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Selection / Decisions

CIS 115 Lecture 7. Selection / Decisions. Control Structures. There are 3 control structures common to most computer languages that determine the flow, or path of execution, of the code: Sequential Selection / Decisions Repetition / Looping. VB Decisions. Visual Basic decision statements

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Selection / Decisions

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  1. CIS 115 Lecture 7 Selection / Decisions

  2. Control Structures There are 3 control structures common to most computer languages that determine the flow, or path of execution, of the code: • Sequential • Selection / Decisions • Repetition / Looping

  3. VB Decisions • Visual Basic decision statements • If…Then • one-way selection structure • If…Then…Else • two-way selection structure • If…Then…ElseIf • multi-way selection structure • If…Then…ElseIf…Else • multi-way selection structure • Select Case • multi-way selection structure

  4. If…Then Decision Structure • If…Then decision structure provides one choice • Evaluate the condition: True or False Ex: Is it cold outside? • True – execute code Ex: If yes, wear a coat • False – do not execute code Ex: If no, Condition True False Conditional Code

  5. If…Then Statement Syntax If condition Then statement[s] End If • Syntax explanation: • If , Then, and End – Keywords • Condition – True/False value, variable, function call or expression • Statement[s] – one or more code statements to be executed if condition is true

  6. Conditions • The execution of an If block is controlled by a condition • Must be (or evaluate to) either true or false • Can be a value, variable, or function call (Boolean DataType) • Can be formed by using the six Relational operators and the three Logical operators

  7. Boolean Variables • A flagis a Boolean variable that signals when some condition exists in the program • Since a Boolean variable is either True or False, it can be used as the condition of an If • Note that an operator is not required (there is alternate syntax that does use operator) If blnQuotaMet Then lblMessage.Text = “Congratulations you have met your sales quota" End If

  8. Boolean Functions • Boolean Functions return a single True or False Value • Since Boolean Functions return either True or False, a Boolean Function Call can be used as the condition of an If • Note that an operator is not required (there is alternate syntax that does use operator) If isNumeric(strInput) Then intNumber = Val(strInput) End If

  9. Relational Operators • Often a condition is formed using a relational operator • A relational operator determines if a specific relationship exists between two values • > Greater than • < Less than • = Equal to • <> Not equal to • >= Greater than or equal to • <= Less than or equal to

  10. Relational Operators (cont.) • Relational operators are binary – meaning they use two operands • Either or both relational operator operands may be values, variables, expressions or function calls length <= 10 (Is length less than or equal to 10) len * wid > max + 1 (Is len * wid greater than max + 1) Val(txtNum.Text) = 0 (Is Val result equal to 0 – not assignment) • Relational operators yield a True or False result

  11. Relational Operators (cont.) • Either or both relational operator operands may be expressions • Math operators are evaluated before relational operators • x+y and a-b are evaluated first • Each result is then compared using the > operator • Either or both relational operator operands may be function calls If x + y > a - b Then lblMessage.Text = "It is true!" End If If Val(txtInput.Text) < getMinValue() Then lblMessage.Text = "Invalid: Below Minimum" End If

  12. Logical Operators These operators are used to evaluate boolean values and will yield a boolean result • And • Both operands must be true for the overall expression to be true, otherwise it is false • Or • One or both operands must be true for the overall expression to be true, otherwise it is false • Xor • One operand (but not both) must be true for the overall expression to be true, otherwise it is false • Not • Reverses the logical value of an expression

  13. The And Operator The truth table for the And Operator Expression 1Expression 2Expression 1 And Expression 2 True False False False True False False FalseFalse True TrueTrue AndAlso operator works identically but does not test minutes>12 if temperature<20 is false If temperature < 20 And minutes > 12 Then lblMessage.Text = “Temperature is in the danger zone." End If

  14. The Or Operator The truth table for the Or Operator Expression 1Expression 2Expression 1 Or Expression 2 True False True False True True True TrueTrue False FalseFalse OrElse operator works identically but does not test temperature>100 if temperature<20 is true If temperature < 20 Or temperature > 100 Then lblMessage.Text = “Temperature is in the danger zone." End If

  15. The Xor Operator The truth table for the Xor Operator Expression 1Expression 2Expression 1 Xor Expression 2 True False True False True True True True False False FalseFalse If total > 1000 Xor average > 120 Then lblMessage.Text = “You may try again." End If

  16. The Not Operator The truth table for the Not Operator Expression 1Not Expression 1 True False False True If Not temperature > 100 Then lblMessage.Text = "You are below the max temp." End If

  17. Example: Checking Numerical Ranges • Checking for a value inside a range uses And • Checking for a value outside a range uses Or • Must pay careful attention to differences in resulting range using: <vs<= or >vs>= • Check problem requirements for ranges carefully If x >= 20 And x <= 40 Then lblMessage.Text = “Value is in the acceptable range." End If If x < 20 Or x > 40 Then lblMessage.Text = “Value is outside the acceptable range." End If

  18. Precedence of Logical Operators • Logical operators have an order of precedence just as arithmetic operators do • From highest to lowest precedence • Not • And • Or • Xor • As with arithmetic operations, parentheses are often used to clarify order of operations

  19. Relational & Logical Operators Combined • For example, in the statement • If x < 0 And y > 100 Or z = 50 • x < 0 And y > 100is evaluated first • If the And condition is true, we then evaluate • True Or z = 50 • If the And condition is false, we then evaluate • False Or z = 50 • If the Or condition is to be evaluated first parentheses must be used • If x < 0 And (y > 100 Or z = 50)

  20. All Operators Precedence • Parenthesis • Arithmetic • Exponential • Multiplication / Division • Integer Division • MOD • Addition / Subtraction • String Concatenation • Relational Operators (< , > , >= , <= , <>) • Logical Operators • Not • And • Or, Xor)

  21. Arithmetic, Relational, & Logical Operators Combined • Evaluate the following if a=5, b=7, x=100, y=30 • If x > a * 10 And y < b + 20 • Evaluating the math operators leaves us with • If x > 50 And y < 27 • Evaluating the relational operators leaves • If True And False • Evaluating the logical operators leaves • False • Parentheses make order of operations clear • If (x > (a * 10)) And (y < (b + 20))

  22. If…Then Examples If (intSales > 50000) Then blnGetsBonus = True End If ----------------------------------------------------------------- If ((blnGetsBonus)Or((intMissedDays < 2)And(intSales > 30000))) intDaysOff = intDaysOff + 1 intEmpRating += 1 End If ----------------------------------------------------------------- If (Not(isNumeric(txtInput.text))) Then txtInput.text = “” MsgBox(“Please enter a number in the textbox”) End If ----------------------------------------------------------------- If (intGrade >= 80)And(intGrade < 90) Then lblMessage.text = “B” ----------------------------------------------------------------- If ((Val(txtGrade.text) < 0)Or(Val(txtGrade.text) > 100)) Then lblMessage.text = “Invalid Grade: Not in the range 0-100” End If

  23. If…Then vs If…Then…Else • The If…Then…Else construct will execute one group of statements or another group (do this or do that) The If…Then construct will execute or ignore a group of statements (do something or do nothing) Condition False True Statement(s) If False Statement(s) If True Condition True False Statement(s) If True

  24. If…Then…Else Decision Structure • If…Then…Else provides two choices • Evaluate condition: True or False • True – execute code in If…Then block • False – execute code in Else Block • One of the two choices must be selected • They are mutually exclusive Condition True False Statement(s) If False Statement(s) If True

  25. If…Then…Else Syntax If condition Then statement[s]1 Else statement[s]2 End If • Syntax explanation: • If , Then, Else, and End – Keywords • Condition – True/False value, variable, function call or expression • Statement[s]1 – executed if condition is True • Statement[s]2 – executed if condition is False

  26. If…Then…Else Examples If (intSales > 50000) Then blnGetsDoubleBonus = True decBonus = 4000.00 Else decBonus = 2000.00 End If ----------------------------------------------------------------- If (Not(isNumeric(txtInput.text))) Then MsgBox(“You did not enter a valid number – program will end”) End Else intNumber = Val(txtInput.text) End If ----------------------------------------------------------------- If (intTemp >= 60)And(intTemp < 90)And(VisibRating() > 5) Then lblMessage.text = “Go - Weather conditions are ideal” Else lblMessage.text = “Wait - Weather conditions unacceptable” End If

  27. If…Then…ElseIf Decision Structure • If…Then…ElseIf allows for multiple mutually exclusive choices • Each of the conditions is tested in sequence • When a condition is true, the corresponding code is executed and the remaining conditions are ignored C1 True Statement(s)1 False True C2 Statement(s)2 False True C3 Statement(s)3 False

  28. If…Then…ElseIf Conditions If it is very cold Then Wear a coat Elseif it is chilly Wear a light jacket Elseif it is windy Wear a windbreaker Elseif it is hot Wear no jacket • The order of the conditions is vital • Wrong order can result in wrong decision • What if it’s chilly and windy? • If windy is tested before chilly, you’d go out with a windbreaker when you need a jacket

  29. If…Then…ElseIf Syntax • Syntax explanation: • If , Then, ElseIf, and End – • Keywords • Condition1 thru n – True/False • value, variable, function • call or expression • Statement[s]1 – executed if • condition1 is True • Statement[s]2 – executed if condition1 is False and if condition2 is True • Statement[s]n – executed if condition1 thru (n-1) is False and if conditionn is True If condition1 Then statement[s]1 ElseIf condition2 statement[s]2 • • • ElseIf conditionn statement[s]n End If

  30. If…Then…ElseIf Examples If radCredCrd.checked Then CredCrdPayment(decSubTot) • ElseIfradDebCrd.checked Then • DebCrdPayment(decSubTot) • ElseIfradCheck.checked Then • CheckPayment(decSubTot) End If If sngAvg < 59.5 Then lblGrade.Text = "F" ElseIf sngAvg < 69.5 Then lblGrade.Text = "D" ElseIf sngAvg < 79.5 Then lblGrade.Text = "C" ElseIf sngAvg < 89.5 Then lblGrade.Text = "B" ElseIf sngAvg <= 100 Then lblGrade.Text = "A" End If • In each example, does the order of the conditions matter? • What happens if the order is reversed in each example?

  31. If…Then…ElseIf…Else (Trailing Else) • If…Then…ElseIf… Else is simply an If…Then…ElseIf with an Else at the end • Called a Trailing Else • If the initial If and none of the ElseIf conditions are True, the trailing Else statement(s) will be executed C1 True Statement(s)1 False True C2 Statement(s)2 False True C3 Statement(s)3 False Statement(s)Else

  32. If…Then…ElseIf …Else Syntax • Syntax explanation: • Same as If..Then…ElseIf • thru statement[s]n • Statement[s]Else – • executed if condition1 thru n • (all previous conditions) are False If condition1 Then statement[s]1 • • • ElseIf conditionn statement[s]n Else statement[s]Else End If

  33. If…Then…ElseIf…Else Examples If intCredScr >= 700 Then strLoanType = “Prime” intLoanRate = 1 • ElseIfintCredScr >= 600 Then • strLoanType = “Standard” • intLoanRate = 2 • ElseIfintCredScr >= 500 Then • strLoanType = “Risk” • intLoanRate = 3 • ElseIfintCredScr >= 400 Then • strLoanType = “HiRisk” • intLoanRate = 4 Else MsgBox(“Not Qualified“) End End If If sngAvg < 59.5 Then lblGrade.Text = "F" ElseIf sngAvg < 69.5 Then lblGrade.Text = "D" ElseIf sngAvg < 79.5 Then lblGrade.Text = "C" ElseIf sngAvg < 89.5 Then lblGrade.Text = "B" ElseIf sngAvg <= 100 Then lblGrade.Text = "A“ Else lblGrade.Text = "Invalid" End If

  34. Nested If • If Statements Within If Statements • Any type of statement may be used inside the statement(s) portion of any form of If • This includes other If statements • If statements within If statements create a more complex decision structure called a Nested If

  35. Nested If Example • A customer qualifies for a special rate loan if: • If credit score is higher than 650 and • Income is more than 30000 Or Debt is less than 1000 • Or If credit score is higher than 700 If intCredScr > 650 Then If decIncome > 30000 Then lblMessage.Text = “qualified" ElseIf DecDebt < 1000 lblMessage.Text = “qualified“ Else lblMessage.Text = “not qualified“ End If ElseIf intCredScr > 700 Then lblMessage.Text = “qualified" Else lblMessage.Text = “not qualified“ End If

  36. Select Case Statement • Similar to If…Then…ElseIf • Performs a series of tests • Conditionally executes the first true condition • Select Case is different in that: • A single test expression may be evaluated • The test expression is listed once • The possible values of the expression are then listed with their conditional statements • Case Else may be included and executed if none of the values match the expression

  37. Select Case Statement Examples Select Case Val(txtInput.Text) Case 1 MsgBox("Day 1 is Monday.") Case 2 MsgBox("Day 2 is Tuesday.") Case 3 MsgBox("Day 3 is Wednesday.") Case 4 MsgBox("Day 4 is Thursday.") Case 5 MsgBox("Day 5 is Friday.") Case 6 MsgBox("Day 6 is Saturday.") Case 7 MsgBox("Day 7 is Sunday.") Case Else MsgBox("The value is invalid.") End Select Select Case strAnimal Case "Dog“,"Cat" MsgBox("House Pet") Case "Cow“,"Pig“,"Goat" MsgBox("Farm Animal") Case "Lion“,"Tiger“,"Bear" MsgBox("Oh My!") End Select Select Case intScore Case Is >= 90 strGrade = “A” Case 80 to 89 strGrade = “B” Case 70 to 79 strGrade = “C” Case 60 to 69 strGrade = “D” Case 0 to 59 strGrade = “F” End Select

  38. Example Decision Problems Write a program that will prompt the user to input a number. Check for valid input. If the input is invalid (non-numeric) – give an error message via MsgBox and end the Event Procedure. If valid – assign the number to a variable and output the number to the user Now experiment with validity checking for more restrictive input criteria with numbers (ex: only numbers from 1-100, only integers, only positive integers, etc.) and text (ex: only single characters, only the letters a-d, etc.)

  39. Example Decision Problems Write a program that inputs 2 values and displays their positive difference. For example, if the first input is 6 and the second input is 9, then the positive difference is 3 (note: 3 is still the answer if the first input is 9 and the second input is 6). Now add the code to handle invalid (non-numeric) input .

  40. Example Decision Problems Write a VB application to have the user input via textbox an integer from 1 t0 100,000 (inclusive). Determine if the input is a valid. If invalid, give an error message, clear the textbox, and end the event procedure (discuss). If valid, use a boolean function to determine if the integer is even or odd and use an integer function to determine if the integer is a perfect square (return the root if yes, return -1 if no). Report your results via label. Write a VB application to have the user input a 2 digit binary number via input box. Determine if the input is a valid 2 digit binary number. If not give a specific error message and terminate the app. If valid, convert the number to a decimal value (try using a function to do this) and report the results via message box. (try with 3 digits)

  41. Homework • Lab 6 and Homework 6 • Visual Basic – Decisions • See handout for details and due date • Questions?

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