UNIT 4 Lesson 13 If statements

If…Then…Else control structure We shall learn how to write code that can make decisions and control the program flow. Decision making process is an integral part of programming. The ability to make decision helps solve problems intelligently and provide useful output to the user. For example, we can write a program that can ask the computer to perform certain task until a certain condition is met, or a program that will reject non-numeric data. In order to control the program flow and to make decisions, we need to use the conditional operators and the logical operators together with the If…Then…Else control structure.

Conditional Operators The conditional operators are powerful tools that resemble mathematical operators. These operators allow a program to compare data values and then decide what actions to take, whether to execute a program or terminate the program. They are also known as numerical comparison operators. They are used to compare two values to see whether they are equal, one value is greater or less than the other value. The comparison will return a true or false result.

Relational Operators = > < >= <= <> OPERATOR DESCRIPTION  =  Equal to  >  Greater than  <  Less than  >=  Equal to or Greater than <=  Less than or Equal to  <>  Not equal to

Logical Operators Sometimes we might need to make more than one comparison before a decision can be made and an action taken. In this case, using numerical comparison operators alone is not sufficient, we need to use additional operators, and they are the logical operators. OPERATOR DESCRIPTION And Both sides must be true Or One side or other must be true Xor One side or other must be true but not both Not Negates true The above operators can be used to compare numerical data as well as non-numeric data such as text(string).  In making strings comparison, there are certain rules to follows: Upper case letters are less than lowercase letters, “A”<”B”<”C”<”D”…….<”Z” and number are less than letters.

Using the If control structure with the Comparison Operators To effectively control the program flow, we shall use the If control structure together with the conditional operators and logical operators. There are basically three types of If control structures, namely If….Then statement, If…Then…Else statement If…Then…ElseIf statement.

If….Then Statement This is the simplest control structure which instructs the computer to perform a certain action specified by the expression if the condition is true. However, when the condition is false, no action will be performed. The syntax for the if…then statement is If condition Then Visual Basic 2013 expressions End If Example Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim intMyNumber As Integer intMyNumber = txtInput.Text If intMyNumber > 100 Then lblDisplay.Text = ” You win a lucky prize” End If End Sub * When you run the program and enter a number that is greater than 100, you will see the “You win a lucky prize” statement. On the other hand, if the number entered is less than or equal to 100, you don’t see any display.

If…Then…Else Statement Using just If….Then statement is not very useful in programming and it does not provide choices for the users. In order to provide a choice, we can use the If….Then…Else Statement. This control structure will ask the computer to perform a certain action specified by the Visual Basic 2013 expression if the condition is met. And when the condition is false, an alternative action will be executed. The syntax for the if…then.. Else statement is Example Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim intMyNumber As Integer intMyNumber = txtInput.Text If intMyNumber > 100 Then lblDisplay.Text = ” Congratulation! You win a lucky prize” Else lblDisplay.Text = ” Sorry, You did not win any prize” End If End Sub * When you run the program and enter a number that is greater than 100, the statement “Congratulation! You win a lucky prize” will be shown. On the other hand, if the number entered is less than or equal to 100, you will see the “Sorry, You did not win any prize” statement

Example Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim intMyNumber, intMyAge As Integer intMyNumber = txtNum.Text myAge = txtAge.Text If intMyNumber > 100 And intMyAge > 60 Then lblDisplay.Text = “Congratulation! You win a lucky prize” Else lblDisplay.Text = “Sorry, You did not win any prize” End If End Sub * This program use the logical operator And beside the conditional operators. This means that both the conditions must be fulfilled in order for the conditions to be true, otherwise the second block of code will be executed. In this example, the number entered must be more than 100 and the age must be more than 60 in order to win a lucky prize, any one of the above conditions not fulfilled will disqualify the user from winning a prize.

The If…Then Statement If…Then statement is a decision structure that executes code when a condition is true. Example: If intGuess = 10 Then lblGuessCheckedMessage.Text = “You guessed it!” End If If the value of intGuess is 10, the Text property of the label object is executed. If the value of intGuess in not equal to 10, then the assignment statement is not executed and program flow continue on to the next line after the End If. You can have multiple lines of code between the Then and End If

Equal Symbol (=) Has two purposes: Comparison – compare two values and return true or false. (If Statements) Assignment – Takes the value on the right side and set the left side memory location to the value.

Boolean Expression & Relational Operators The condition of the If condition Then statement is a Boolean expression. Boolean expression – evaluates to either True or False. A Boolean variable may also be used as the condition of the If…Then statement. Example: Dim blnAns As Boolean blnAns = True If blnAns Then Some type of code goes here End If

Relational Operators Six relational operators: = equal sign < less than <= less than or equal to > Greater than >= Greater than or equal to <> Not equal to

Roundoff Error If…Then statement should never make an equality (=) comparison between floating point numbers (numbers with decimals) because of the possibility of a roundoff error.

The If…Then…Else Statement Optional to an If…Then statement Executed when the If condition evaluates to False. If…Then…Else form: If condition Then statement Else End If

Example of an If…Then…Else Statement Private Sub cmdCheckGuess_Click() Const intNum As Integer = 10 Dim intGuess As Integer intGuess = txtGuess.Text If intGuess = intNum Then lblGuessMessage.Text= “You guessed it” Else lblGuessMessage.Text = “Try again” End If End Sub Typing a number other than 10 will display “Try again”

Indentation Indentation helps readability Indentation has no effect on the execution of the statement. For an If…Then…Else all executable code inside the statement should be indented. Example: If condition Then executable code Else End If

Nested If…Then…Else Statements

Nested If…Then…Else Nested If…Then…Else Statement - is an If statement inside another If statement or an Else statement. There are many combinations that you can use with nested If statements that you can not cover all of them.

Examples If Boolean Condition Then Code1 Statement Code2 Statement ‘ nested if statement End If Else Code3 Statement

Example Continued If intGuess = intSecretNumber Then lblGuessCheckedMessage.text = “you guessed it! Else If intGuess < intSecretNumber Then lblGuessCheckedMessage.text = “too low” lblGuessCheckedMessage.text = “too high” End If (nested statements should be indented for good programming style)

Examples If condition1 Then Code1 Else If condition2 Then Code2 Code3 End If When conditional1 is false, the else portion will run and check condition2. If codition2 is false then Code3 will run. If condition1 is true, Code1 will run and everything else is skipped. If condition1 is false and condition2 is true then Code2 will run.

Example If condition1 Then If condition2 Then If condition3 Then Code1 Else Code2 End If Code3 Code4

The If…Then…ElseIf Statement Is used to decide among three or more actions where only the first If or ElseIf condition that is true runs and all others are skipped. Notice there is no space in the ElseIf An ElseIf can contain an If statement The Else statement is optional

Example If condition1 Then ElseIf condition2 Then Code1 ElseIf condition2 Then Code2 ElseIf condition3 Then Code3 ElseIf condition4 Then Code4 Else Code5 End If

Example Continued (ElseIf) Used to decide among 3, 4, or more actions This example includes three possible decisions in the IF..Then…ElseIf: If intGuess = intSecretNumber Then lblGuessCheckedMessage.text= “you guessed it” ElseIf intGuess < intSecretNumber Then lblGuessedCheckedMessage.text=“too low” Else lblGuessCheckedMessage.text= “too high” End If

Programming Style The use of If…Then… ElseIf is better and easier to read then the nested If…Then…Else Beware of the range of the conditions in an If…Then…ElseIf because the first condition that evaluates true will run and then the rest of the ElseIf statements are skipped.

Generating Random Numbers

Generating Random Numbers Built-in function – Built-in meaning that Visual Basic already has it and it doesn’t need to be coded. Rnd function – generates a random number greater than or equal to 0 and less than 1.

Functions Visual Basic has many built-in functions. Function – is a procedure that performs a task and returns a value. Predefined functions are used by including the function name in a statement. When the line of code containing the function name is executed, the statements that make up the function are executed.

Function Arguments Arguments are data that functions requires to perform its task. The data is “passed” to the function as arguments enclosed in parentheses after the function name. Example: Int(-5.4) which will return –6.

RND function Using Rnd alone generates random numbers greater than or equal to 0 and less than 1. If you want to generate numbers in a greater range, simply multiply Rnd by the upper number of the range you want. Example: lblRanNum.Text = Rnd * 10 Generate numbers in a range with an upper and lower limt do the following. (HighNumber – LowNumber + 1) * Rnd + LowNumber Where HighNumber and LowNumber represents the limits of the range.

Int function Returns the Integer portion of a number without rounding. Example: Int(21 * Rnd + 10) ‘will return the integer portion only Again, This number is found by using the equation (HighNumber-LowNumber + 1) * Rnd +LowNumber

Fix Function Very similar to the Int function Both will work the same for positive numbers by returning the Integer portion only. Negative numbers Integer function returns the Integer less than or equal to its negative argument. Fix Function returns the first integer greater than or equal to its negative argument Example: Fix(-5.4) would return –5. Int(-5.4) would return –6.

Randomize Programs that uses Rnd function should include a Randomize statement. The Randomize statement initializes the Visual Basic Rnd function so different random numbers are generated in a program from run to run. Best place for the Randomize statement is in the Form_Load() procedure because it occurs only once. Example: Private Sub Form_Load() Randomize End Sub

How Random Numbers are Generated Uses the computer’s clock to generate a seed value. A seed is a value used by the Rnd function to generate a pseudorandom (not truly random) sequence of numbers. Each time the Rnd is used, the next number in the sequence is returned.

Scope Scope of a variable or constant – refers to its accessibility among procedures. Is where the variable or constant can be accessed or used. Example: the scope of a variable declared in an event procedure is limited to that procedure. No other procedure can refer to that variable or change its value. Variables declared in procedures are considered “local” variables of that procedure.

Global Variables Global variables declared in the General section of a form module are accessible to every procedure. This is known as “Global” variables. Global variable declaration should use the keyword Private in place of the keyword Dim. Global Constants declarations should use the keyword Private in front of the keyword Const. Global variables are declared right after the Option Explicit statement outside of any procedures.

Programming Style Local variables should be used whenever possible because they don’t interfere with other procedures. Two procedures can contain the same variable names and each has their own values. Global variables that are not declared as constant can cause debugging problems because any procedure can change the global variable value. You can do it without a constant but you must make sure errors do not occur—Check Code

Logical Operators 4 types of logical operators And Or Not Xor A logical operator joins two expressions to create an expression that evaluates to either true or false Example: dblnum > 0 And dblnum <= 10 will evaluate to true if dblNum is in the range 0<dblnum<=10. It would evaluate false otherwise.

And Expression1 And Expression2 True And True = True False And True = False True And False = False False And False = False So the only time an And operator evaluates to true is when both expressions are true.

Or Expression1 Or Expression2 True Or True = True False Or True = True True Or False = True False Or False = False So if one or both expressions evaluates to true than it equals true. The only time it evaluates to false if both expressions are false.

Not Expression Result Not True False Not False True Note that the Not changes it to the opposite.

Xor Expression1 Or Expression2 True Or True = False False Or True = True True Or False = True False Or False = False

Operator Precedence Not is evaluated before an And An And is evaluated before an Or Or is evaluated before Xor Xor is last You can use parentheses to change the order.

Algorithms Algorithm – is a series of steps that tell how to solve a problem. Pseudocode – half English and half code of instructions to solve a problem. Flow Chart – use of symbols to show the flow of code.

Errors Compile Error –Syntax Errors Run Time errors – occurs at runtime. Dividing a variable by zero. Logic errors – programs runs but the output is incorrect. Usually bad formula.

If…Then…ElseIf Statement If there are more than two alternative choices, using just If…Then…Else statement will not be enough. In order to provide more choices, we can use the If….Then…ElseIf Statement. The general structure for the if…then.. Elseif statement is If condition Then Visual Basic 2013 expression ElseIf condition Then . Else End If

Example Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim intMark As Integer Dim strGrade as String intMark = txtInput.Text If intMark >=80 Then Grade = “A” ElseIf Mark >= 60 And Mark < 80 Grade= “B” ElseIf Mark >= 40 And Mark < 60 Grade = “C” Else Grade = “D” End If End Sub

Lesson 14 Select Case Statement UNIT 4 Lesson 14 Select Case Statement

Select Case Control Structure The Select Case control structure is slightly different from the If….then…Else control structure. The difference is that the Select Case control structure basically only makes decision on one expression or dimension. On the other hand, the If…Then…Else statement may evaluate only one expression, each If statement may also compute entirely different dimensions. Select Case is preferred when there exist multiple conditions as using If…Then..ElseIf statements will become too complicated.

The Select Case…End Select Structure The structure of the Select Case control structure in is as follows: Select Case test expression Case expression list 1 Block of one or more Visual Basic 2013 statements Case expression list 2 Block of one or more Visual Basic 2013 Statements Case expression list 3 . . Case Else Block of one or more Visual Basic 2013 Statements End Select

The usage of Select Case examples Example: Examination Grades Private Sub Compute_Click( ) Dim strGrade As String strGrade = txtgrade.Text Select Case strGrade Case “A” lblDisplay.Text=”High Distinction” Case “A-” lblDisplay.Text=”Distinction” Case “B” lblDisplay.Text=”Credit” Case “C” lblDisplay.Text=”Pass” Case Else lblDisplay.Text=”Fail” End Select

Example In this example, you can use the keyword Is together with the comparison operators. Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim dblMark As Double dblMark = txtMrk.Text Select Case strMark ’Examination Marks Case Is >= 85 lblDisplay.Text= “Excellence” Case Is >= 70 lblDisplay.Text= “Good” Case Is >= 60 lblDisplay.Text = “Above Average” Case Is >= 50 lblDisplay.Text= “Average” Case Else lblDisplay.Text = “Need to work harder” End Select End Sub

Example Rewritten as follows: Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim dblMark As Double dblMark = txtInput.Text Select Case dblMark ‘Examination Marks Case 0 to 49 lblDisplay.Text = “Need to work harder” Case 50 to 59 lblDisplay.Text = “Average” s Case 60 to 69 lblDisplay.Text= “Above Average” Case 70 to 84 lblDisplay.Text = “Good” Case 85 to 100 lblDisplay.Text= “Excellence” Case Else lblDisplay.Text= “Wrong entry, please reenter the mark” End Select End Sub

Example Grades in high school are usually presented with a single capital letter such as A, B, C, D or E. The grades can be computed as follow: Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim dblMark As Double dblMark = txtInput.Text Select Case dblMark ‘Examination Marks Case 0 To 49 lblDisplay.Text = “E” Case 50 To 59 lblDisplay.Text = “D” Case 60 To 69 lblDisplay.Text = “C” Case 70 To 79 lblDisplay.Text = “B” Case 80 To 100 lblDisplay.Text = “A” Case Else lblDisplay.Text = “Error, please reenter the mark” End Select End Sub

Errors Roundoff Errors The condition of an If – Then statement should never make an equality comparison between floating points (numbers with decimals) because of the possibility of roundoff error.   Roundoff Error – occurs because floating point numbers cannot be exactly represented in binary notation by the computer. Example: Repeating decimals Logical Errors Logical Errors are program that are syntactically correct but produces the wrong answer. Logical errors are usually because of a math error or bad formula.

Programming Style Proper indentation of code is necessary for readability. All statements of an If – Then structure should be indented.

Generating Random Numbers Many programs especially games require a way to generating random numbers. In VB, we use the built-in Rnd function to generate numbers between 0 and less than 1 (not including 1). What are functions? – In VB, there are many built-in functions. A function is a type of procedure that performs a task and returns a value. A line of code will have the function name in it and when the program reaches the built-in function, the program will run the function and return the value in place of the function name. What are function arguments? Arguments is a way to pass a value to a function and have the function use the value to generate a new value that is returned to the function call in the statement. To use the Rnd function, you must type in the Randomize statement in the form_load procedure. Though it could be type in other procedure, it is better in the form_load procedure because it automatically runs when the user starts the program.

Example: Private Sub Form_Load() Randomize End Sub   Rnd function Examples: LblDisplay.text = Rnd * 10 ‘ Generates random numbers between 0 and less than 10 To generate a number in a range use the following formula. (High Number – Low Number + 1) * Rnd + Low Number Example: Generate numbers greater than or equal to 10 and less than 31 Use the following expression (30 – 10 + 1) * Rnd + 10

Int & Fix Functions Int Function - returns the integer portion of a number without rounding. Example: Int((30 – 10 + 1 ) * Rnd + 10) ‘ returns an Integer portion only   Fix Function – very similar to the Int function accepts for negative numbers. When working with negative numbers, the Int function will returns the first negative integer less than or equal to its argument (number inside the parentheses), while the Fix returns the first negative integer that is greater than or equal to its argument. Examples: Int(-5.4) -> -6 Fix(-5.4) -> -5