Selection Structure

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Selection Structure

• If . . . Then . . Else
• Case

2
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.

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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

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If..ThenElse Statement

• If condition Then
• instructions when the condition is
  true
• Else
• instructions when the condition is false
• End If

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Relational Operators

• gt
• gt
• lt
• lt
• ltgt

• 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.
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Expressions Containing Relational Operators

• 10 3 lt 5 2
• 5 2 is evaluated first, giving 10
• 10 3 is evaluated second, giving 13
• 13 lt 10 is evaluated last, giving false

• 7 gt 3 4 / 2
• 3 4 is evaluated first, giving 12
• 12 / 2 is evaluated second, giving 6
• 7 gt 6 is evaluated last, giving true

All expressions containing a relational operator
will result in either a true or false answer only.
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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 gt 123
• Write a condition that checks if the value stored
  in the strName variable is Mary Smith
• UCase(strName) MARY SMITH

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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.
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Expressions Containing the And Logical Operator

• 3 gt 2 And 6 gt 5
• 3 gt 2 is evaluated first, giving true
• 6 gt 5 is evaluated second, giving true
• true And true is evaluated last, giving true

• 10 lt 25 And 6 gt 5 1
• 5 1 is evaluated first, giving 6
• 10 lt 25 is evaluated second, giving true
• 6 gt 6 is evaluated third, giving false
• true And false is evaluated last, giving false

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Expression Containing the Or Logical Operator

• 8 4 2 Or 7 lt 5
• 4 2 is evaluated first, giving 8
• 8 8 is evaluated second, giving true
• 7 gt 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.
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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.

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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 gt 75 And sngProj gt 35
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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 gt 65
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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
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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.

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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

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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
• End If

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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.
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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 End If
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Nested If Example 2
Write a selection structure that assigns a bonus
to the sngBonus variable. The bonus is determined
by the salespersons 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.
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Nested If Example 2
If intCode 1 Then If sngSales gt 10000
Then sngBonus 500 Else sngBonus 200 End
If Else If intCode 2 Then If sngSales gt
20000 Then sngBonus 600 Else sngBonus
550 Else sngBonus 150 End If End If
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Nested If Example 2
If intCode 1 And sngSales gt 10000 Then
sngBonus 500 Else If intCode 1 And sngSales
lt 10000 Then sngBonus 200 Else If intCode
2 And sngSales gt 20000 Then sngBonus
600 Else If intCode 2 And sngSales lt 20000
Then sngBonus 550 Else sngBonus
150 End If End If End If End If
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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

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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

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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

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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.
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Select Case Example 1
Select Case intCode Case 1, 3 sngTax
.04 Case 2 sngTax .05 Case Else sngTax
.02 End Select
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Select Case Example 2
Write a selection structure that assigns a bonus
to the sngBonus variable. The bonus is determined
by the salespersons 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.
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Select Case Example 2
Select Case intCode Case 1 Select Case
sngSales Case Is gt 10000 sngBonus
500 Case Else sngBonus 200 End
Select Case 2 Select Case sngSales Case Is
gt 20000 sngBonus 600 Case
Else sngBonus 550 End Select Case
Else sngBonus 150 End Select
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Select Case Example 2
Select Case True Case intCode 1 And sngSales
gt 10000 sngBonus 500 Case intCode 1 And
sngSales lt 10000 sngBonus 200 Case intCode
2 And sngSales gt 20000 sngBonus 600 Case
intCode 2 And sngSales lt 20000 sngBonus
550 Case Else sngBonus 150 End Select