Introductory Notes: Condition Statements

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1 Brigham Young University - Idaho College of Physical Sciences and Engineering Department of Mechanical Engineering Introductory Notes: Condition Statements The simplest of all computer programs perform a set of instructions sequentially, starting at the top of the program and proceeding down one line at a time until the end of the program is reached. Most programs, however, require that decisions may be during the time of execution. This may require testing conditions to determine what procedure to perform next, skipping over some statements, and executing some statements multiple times. In Excel, we were introduced to this concept when we learned how to use the IF function. We will extend those concepts in the class. A quick summary of the commands we ll be working with is provided below. More detailed notes and examples from the online help files have also been included following the summary section. Relational Operators Relational or comparison operators are used in expressions to perform a test. They are most commonly used in conjunction with an IF statement. Common relational operators include: < less than <= less than or equal to > greater than >= greater than or equal to <> not equal to = equal to Logical Operators Logical operators are used in complex expressions to perform a test. In essence they are used to check for the validity of multiple conditions being met. Common logical operators include AND, OR, and NOT: AND returns true if all parts of the expression are true, otherwise returns false OR returns true if any part of the expression is true, otherwise returns false NOT returns true if expression is false, returns false if expression is true You may need to use parenthesis to control order of operation for more complex expressions.

2 If-Then-Else Statement If statements are used to execute a statement or block of statements when a condition is true. Logical and relational operators are used to form expressions which are evaluated as either true or false. Syntax for the If statement is as follows: If condition Then [statements ] [ElseIf condition Then] [statements ] [Else] [statements ] When a condition is true, VBA executes the corresponding block of statements. Execution of the program is then transferred to the end of the If structure. A one line version of the is also available for cases where a single statement is to be executed: If condition Then statement GoTo A GoTo statement offers a straightforward means of changing the program flow. This statement transfers program control to a new statement, identified by a label. A label is a text string followed by a colon. In general avoid the use of GoTo, using it only when you have no other way to perform an action. In practice, the only time you really need to use a GoTo statement if for error trapping.

3 Online Help File Notes Not Operator Used to perform logical negation on an expression. Syntax result = Not expression The Not operator syntax has these parts: Part result expression Description Required; any numeric variable. Required; any expression. Remarks The following table illustrates how result is determined: If expression is True False Null Then result is False True Null In addition, the Not operator inverts the bit values of any variable and sets the corresponding bit in result according to the following table: If bit in expression is o Then bit in result is Example This example uses the Not operator to perform logical negation on an expression. Dim A, B, C, D, MyCheck A = 10: B = 8: C = 6: D = Null ' Initialize variables. MyCheck = Not(A > B) ' Returns False. MyCheck = Not(B > A) ' Returns True. MyCheck = Not(C > D) ' Returns Null. MyCheck = Not A ' Returns -11 (bitwise comparison).

4 Or Operator Used to perform a logical disjunction on two expressions. Syntax result = expression1 Or expression2 The Or operator syntax has these parts: Part result expression1 expression2 Description Required; any numeric variable. Required; any expression. Required; any expression. Remarks If either or both expressions evaluate to True, result is True. The following table illustrates how result is determined: If expression1 is And expression2 is Then result is True True True True False True True Null True False True True False False False False Null Null Null True True Null False Null Null Null Null The Or operator also performs a bitwise comparison of identically positioned bits in two numeric expressions and sets the corresponding bit in result according to the following table: If bit in expression1 is And bit in expression2 is Then result is Example This example uses the Or operator to perform logical disjunction on two expressions. Dim A, B, C, D, MyCheck A = 10: B = 8: C = 6: D = Null ' Initialize variables. MyCheck = A > B Or B > C ' Returns True. MyCheck = B > A Or B > C ' Returns True. MyCheck = A > B Or B > D ' Returns True. MyCheck = B > D Or B > A ' Returns Null. MyCheck = A Or B ' Returns 10 (bitwise comparison).

5 And Operator Used to perform a logical conjunction on two expressions. Syntax result = expression1 And expression2 The And operator syntax has these parts: Part result expression1 expression2 Description Required; any numeric variable. Required; any expression. Required; any expression. Remarks If both expressions evaluate to True, result is True. If either expression evaluates to False, result is False. The following table illustrates how result is determined: If expression1 is And expression2 is The result is True True True True False False True Null Null False True False False False False False Null False Null True Null Null False False Null Null Null The And operator also performs a bitwise comparison of identically positioned bits in two numeric expressions and sets the corresponding bit in result according to the following table: If bit in expression1 is And bit in expression2 is The result is Example This example uses the And operator to perform a logical conjunction on two expressions. Dim A, B, C, D, MyCheck A = 10: B = 8: C = 6: D = Null ' Initialize variables. MyCheck = A > B And B > C ' Returns True. MyCheck = B > A And B > C ' Returns False. MyCheck = A > B And B > D ' Returns Null. MyCheck = A And B ' Returns 8 (bitwise comparison).

6 If...Then...Else Statement Conditionally executes a group of statements, depending on the value of an expression. Syntax If condition Then [statements] [Else elsestatements] Or, you can use the block form syntax: If condition Then [statements] [ElseIf condition-n Then [elseifstatements]... [Else [elsestatements]] The If...Then...Else statement syntax has these parts: Part condition statements condition-n elseifstatements elsestatements Description Required. One or more of the following two types of expressions: A numeric expression or string expression that evaluates to True or False. If condition is Null, condition is treated as False. An expression of the form TypeOf objectname Is objecttype. The objectname is any object reference and objecttype is any valid object type. The expression is True if objectname is of the object type specified by objecttype; otherwise it is False. Optional in block form; required in single-line form that has no Else clause. One or more statements separated by colons; executed if condition is True. Optional. Same as condition. Optional. One or more statements executed if associated condition-n is True. Optional. One or more statements executed if no previous condition or condition-n expression is True. Remarks You can use the single-line form (first syntax) for short, simple tests. However, the block form (second syntax) provides more structure and flexibility than the single-line form and is usually easier to read, maintain, and debug. Note With the single-line form, it is possible to have multiple statements executed as the result of an If...Then decision. All statements must be on the same line and separated by colons, as in the following statement: If A > 10 Then A = A + 1 : B = B + A : C = C + B A block form If statement must be the first statement on a line. The Else, ElseIf, and parts of the statement can have only a line number or line label preceding them. The block If must end with an statement. To determine whether or not a statement is a block If, examine what follows the Then keyword. If anything other than a comment appears after Then on the same line, the statement is treated as a single-line If statement. The Else and ElseIf clauses are both optional. You can have as many ElseIf clauses as you want in a block If, but none can appear after an Else clause. Block If statements can be nested; that is, contained within one another. When executing a block If (second syntax), condition is tested. If condition is True, the statements following Then are executed. If condition is False, each ElseIf condition (if any) is evaluated in turn. When a True condition is found, the statements immediately following the associated Then are executed. If none of the ElseIf conditions are True (or if there are no ElseIf clauses), the statements following Else are executed. After executing the statements following Then or Else, execution continues with the statement following.

7 Tip Select Case may be more useful when evaluating a single expression that has several possible actions. However, the TypeOf objectname Is objecttype clause can't be used with the Select Case statement. Note TypeOf cannot be used with hard data types such as Long, Integer, and so forth other than Object. Example This example shows both the block and single-line forms of the If...Then...Else statement. It also illustrates the use of If TypeOf...Then...Else. Dim Number, Digits, MyString Number = 53 If Number < 10 Then Digits = 1 ElseIf Number < 100 Then ' Initialize variable. ' Condition evaluates to True so the next statement is executed. Else Digits = 2 Digits = 3 ' Assign a value using the single-line form of syntax. If Digits = 1 Then MyString = "One" Else MyString = "More than one" Use If TypeOf construct to determine whether the Control passed into a procedure is a text box. Sub ControlProcessor(MyControl As Control) If TypeOf MyControl Is CommandButton Then Debug.Print "You passed in a " & TypeName(MyControl) ElseIf TypeOf MyControl Is CheckBox Then Debug.Print "You passed in a " & TypeName(MyControl) ElseIf TypeOf MyControl Is TextBox Then Debug.Print "You passed in a " & TypeName(MyControl) End Sub

8 Using If...Then...Else Statements You can use the If...Then...Else statement to run a specific statement or a block of statements, depending on the value of a condition. If...Then...Else statements can be nested to as many levels as you need. However, for readability, you may want to use a Select Case statement rather than multiple levels of nested If...Then...Else statements. Running Statements if a Condition is True To run only one statement when a condition is True, use the single-line syntax of the If...Then...Else statement. The following example shows the single-line syntax, omitting the Else keyword: Sub FixDate() End Sub mydate = #2/13/95# If mydate < Now Then mydate = Now To run more than one line of code, you must use the multiple-line syntax. This syntax includes the statement, as shown in the following example: Sub AlertUser(value as Long) End Sub If value = 0 Then AlertLabel.ForeColor = "Red" AlertLabel.Font.Bold = True AlertLabel.Font.Italic = True Running Certain Statements if a Condition is True and Running Others if It's False Use an If...Then...Else statement to define two blocks of executable statements: one block runs if the condition is True, the other block runs if the condition is False. Sub AlertUser(value as Long) If value = 0 Then Else End Sub AlertLabel.ForeColor = vbred AlertLabel.Font.Bold = True AlertLabel.Font.Italic = True AlertLabel.Forecolor = vbblack AlertLabel.Font.Bold = False AlertLabel.Font.Italic = False

9 Testing a Second Condition if the First Condition is False You can add ElseIf statements to an If...Then...Else statement to test a second condition if the first condition is False. For example, the following function procedure computes a bonus based on job classification. The statement following the Else statement runs if the conditions in all of the If and ElseIf statements are False. Function Bonus(performance, salary) If performance = 1 Then Bonus = salary * 0.1 ElseIf performance = 2 Then Bonus = salary * 0.09 ElseIf performance = 3 Then Else End Function Bonus = salary * 0.07 Bonus = 0

10 GoTo Statement Branches unconditionally to a specified line within a procedure. Syntax GoTo line The required line argument can be any line label or line number. Remarks GoTo can branch only to lines within the procedure where it appears. Note Too many GoTo statements can make code difficult to read and debug. Use structured control statements (Do...Loop, For...Next, If...Then...Else, Select Case) whenever possible. Example This example uses the GoTo statement to branch to line labels within a procedure. Sub GotoStatementDemo() Dim Number, MyString Number = 1 ' Initialize variable. ' Evaluate Number and branch to appropriate label. If Number = 1 Then GoTo Line1 Else GoTo Line2 Line1: MyString = "Number equals 1" GoTo LastLine ' Go to LastLine. Line2: ' The following statement never gets executed. MyString = "Number equals 2" LastLine: Debug.Print MyString ' Print "Number equals 1" in ' the Immediate window. End Sub