Selection Structure If... Then.. Else Case

Selection Structure Use to make a decision or comparison and then, based on the result of that decision or comparison, to select one of two paths. The condition must result in either a true (yes) or false (no) answer. If the condition is true, the program performs one set of tasks. If the condition is false, there may or may not be a different set of tasks to perform.

Selection Structure Pseudocode If condition is true Then perform these tasks End If Perform these tasks whether condition is true or false If condition is true then perform these tasks Else perform these tasks End If Perform these tasks whether condition is true or false

If..Then…Else Statement If condition Then [instructions when the condition is true] [Else [instructions when the condition is false]] End If

Relational Operators = > >= < <= <> Equal to Greater than Greater than or equal to Less than Less than or equal to Not equal to These operators are evaluated from left to right, and are evaluated after any mathematical operators.

Expressions Containing Relational Operators < 5 * 2 5 * 2 is evaluated first, giving is evaluated second, giving < 10 is evaluated last, giving false 7 > 3 * 4 / 2 3 * 4 is evaluated first, giving / 2 is evaluated second, giving 6 7 > 6 is evaluated last, giving true All expressions containing a relational operator will result in either a true or false answer only.

Examples of Relational Operators used in the condition Write a condition that checks if the value stored in the intNum variable is greater than 123 intNum > 123 Write a condition that checks if the value stored in the strName variable is “Mary Smith” UCase(strName) = “MARY SMITH”

Logical Operators Not And Or Reverses the truth value of condition; false becomes true and true becomes false All conditions connected by the And operator must be true for the compound condition to be true Only one of the conditions connected by the Or operator needs to be true for the compound condition to be true. These operators are evaluated after any mathematical and relational operators. The order of precedence is Not, And, Or.

Expressions Containing the And Logical Operator 3 > 2 And 6 > 5 3 > 2 is evaluated first, giving true 6 > 5 is evaluated second, giving true true And true is evaluated last, giving true is evaluated first, giving 6 10 < 25 is evaluated second, giving true 6 > 6 is evaluated third, giving false true And false is evaluated last, giving false

Expression Containing the Or Logical Operator 8 = 4 * 2 Or 7 < 5 4 * 2 is evaluated first, giving 8 8 = 8 is evaluated second, giving true 7 > 5 is evaluated third, giving false true Or false is evaluated last, giving true All expressions containing a relational operator will result in either a true or false answer only.

Evaluation of Expressions Containing Logical Operators If you use the And operator to combine two conditions, Visual Basic does not evaluate the second condition if the first condition is false. If you use the Or operator to combine two conditions, Visual Basic does not evaluate the second condition if the first condition is true.

Example of Logical Operators used in the condition To pass a course, a student must have an average test score of at least 75 and an average project score of at least 35. Write the condition using the variables sngTest and sngProj. sngTest >= 75 And sngProj >= 35

Example of Logical Operators used in the condition Only people living in the state of Michigan who are over 65 years old receive a discount. Write the condition using the variables strState and intAge. UCase(strState) = “MICHIGAN” And intAge > 65

Example of Logical Operators used in the condition Only employees with job codes of 34 and 67 will receive a raise. Write the condition using the variable intCode. intCode = 34 Or intCode = 67

Nested Selection Structure A nested selection structure is one in which either the true path or the false path includes yet another selection structure. Any of the statements within either the true or false path of one selection structure may be another selection structure.

Nested If in the true path If condition1 Then [instructions when condition1 is true] If condition2 Then [instructions when both condition1 and condition2 are true] [Else [instructions when condition1 is true and condition2 is false]] End If Else [instructions when condition1 is false]] End If

Nested If in the false path If condition1 Then [instructions when condition1 is true] Else If condition2 Then [instructions when condition1 is false and condition2 is true] [Else [instructions when both condition1 and condition2 are false]] End If

Nested If Example 1 Write a selection structure that assigns a sales tax rate to the sngTax variable. The tax rate is determined by the state code stored in the intCode variable. Codes of 1 and 3 represent a 4% rate; a code of 2 represents a 5% rate. All other codes represent a 2% rate.

Nested If Example 1 If intCode = 1 Or intCode = 3 Then sngTax =.04 Else If intCode = 2 Then sngTax =.05 Else sngTax =.02 End If

Nested If Example 2 Write a selection structure that assigns a bonus to the sngBonus variable. The bonus is determined by the salesperson’s code (intCode) and, in some cases, by the sales amount (sngSales). If the code is 1 and the salesperson sold at least $10,000, then the bonus is $500; otherwise these salespeople receive $200. If the code is 2 and the salesperson sold at least $20,000, then the bonus is $600; otherwise these salespeople receive $550. All others receive $150.

Nested If Example 2 If intCode = 1 Then If sngSales >= Then sngBonus = 500 Else sngBonus = 200 End If Else If intCode = 2 Then If sngSales >= Then sngBonus = 600 Else sngBonus = 550 Else sngBonus = 150 End If

Nested If Example 2 If intCode = 1 And sngSales >= Then sngBonus = 500 Else If intCode = 1 And sngSales < Then sngBonus = 200 Else If intCode = 2 And sngSales >= Then sngBonus = 600 Else If intCode = 2 And sngSales < Then sngBonus = 550 Else sngBonus = 150 End If

Case Form of the Selection Structure Referred to as the extended selection structure Easier than the nested If to write and understand Typically used when a selection structure has several paths from which to choose

Select Case Statement Select Case testexpression [Case expressionlist1 [instructions for the first Case]] [Case expressionlist2 [instructions for the second Case]] [Case expressionlistn [instructions for the nth Case]] [Case Else [instructions for when the testexpression does not match any of the expressionlists]] End Select

To and Is Keywords Use the To keyword to specify a range of values when you know both the minimum and maximum values Use the Is keyword to specify a range of values when you know only one value, either the minimum or the maximum

Select Case Example 1 Write a selection structure that assigns a sales tax rate to the sngTax variable. The tax rate is determined by the state code stored in the intCode variable. Codes of 1 and 3 represent a 4% rate; a code of 2 represents a 5% rate. All other codes represent a 2% rate.

Select Case Example 1 Select Case intCode Case 1, 3 sngTax =.04 Case 2 sngTax =.05 Case Else sngTax =.02 End Select

Select Case Example 2 Write a selection structure that assigns a bonus to the sngBonus variable. The bonus is determined by the salesperson’s code (intCode) and, in some cases, by the sales amount (sngSales). If the code is 1 and the salesperson sold at least $10,000, then the bonus is $500; otherwise these salespeople receive $200. If the code is 2 and the salesperson sold at least $20,000, then the bonus is $600; otherwise these salespeople receive $550. All others receive $150.

Select Case Example 2 Select Case intCode Case 1 Select Case sngSales Case Is >= sngBonus = 500 Case Else sngBonus = 200 End Select Case 2 Select Case sngSales Case Is >= sngBonus = 600 Case Else sngBonus = 550 End Select Case Else sngBonus = 150 End Select

Select Case Example 2 Select Case True Case intCode = 1 And sngSales >= sngBonus = 500 Case intCode = 1 And sngSales < sngBonus = 200 Case intCode = 2 And sngSales >= sngBonus = 600 Case intCode = 2 And sngSales < sngBonus = 550 Case Else sngBonus = 150 End Select