Selection Control Structures

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CHAPTER 4 Selection Control
Structures
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OBJECTIVES

• To introduce the 4 types of selection control
  structures
• To introduce the case structure
• To familiarize students with the usage of
  selection control structures

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Types of IF statements

• Simple If
• Null Else
• Combined If
• Nested If

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Simple IF

• If . Then
• Else
• EndIf

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Simple IF

• If (condition is true) Then
• execute statement 1
• Else
• execute statement 2
• EndIf

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Simple IF

• In a Simple If, one of two statements is executed
• One will always be executed
• The second statement acts as a default statement

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Null Else

• If .. Then
• EndIf

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Null Else

• If (condition is true) Then
• execute statement
• EndIf

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Null Else

• In a Null Else, one or no statement is executed
• This structure allows the program block to NOT
  execute selectively

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Combined IF

• Uses the keywords AND and OR
• Allows more complex selection conditions to be
  performed

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Combined IF - AND

• If AND . Then
• Else
• EndIf

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Combined IF - AND

• If (condition 1 is true) AND
• (condition 2 is true) Then
• execute statement 1
• Else
• execute statement 2
• EndIf

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Combined IF - AND

• With AND, both conditions have to be true for the
  first statement to be executed
• The default statement executes otherwise (or in
  the case of a Null Else, nothing happens)

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Combined IF - OR

• If . OR .. Then
• Else
• EndIf

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Combined IF - OR

• If (condition 1 is true) OR
• (condition 2 is true) Then
• execute statement 1
• Else
• execute statement 2
• EndIf

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Combined IF - OR

• With OR, either condition has to be true for the
  first statement to be executed
• Thus, the more conditions there are, the higher
  the chances for execution of the first statement

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Nested IF

• Two or more Simple If/Null Else structures used
  together
• Enables multiple conditional execution of
  statements
• 2 types linear and non-linear

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Linear Nested IF

• If Then
• Else If Then
• Else
• EndIf
• EndIf

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Linear Nested IF

• If (condition 1 is true) Then
• execute statement 1
• Else If (condition 2 is true) Then
• execute statement 2
• Else
• execute statement 3
• EndIf
• EndIf

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Non-Linear Nested IF

• If Then
• If Then
• Else
• EndIf
• Else
• EndIf

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Non-Linear Nested IF

• If (condition 1 is true) Then
• If (condition 2 is true) Then
• execute statement 1
• Else
• execute statement 2
• EndIf
• Else
• execute statement 3
• EndIf

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Notes on Nested IF

• Non-Linear Nested Ifs are messy and its usage is
  not encouraged
• Usually can be rewritten using Combined IF
  structures
• Likewise, Linear Nested Ifs can sometimes be
  rewritten using the Case structure

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Non-Linear Nested IF

• If (condition 1 is true) Then
• If (condition 2 is true) Then
• execute statement 1
• EndIf
• EndIf

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Non-Linear Nested IFRewritten

• If (condition 1 is true) AND
• (condition 2 is true) Then
• execute statement 1
• EndIf

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Case

• Rewrites the Linear Nested If
• Basically compares variables for the one to
  execute.
• Usually used for menus
• Does not use conditions

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Case

• Case Of single variable
• value_1 execute statement 1
• value_2 execute statement 2
• value_n execute statement n
• value_other execute default statement
• Endcase

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Case Example

• Case Of char_stored
• A Display A
• B Display B
• C Display C
• other Display X
• Endcase

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Questions?