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Chapter 6: The Repetition Structure Programming with Microsoft Visual Basic .NET, Second Edition 1 The Repetition Structure (Looping) Lesson A Objectives Code the repetition structure using the For.Next and Do.Loop statements Write pseudocode for the repetition structure Create a flowchart for the repetition structure Initialize and update counters and accumulators 2 Programming with Microsoft Visual Basic .NET, Second Edition The Repetition Structure Use the repetition structure to repeatedly process one or more program instructions until some condition is met, at which time the repetition ends The repetition structure is referred to as a loop 3 Programming with Microsoft Visual Basic .NET, Second Edition The Repetition Structure (continued) Pretest loop: evaluation occurs before the instructions within the loop are processed Posttest loop: evaluation occurs after the instructions within the loop are processed 4 Programming with Microsoft Visual Basic .NET, Second Edition The ForNext Loop Use the ForNext statement to code a loop that repeats for a specific number of times Figure 6-2: Syntax and examples of the For.Next statement 5 Programming with Microsoft Visual Basic .NET, Second Edition The ForNext Loop (continued) Figure 6-2: Syntax and examples of the For.Next statement (continued) 6 Programming with Microsoft Visual Basic .NET, Second Edition The ForNext Loop (continued) counter is a numeric variable that keeps track of how many times the loop instructions are repeated startvalue, endvalue, and stepvalue Must be numeric Can be positive or negative, integer or non-integer Default stepvalue is 1 7 Programming with Microsoft Visual Basic .NET, Second Edition The ForNext Loop (continued) Figure 6-4: Pseudocode and flowchart for the first example shown in Figure 6-2 8 Programming with Microsoft Visual Basic .NET, Second Edition The ForNext Loop (continued) Dim count As Integer For count = 0 to 3 Step 1 Debug.WriteLine(co unt) Next count Dim count As Integer For count = 3 to 0 Step - 1 Debug.WriteLine(coun t) Next count Dim count As Integer For count = 0 to 10 Step 2 Debug.WriteLine(co unt) Next count Dim loc As Single For loc = 0.5 To 15 Step 0.5 Debug.WriteLine(loc) Next loc ForNext loop examples: 9 Programming with Microsoft Visual Basic .NET, Second Edition The DoLoop Statement Unlike the ForNext statement, the DoLoop statement can be used to code both a pretest loop and a posttest loop The DoLoop statement begins with the Do clause and ends with the Loop clause 10 Programming with Microsoft Visual Basic .NET, Second Edition The DoLoop Statement (continued) Figure 6-7: Syntax and examples of the Do.Loop statement 11 Programming with Microsoft Visual Basic .NET, Second Edition The DoLoop Statement (continued) Figure 6-7: Syntax and examples of the Do.Loop statement (continued) 12 Programming with Microsoft Visual Basic .NET, Second Edition The DoLoop Statement (continued) Figure 6-9: Flowcharts for the examples shown in Figure 6-7 13 Programming with Microsoft Visual Basic .NET, Second Edition The DoLoop Statement (continued) Figure 6-9: Flowcharts for the examples shown in Figure 6-7 (continued) 14 Programming with Microsoft Visual Basic .NET, Second Edition Using Counters and Accumulators Counters and accumulators are used within a repetition structure to calculate subtotals, totals, and averages A counter is a numeric variable used for counting something and is typically updated by 1 An accumulator is a numeric variable used for accumulating and is updated by an amount that varies 15 Programming with Microsoft Visual Basic .NET, Second Edition Using Counters and Accumulators (continued) Initializing: assigning a beginning value to the counter or accumulator Updating (incrementing): adding a number to the value stored in the counter or accumulator 16 Programming with Microsoft Visual Basic .NET, Second Edition Nested Repetition Structures Lesson B Objectives Nest repetition structures 17 Programming with Microsoft Visual Basic .NET, Second Edition Nesting Repetition Structures In a nested repetition structure, one loop, referred to as the inner loop, is placed entirely within another loop, called the outer loop A clock uses nested loops to keep track of the time 18 Programming with Microsoft Visual Basic .NET, Second Edition Nesting Repetition Structures (continued) Figure 6-16: Nested loops used by a clock 19 Programming with Microsoft Visual Basic .NET, Second Edition The Grade Calculator Application Professor Arkins needs an application that allows him to assign a grade to any number of students Each students grade is based on three test scores, with each test worth 100 points The application should total the test scores and then assign the appropriate grade, using the table shown on the next slide 20 Programming with Microsoft Visual Basic .NET, Second Edition The Grade Calculator Application (continued) Total points earnedGrade 270300A 240269B 210239C 180209D below 180F 21 Programming with Microsoft Visual Basic .NET, Second Edition The Grade Calculator Application (continued) uiAssignGradeButtons Click event procedure Allows Professor Arkins to enter each students test scores, and then assign the appropriate grade Contains two loops, one nested within the other A For.Next statement controls the inner loop 22 Programming with Microsoft Visual Basic .NET, Second Edition The Grade Calculator Application (continued) uiAssignGradeButtons Click event procedure (continued) A Do.Loop statement controls the outer loop The inner loop is a pretest loop The outer loop is a posttest loop 23 Programming with Microsoft Visual Basic .NET, Second Edition The Grade Calculator Application (continued) Figure 6-20: Sample run of the application that contains the procedure 24 Programming with Microsoft Visual Basic .NET, Second Edition Coding the Shoppers Haven Application Lesson C Objectives Select the existing text in a text box Prevent a form from closing 25 Programming with Microsoft Visual Basic .NET, Second Edition Shoppers Haven The manager of Shoppers Haven wants an application that the store clerks can use to calculate the discounted price of an item, using discount rates from 10% through 30% in increments of 5% The clerks will enter the items original price The application should display the discount rates and the discounted prices in the interface 26 Programming with Microsoft Visual Basic .NET, Second Edition Shoppers Haven (continued) Figure 6-21: User interface for the Shoppers Haven application 27 Programming with Microsoft Visual Basic .NET, Second Edition Shoppers Haven (continued) Figure 6-22: TOE chart for the Shoppers Haven application 28 Programming with Microsoft Visual Basic .NET, Second Edition Shoppers Haven (continued) Figure 6-23: Pseudocode for the Calculate buttons Click event procedure 29 Programming with Microsoft Visual Basic .NET, Second Edition Shoppers Haven (continued) Figure 6-27: Discounted prices shown in the Shoppers Haven application 30 Programming with Microsoft Visual Basic .NET, Second Edition Selecting the Existing Text in a Text Box Use the SelectAll method to select all of the text contained in a text box Syntax: textbox.SelectAll() textbox is the name of the text box whose text you want to select 31 Programming with Microsoft Visual Basic .NET, Second Edition Selecting the Existing Text in a Text Box (continued) Enter the SelectAll method in a text box controls Enter event A text box controls Enter event occurs when the user tabs to the control, and when the Focus method is used in code to send the focus to the control The uiOriginalTextBox controls Enter event is responsible for highlighting the existing text in the control 32 Programming with Microsoft Visual Basic .NET, Second Edition Selecting the Existing Text in a Text Box (continued) Figure 6-29: Text selected in the Shoppers Haven application 33 Programming with Microsoft Visual Basic .NET, Second Edition Coding the TextChanged Event Procedure A controls TextChanged event occurs when the contents of a controls Text property change Use the uiOriginalTextBoxs TextChanged event to clear the contents of the uiDiscPricesLabel when the user changes the original price 34 Programming with Microsoft Visual Basic .NET, Second Edition Coding the ShoppersForms Closing Event Procedure A forms Closing event occurs when a form is about to be closed In the Shoppers Haven application, the Closing event procedure is responsible for: Verifying that the user wants to exit the application Taking an action based on the users response 35 Programming with Microsoft Visual Basic .NET, Second Edition Coding the ShoppersForms Closing Event Procedure (continued) Figure 6-31: Pseudocode for the ShoppersForms Closing event procedure 36 Programming with Microsoft Visual Basic .NET, Second Edition Coding the ShoppersForms Closing Event Procedure (continued) Figure 6-33: Message box displayed by the forms Closing event 37 Programming with Microsoft Visual Basic .NET, Second Edition Summary Repetition structure (loop): the co