combined with the logical operators, produce advanced compound conditions. The next hour explains how to write looping statements so that your programs can repeat logic as often as needed. Q&A
WorkshopThe quiz questions and exercises are provided for your further understanding. See Appendix C, "Answers," for answers. Quiz
If (intA < 1) And (intC >= 8) Then lblDraft.Caption = "Overdrawn" Else lblDraft.Caption = "Underdrawn" End Else Exercises
If Not(X < 10) Or Not(Y >= 20) Then |
