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Lecture Set 5

Lecture Set 5. Control Structures Part A - Decisions Structures. Objectives. You have seen almost all of this before … Use the Boolean data type in decision-making statements Use If statements and Select Case statements to make decisions

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Lecture Set 5

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  1. Lecture Set 5 Control Structures Part A - Decisions Structures

  2. Objectives • You have seen almost all of this before … • Use the Boolean data type in decision-making statements • Use If statements and Select Case statements to make decisions • Use logical operators to create complex conditional expressions • Learn how to write and evaluate conditional expressions (sometimes called Boolean expressions because they have True/False values)

  3. Introduction to Decision-making - 1 • Programming “simply involves the us of three basic control structures -- • The sequence structure • The decision structure • The repetition structure • Recursion aside, these are the only three constructs you need to build algorithms • This is it -- End of story!

  4. Introduction to Decision-making - 2 • This chapter discusses the decision-making structures provided by Visual Basic • Statements execute conditionally based on the outcome of a decision • Three kinds of decision-making or alternative path execution structures • Single alternative decision structures (if-then) • Double alternative structures (if-then-else) • Multiple alternative structures (including the select) • Much like Java and C++ - not much new under the sun

  5. Introduction to Boolean Data • Boolean data operates similarly to an on/off switch • True signifies on • False signifies off • Many properties are of the Boolean data type (can store only Boolean values) • True or False • Visible and Enabled for example

  6. Declaring a Boolean Variable • Declare a Boolean variable • Uninitialized Boolean variables have a value of False Dim Valid As Boolean • Declare and initialize a Boolean variable Dim BrowseMode As Boolean = True • Declare multiple Boolean variables Dim Test1, Test2 As Boolean

  7. Boolean Assignment Statements • The keywords True and False are used in Boolean assignment statements • Boolean expressions may also be used • Example: Dim Valid As Boolean Valid = True Valid = False

  8. Decision-making Statements • Applications need the capability to execute one group of statements in certain circumstances and other statements in other circumstances • These statements are called decision-making statements or decision structures or alternative structures • The If statement is used to make decisions

  9. Decision-making (Pseudocode) If the input date is greater than the current date then Display a message box indicating the input is invalid. End of If statement

  10. Flowchart of a Decision-making Statement

  11. If Statements and Comparison Operators • A conditional statement executes one group of statements when a condition is True and another group of statements when a condition is False • Comparison operators are used in conditional statements • Conditional operations always produce a Boolean result

  12. Comparison Operators • Equal to (=) • Not equal to (<>) • Less than (<) • Greater than (>) • Less than or equal to (<=) • Greater than or equal to (>=)

  13. Logical Operators Name Description And Returns True if both expressions are True. This operator always evaluates both expressions. Or Returns True if either expression is True. This operator always evaluates both expressions. AndAlso Returns True if both expressions are True. This operator only evaluates the second expression if necessary. OrElse Returns True if either expression is True. This operator only evaluates the second expression if necessary. Not Reverses the value of the expression.

  14. Using Comparison Operators (Example) • Example: Dim Result As Boolean Dim Value1 As Integer = 3, Value2 As Integer = 5 Result = Value1 < Value2 ' True Result = Value1 + 2 < Value2 – 1 ' False

  15. Using Comparison Operators (Example, continued) • Parentheses can clarify the order of evaluation • The following two statements are equivalent: Result = Value1 + 2 < Value2 – 1 Result = (Value1 + 2) < (Value2 – 1)

  16. Evaluating a Condition

  17. Comparison Operators and If Statements • Comparison operators are most commonly used with an If statement • A group of statements executes only when a condition is True • This form of If statement is called a one-way If statement • The statements that execute as a result of a condition are called a statement block

  18. One-Way If Statement (Syntax) • AKA “Single Alternative Decision Structures” If condition Then statements ‘a block of one or more statements End If Next sequential statement ‘(NSS) • condition must evaluate to a Boolean value • If the condition is True, statements execute • If the condition is False, statements do not execute • Execution continues at the statement following the End If (the NSS) • statements make up a statement block

  19. One-Way If Statement (Example) Dim CurrentValue As Boolean = True If CurrentValue = True Then ' Statements that execute when ' CurrentValue is True End If ' statements

  20. One-Way If Statement

  21. Comparison Operations with Dates • Yep! We are a bit ahead of ourselves. But you will get a “taste” of using the date type • Comparison operations can be performed on dates • Dates in the past are less than dates in the future • Example: Dim StartDate As DateTime = #3/22/2007# Dim EndDate As DateTime = #3/24/2007# If StartDate < EndDate = True Then EndDate = System.DateTime.Today End If

  22. Comparison Operations with Numeric Data Types • Comparison operations can be performed on numeric data • Example: Dim Value1 As Integer = 90 If Value1 < 100 Then ' Statements execute when ' Value1 is less than 100 End If ' statements

  23. Comparison Operations with Strings • Comparison operations can be performed with strings • Strings are compared character-by-character from left to right • String comparisons are performed in two ways • Case sensitive (binary comparison) • A < B < E < Z < a < b < e < z • Option Compare Binary • Case insensitive (text comparison) • (A=a) < (B=b) < (E=e) < (Z=z) • Option Compare Text

  24. String Equality Using Text and Binary Comparison (aha – something NEW)

  25. Binary and Text Comparisons (optional) • It’s all a matter of what you are used to … • If you turn on Option Compare Binary in a module you get case sensitive comparisons • If you use Option Compare Text you get case insensitive comparisons • The default setting for a new project is Option Compare Binary • You can change this setting for an entire project by going to the Project Properties dialog box • But you probably will not want to do this

  26. Introduction to Two-way If Statements • AKA “Double alternative decision structures” (if-then-else) • One statement block executes when a condition is True and another statement block executes when the condition is False • This form of If statement is commonly referred to as an If . . . Then . . . Else statement

  27. Two-way If Statements (Syntax) If condition Then statements(True) Else statements(False) End If statements • Statements(True) execute if the condition is True • Statements(False) execute if the condition is False

  28. Two-way If Statements (Example) • If Grade is greater than 75, set Pass to True. Otherwise, set Pass to False Dim Pass As Boolean Dim Grade As Integer = 80 If Grade > 75 Then Pass = True Else Pass = False End If ' statements

  29. Two-way If Statement

  30. Introduction to Multiway If Statements • AKA – “Multiple Alternative Decision Structures” • Multiway If statements have three or more possible outcomes

  31. Multiway If Statements (Syntax) If condition1 Then [statements] [ElseIf condition2 Then [elseifStatements]] [Else] [elseStatements]] End If statements

  32. Multiway If Statements (Dissection) • condition1 is first tested • If True, then the first statement block executes • Execution continues as the statement following the decision-making statement • If False, condition2 is tested and then the remaining conditions are tested • If no conditions are True, then the statements in the Else block execute • The Else block is optional

  33. Multiway If Statement

  34. Multiway If Statement (Example) Dim NumericGrade As Integer = 84 Dim LetterGrade As String If NumericGrade >= 90 Then LetterGrade = "A" ElseIf NumericGrade >= 80 Then LetterGrade = "B" ElseIf NumericGrade >= 70 Then LetterGrade = "C" ElseIf NumericGrade >= 60 Then LetterGrade = "D" Else LetterGrade = "F" End If ' statements

  35. Notes About If Statements • If statements can be written in different ways • Chose the If statement that is most readable • This decision can be subjective • The Code Editor automatically indents blocks in an If statement • The Code Editor automatically inserts the End If • If statements can be nested • One If statement can contain another If statement

  36. Introduction to Select Case Statements • Select Case statements are similar to multiway If statements • The same expression must be used in each condition • Select Case statements are faster than comparable multiway If statements • Select Case statements tend to be more readable than comparable multiway If statements

  37. Select Case Statement (Syntax) Select Case testExpression Case expressionList-1 statement-block1 [Case expressionList-2 statement-block2] [Case expressionList-n statement-blockn] [Case Else statements] End Select ' statements

  38. Select Case Statement (Dissection) • testExpression is evaluated once • Each expressionList is then tested. If True, the corresponding statement-block executes and the Select Case statement ends • Each expressionList is tested in order • When an expressionList is found to be True, the statement block executes and the Select Case statement ends • If no expessionList is True, then the statements in the Case Else block execute

  39. Select Case Statement (Example) Dim Quarter As Integer = 1 Dim QuarterString As String Select Case Quarter Case 1 QuarterString = "First" Case 2 QuarterString = "Second" Case 3 QuarterString = "Third" Case 4 QuarterString = "Fourth" Case Else QuarterString = "Error" End Select ' statements

  40. Select Case Statement

  41. Select Case Statement (Variations) • The To clause is used to test a range of values • Case 90 to 100 • The Is clause is used with comparison operators • Case is > 90 • A list of values can be created with a comma separated list • Case 1, 3, 5

  42. Logical Operators (Introduction) • Logical operators are used in conjunction with comparison and arithmetic operators • Logical operators perform the same task as a conjunction (and) or a disjunction (or) in English • The logical operators are And, Or, Not, Xor • See Table 7-2 for examples

  43. Logical Operators (Precedence) • Logical operators (Not, And, Or, Xor) have an order of precedence • Arithmetic operators are evaluated first • Comparison operators are evaluated second • Logical operators are evaluated last (of all the BINARY operators), from left to right in the following order: • Not, And, Or, Xor

  44. Logical Operators (Example) • Evaluation of an expression (be sure you know how to do these): Dim Result As Boolean Result = (3 + 4) > 6 And (4 + 1) < 6 Result = 7 > 6 And 5 < 6 Result = True And True Result = True • If I remove the parentheses in line 1, does it change the result? Why? How?

  45. Logical Operators (Example, continued) • Evaluation of an expression using And and Xor Dim Result As Boolean Result = (7 > 6) And (5 > 3) Xor (3 > 2) Result = True And True Xor True Result = True Xor True Result = False

  46. The Not Operator • The Not operator is a unary operator • Examples: • Result = Not (True) ' False • Result = Not (False) ' True • Result = Not (4 > 3) ' False

  47. Using Logical Operators • Logical operators are typically combined with comparison and arithmetic operators in decision-making statements • Example: If Input >= CurrentMin And _ Input <= CurrentMax Then Valid = True Else Valid = False End If ' statements

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