Selection Structures

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Selection Structures

• Tonga Institute of Higher Education

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Introduction

• Programs must be able to adapt to conditions
  encountered when the program is running.
• Selection structures allow programs to adapt to
  different conditions.
• IFElse Statement
• Select Statement

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Source Code Execution Order

• Generally, source code is run in order

This is run first
This is run second
This is run third

• But we dont always need to run in order

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Selection Structures/Statements

• Selection structures allow programs to run
  different code based on runtime conditions.
• If Statement
• Select Statement

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If Statement / Structure

• This allows a program to run different code based
  on a condition.
• They require 3 things
• The condition to test for.
• The code to run if the condition is true.
• The code to run if the condition is false.
• Example
• Condition If my name is Dave.
• If True Print Your name is Dave!
• If False Print Your name is not Dave!

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If Statement Overview

• Example
• Condition If my name is Dave.
• If True Print Your name is Dave!
• If False Print Your name is not Dave!

Keywords are blue
Code to run if True
Keywords are blue
Code to run if False
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IF Statements - 1
You can execute many lines of code
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IF Statements - 2
You dont need an Else statement
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Nested If Statements
Check for different conditions.
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Comparison Operators

• The return of a comparison operator is a boolean
  (true/false)
• Equal
• Not Equal ltgt
• Less Than lt
• Less Than or Equal To lt
• Greater Than gt
• Greater Than or Equal To gt

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Demonstration

• If Statements

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Logical Operators

• Logical operators return a true or false value as
  a result of performing an operation on two
  booleans.
• And
• Or
• Use these to evaluate multiple conditions.
• Example We own a restaurant. We want to buy
  chicken and pigs from large suppliers. Our
  criteria for finding a supplier could be
• ChickenQuantity gt 1000 And PigQuantity gt 1000
• Or
• ChickenQuantity gt 5000
• Or
• PigQuantity gt 5000

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Logical Operators - And

• And
• Both conditions must be true to return true.
  Otherwise, false is returned.

Condition 1 Operator Condition 2 Result
True And True True
True And False False
False And True False
False And False False
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Logical Operators - Or

• Or
• At least one condition must be true to return
  true. Otherwise, false is returned.

Condition 1 Operator Condition 2 Result
True Or True True
True Or False True
False Or True True
False Or False False
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Practice!

• Dim Name1 as String Dave
• Dim Name2 as String Cathy
• Dim Name3 as String Jenny
• (Name1 Bob And Name2 Cathy) Or Name3
  Jenny
• Name1 Dave Or Name2 ltgt Bob
• (Name1 ltgt Dave And Name2 Bob) Or Name3
  Jenny

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Demonstration

• If Statements with Boolean Logic

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Select Statement / Structure

• This allows a program to run different code based
  on a condition.
• Improves readability of very long nested if
  statements

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Demonstration

• Select Statements