1. Chapter 4 The If…Then Statement
4/22/2017 5:29 AM
Chapter 4 The If…Then Statement
Conditional control structure, also called a decision structure
Executes a set of statements when a condition is true
The condition is a Boolean expression
For example, the statement If x = 5 Then y = 20 End If assigns the value 20 to y only if x is equal to 5.
The ( = )is used as a relational operator to compare x to a certain value ( 5 in this case )
The general statement is:
If condition Then
statements
End If
The condition of an If…Then should never make an equality comparison between floating point numbers because of the possibility of round off error.
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2. Chapter 4 Relational Operators
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Chapter 4 Relational Operators
Operator Meaning
= equal to
< less than
<= less than or equal to
> greater than
>= greater than or equal to
<> not equal to
Relational operators are used to form Boolean expressions. Boolean expressions evaluate to True or False.
Note that the equal sign is used as both an assignment operator and a relational operator.
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3. Chapter 4 The If…Then…Else Statement
4/22/2017 5:29 AM
Chapter 4 The If…Then…Else Statement
Contains an Else clause that is executed when the If condition evaluates to false. For example, the statement If x = 5 Then y = 20 Else y = 10 End If assigns the value 20 to y if x is equal to 5 or the value 10 if x is not equal to 5.
If condition Then
statements
Else
End if
GPS: indentation
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4. Chapter 4 Nested If…Then…Else Statements
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Chapter 4 Nested If…Then…Else Statements
Should be indented to make the logic clear.
Nested statement executed only when the branch it is in is executed. For example, the statement If x = 5 Then y = 20
Else If x > 5 Then y = 10 Else y = 0 End If End If evaluates the nested If…Then…Else only when x is not equal to 5.
Second is “nested”
GPS: indentation
TIP: when you click a keyword in a control structure all of the keywords in the structure are highlighted.
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5. Chapter 4 The If…Then…ElseIf Statement
4/22/2017 5:29 AM
Chapter 4 The If…Then…ElseIf Statement
Used to decide among three or more actions.
Conditions must be properly ordered for the statement to evaluate as expected. For example, the statement If x < 5 Then y = 20 ElseIf x < 10 Then y = 40 ElseIf x < 15 Then y = 80 End If would give very different results if the conditions were ordered differently.
Used to decide among 3 or more courses of action.Syntax:
If condition Then
statements
ElseIf condition Then …
Else
Statements
End If.
There can be multiple ElseIf clauses, and the last Else is optional
GPS: Decision structures with many branches can become difficult to understand so brief inline comments help
Write TestGrade and keep modifying it:
Private Sub btnCheckGrade_Click(sender As Object, e As System.EventArgs) Handles btnCheckGrade.Click
Dim grade As Integer
grade = Val(Me.txtTestScore.Text)
If grade >= 70 Then
Me.lblComment.Text = "Great Job"
End If
End Sub
© 2012 EMC Publishing, LLC

6. Chapter 4 The Select…Case Statement
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Chapter 4 The Select…Case Statement
The result of an expression determines which statements to execute.
The Case Else code is optional and is executed when none of the previous cases are met: Select Case numLegs Case 2 Me.lblMessage.Text = "human" Case 4 Me.lblMessage.Text = "beast" Case 8 Me.lblMessage.Text = "insect" Case Else Me.lblMessage.Text = "???" End Select
Syntax:
Select expression
Case value
statements …
Case Else
End Select
The expression must evaluate to a built-in data type.
Else clause is optional.
Value type should match the expression type and can be: a single value, a list separated by commas or a range separated by the keyword” To”
Another Example:
Select Case score
Case 0, 10
Me.lblMessage.Text = “Nice try” ‘score is 0 or 10
Case 20 To 23
Me.lblMessage.Text = “Great!” ‘score is 20, 21, 22 or 23
Me.lblMessage.Text = “Invalid score.” ‘score not, 0, 10, 20
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7. Chapter 4 The Select…Case Is Statement
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Chapter 4 The Select…Case Is Statement
Compares the result of an expression to a range of values to determine which statements to execute. For example:
Select Case score Case Is < 10 Me.lblMessage.Text = "Nice try." Case Is < Me.lblMessage.Text = "Good." Case Is >= Me.lblMessage.Text = "Great!" End Select
Compares the result of an expression to a range of values when a relational operator is part of the value.
Write TestGrade using case
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8. Chapter 4 The Rnd() Function
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Chapter 4 The Rnd() Function
Uses a formula to generate a sequence of numbers that are each greater than 0 and less than 1 and then returns one number from the sequence.
A random integer in a range is generated by using the formula: Int(highNum – lowNum + 1) * Rnd() + lowNum)
Random integers are produced by using the Int() function along with the Rnd() function: Int(21 * Rnd() + 10) '10 to 30
The Randomize() statement initializes the random number generator.
Int() requires a numeric value and returns the integer part of that number without rounding.
Fix() works the same as Int() for positive numbers.
- For negative numbers Int() returns the first negative integer less than or equal to its argument. Int(-5.4)->6
Fix() returns the first negative integer that is greater than or equal to its argument Fix(-5.4)->5
Randomize() uses a value based on the computer’s clock as a seed for the random generator.
The seed is a starting value for calculating the sequence of pseudorandom numbers
Write RandomNumbers
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9. A set of steps that outline how to solve a problem.
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Chapter 4 Algorithms
A set of steps that outline how to solve a problem.
Can be implemented in plain English or in a mix of English and program code called pseudocode.
Flowcharts are another option
Algorithms allow a programmer to think through a program before actually typing code, which may reduce errors in logic.
Pseudo code is a mix of English and program code without syntax rules to follow.
It helps reducing logic errors.
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10. Chapter 4 Static Variables
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Chapter 4 Static Variables
Declared with the keyword Static instead of Dim.
Have a lifetime the duration of the program's running time.
Used to extend the lifetime of local variables in a procedure.
Should be explicitly initialized when declared.
A better choice than a global variable because the scope of the variable can be limited.
The lifetime of a local variable is the duration of the procedure in which it was declared.
The lifetime of a global variable is the duration of the program
Static variables are necessary in event procedures with variables that should be retained in memory throughout program execution.
The scope of a static variable is local to the procedure it is declared but its lifetime is the duration of the program.
!!! A variable declared in a Click event procedure is redeclared and reinitialized each time the Click event occurs unless it is declared as Static.
Unless the the variable is assigned a new value, the value is retained throughout program execution.
© 2012 EMC Publishing, LLC

11. Chapter 4 Compound Boolean Expressions
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Chapter 4 Compound Boolean Expressions
More than one Boolean expression in a single condition.
Formed using the And, Or, or Not operators.
Order of operations: Not evaluated first, And second, Or last.
Compound Boolean expressions use more than one Boolean expression to determine if a condition is true or false.
© 2012 EMC Publishing, LLC

12. Chapter 4 And Truth Table
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Chapter 4 And Truth Table
And
Exp1
Exp2
Result
True
False
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13. Or Exp1 Exp2 Result True False Chapter 4 Or Truth Table
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Chapter 4 Or Truth Table Or
Exp1
Exp2
Result
True
False
© 2012 EMC Publishing, LLC

14. Chapter 4 Not Truth Table
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Chapter 4 Not Truth Table
Not
Exp
Result
True
False
Write GuessingGame.
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15. Chapter 4 The MessageBox Class
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Chapter 4 The MessageBox Class
A predefined dialog box that displays a message to the user.
Includes the Show() method for displaying the dialog box. For example:
MessageBox.Show(message)
A message box can display text in the title bar by including a string as second parameter. Ex:
MessageBox.Show(“Good guess!”, “Game”) will display a message box with Game in the title bar
© 2012 EMC Publishing, LLC

16. Chapter 4 Counter Variables
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Chapter 4 Counter Variables
A variable that is incremented by a constant value.
Used for counting guesses, the numbers of values entered, the number of times a button was clicked, and so on.
The value of a counter is updated in a statement similar to:
counter = counter + CONSTANT
Should be initialized when declared and updated by an unchanging amount.
Many algorithms involve counting. Applications written for algorithms that involve counting use a counter variable for storing a number that is incremented by a constant value.
CONSTANT is defined globally, as Const
numTries = numTries + 1 can be written as
numTries += 1
Counters are sometimes counted backwards:
numTries -= 1
Counter should be initialized and then updated by an unchanged amount ( STEP is a good identifier )
A counter in an event procedure should be declared as Static variable so it is initialized only once.
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17. Chapter 4 Assignment Operators
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Chapter 4 Assignment Operators
Operator Operation
+= addition and then assignment
-= subtraction and then assignment
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18. Chapter 4 The CheckBox Control
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Chapter 4 The CheckBox Control
Check boxes allow the user to select options.. Unlike Radio buttons, more than one can be selected.
(Name) should begin with chk.
Text is the text displayed next to the box.
Checked is set to True if the box should be displayed as checked.
An If…Then statement is often used to determine if a check box is checked or cleared.
Related Check boxes are grouped together in a GroupBox.
A Click event is sometimes coded for a check box. This procedure executes when a check box is clicked and usually includes code to determine the state of the check box and then performs actions depending on whether the check box was selected or cleared.
Write: MorningToDo
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19. Chapter 4 Implicit Line Continuation
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Chapter 4 Implicit Line Continuation
A statement typically fits on one line, but can be continued onto the next line using a line-continuation sequence:
If Not (Me.chkBed.Checked And Me.chkLunch.Checked _
And Me.chkHomework.Checked And Me.chkTeeth.Checked) Then
...
In many cases, you can continue a statement on the next consecutive line without using the underscore character (_):
after a comma (,)
after an open parenthesis (() or before a closing parenthesis ())
after an open curly brace ({) or before a closing curly brace (})
after assignment operators (=, &=, :=, +=, -=, *=, /=, \=, ^=)
after binary operators (+, -, /, *, Mod, <>, <, >, <=, >=, And, Or)
A line of code can be about 65,000 characters in length. It is easier to work with lines of 80 characters or less.
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