ISAT 252 Visual Basic Repetition

Assignment Should have read Chapter 5 ( ) on loops Do tutorial 5-4

Objectives Recognize and correctly interpret conditions using both relational and logical operators Write VB code that uses loop structures For/Next Do/Loops Use nested loops correctly Recognize when to use each type of loop

Fundamental Program Structures SequenceSequence Step by step execution of statements SelectionSelection Determine whether or not to execute a statement(s) based on certain condition If –Statements, Case Statements Iteration Repeatedly execute one or several statements based on certain condition Do-Loops, For-Loops All programs are combinations of these three structures.

Loops The process of repeating a series of statements is called looping Loop statements allow us to execute a statement multiple times The group of repeated statements is called the body of the loop An iteration is a single execution of the statements in the loop All loops must have a mechanism to control the number of iterations

Control User controlled loops – occur when the user repeatedly triggers events, such as when entering new data typical in event-driven programs, but not in programs using older languages number of iterations determined by the user Program controlled loops – loop iterations are controlled by the programmer

Types of program control loops Use a control mechanism For / Next Loops counter controlled uses a loop index to count and control the number of iterations changes the loop index automatically Do / Loop condition controlled tests a condition to determine whether to continue looping YOU must change the loop index value Important: You must choose the right kind of loop for the situation

For / Next Loops Syntax: For loopIndexVar = initial value to final value [Step increment] body of loop Next loop index [ ] means optional used when the number of iterations can be determined BEFORE beginning the loop loopIndexVar must be a numeric variable initial value may be any numeric variable, expression, or constant final value may be any numeric variable, expression, or constant loopIndexVar is automatically incremented in each iteration by 1 or by the Step increment if it is there

For Loop Action Statement(s) True loopIndex condition False increment Initialization of loopIndex

Example 1 For intLoopIndex = 1 to 5 intSquare = intLoopIndex * intLoopIndex txtOutput.Text += “x = “ + intLoopIndex.ToString() + Space(5) + _ “x ^ 2 = “ + intSquare.ToString() + vbNewLine Next intLoopIndex output: x = 1 x ^ 2 = 1 x = 2 x ^ 2 = 4 x = 3 x ^ 2 = 9 x = 4 x ^ 2 = 16 x = 5 x ^ 2 = 25 Remember txtOutput.Text += means txtOutput.Text = txtOutput.Text +… What is the initial value?What is the final value?

Employees of Kilobytes, Inc. get a salary increase of 10% every year. Display out the salary for the next 4 years of an employee currently making $20,000. sngSalary = sngIncrease = 0.1 For intIndex = 1 to 4 sngSalary = sngSalary + sngSalary * sngIncrease txtOutput.Text += “Year: “ + intIndex.ToString() + “ Salary: “ + _ sngSalary.ToString(“c”) + vbNewLine Next intIndex output: Year: 1 Salary: $ Year: 2 Salary: Example 2

Increments Default value is 1 Can be changed using the Step Examples – How many times will loop execute? For intIndex = 3 to 6 For intIndex = 3 to 10 Step 2 For sngIndex = 1 to 2 Step 0.1 For intIndex = 10 to 1 Step -1 For intIndex = 10 to 1 Step -2 DO NOT change the value of the loop index, initial value, test value, or increment in the body of the loop

Do-While/ Do-Loops Do {While | Until} Condition loop body Loop Pretest Loop Posttest Loop Do loop body Loop {While | Until} Condition

Do-While, Do-Until : Action statement(s) True condition False do while loop statement(s) False condition True do until loop Loop body executes Until the condition is TRUE Loop body executes While the condition is TRUE Both use a pre-test: Test the condition before executing the body of the loop with a pretest, loop may not execute at all

Do-Loop-While, Do-Loop-Until: Action Both use a post-test: Executes the body of the loop, then test the condition The body of the loop is always executed at least once!! True condition evaluated statement False do loop while False condition evaluated statement True do loop until Loop body executes Until the condition is TRUE Loop body executes While the condition is TRUE

Example Conditions computed values Do Until sngTotalCost >=10000 Do While sngTotalCost < input values Do While intAge < 35 (intAge is input data) Do Until intAge >= 35 end of file or sentinel markers Do Until strUserName = “The End” Do While strUserName <> “The End” Equivalent !!

Example 3: Credit Card payments sngBalance = …‘give initial value to loopIndex before loop starts Do While sngBalance >= sngPayment intPaymentNumber = intPaymentNumber + 1 sngInterest = sngBalance * sngRate sngTotalInterest = sngTotalInterest + sngInterest sngBalance = sngBalance - sngPayment + sngInterest txtOutput.Text += “Payment Number: “ + _ intPaymentNumber.ToString() + Space(4) + _ “New Balance = “ + sngBalance.ToString() + Space(4) _ “Total Interest = “+sngTotalInterest ToString() + vbNewLine Loop

Output: Assume: sngBalance = 100, sngRate = 2% per month intPayment = 20 Payment Number: 1 New Balance = 82 Total Interest = 2 Payment Number: 2 New Balance = Total Interest = 3.64 Payment Number: 3 New Balance = Total Interest = 4.91 Payment Number: 4 New Balance = Total Interest = 5.81 Payment Number: 5 New Balance = 6.33 Total Interest = 6.32

Another Example compute average of grades Initialize total to zero Initialize counter to zero Prompt for the first grade Input the first grade (possibly the sentinel) While the user has not yet entered the sentinel Add this grade to the total Add one to the grade counter Prompt for the next grade Input the next grade End While If the counter is not equal zero Set the average to the total divide by counter Else Display “ No grades were entered” The user may not want to enter any grades. Therefore, we need to check the sentinel after the first input is entered before processing the grades.

The sentinel is -1

Sample Code Private Sub btnEnterGrades_Click() handles btnGrades.click Dim intTotal As Integer 'Sum of all grades inputs Dim intCounter As Integer 'Number of grades input Dim intGrade As Integer 'Current Input Grade Dim sngAverage As Single 'Floating point average Dim strInputMessage As String 'Text displayed in Input Box Dim strGrade As Integer 'Current Input Grade 'Initialization intTotal = 0 intCounter = 0 strInputMessage = "Enter grades: -1 to end" 'Processing Phase intGrade = Cint(InputBox(strInputMessage, "Collect Grades“)) 'Loop until intGrade has a value of -1 Do Until intGrade = -1 intTotal = intTotal + intGrade'Add intGrade to intTotal intCounter = intCounter + 1'Add 1 to intCounter ' Input a new grade ' If the user enter the sentinel, the loop condition ' becomes True intGrade = CInt(InputBox(strInputMessage, "Collect Grades“)) Loop ‘ Termination Phase If intCounter > 0 Then sngAverage = intTotal / intCounter lblAverage.Text = "The class average is " + _ sngAverage.ToString(“f”) Else lblAverage.Text = "No grades were entered" End If End Sub Private Sub btnExit_Click() handles cmdExit.click End End Sub

One more Example sngTotal = 0 Do While sngTotal < 25 sngNum = CSng(Inputbox(“Enter a number”)) If sngNum > 0 then sngTotal = sngTotal + sngNum^2 End If Loop

Nested Loops Can have loops within loops Any combination of types Any level of nesting Do Do While Loop For J Next J Loop Until Important to indent code properly!!

Example 1 Analyze data collected by a temperature sensor which takes readings every hour for one week sngTotal=0.0 For intDayNumber = 1 to 7 For intHourNumber = 1 to 24 (process data for day = intDayNumber and hour = intHourNumber) textOutput.text += “Temperature Day “ + intDayNumber.ToString() + _ “Hour:” + intHourNumber.ToString() + _ sCurrentSensorTemperature.ToString() + vbNewLine sngTotal=sngTotal + sngCurrentSensorTemperature Next intHourNumber Next intDayNumber sngAverage= sngTotal/(7*24) textOutput.text += “Average Temperature:” + sngAverage.ToString()

Generalization of example 1 Given a function of 2 independent integer variables: z=f(x, y) Compute z for values of x and y within a certain range. Outer loop: change X Inner loop: change Y Problem: = Write a VB function that receives as parameters 2 positive whole numbers, x and y, and computes F(x,y)=  i *  (j-1)*j What happens if z is a function of 3 variables? 0  i  x1jy1jy

For intI = 1 To 4 txtOutput.Text += "I = “ + intI + vbNewLine For intJ = 1 To 3 txtOutput.Text += intI.ToString() + "* " + intJ.toString() + _ " = " +(intI * intJ).toString() + vbNewLine Next intJ Next intI

Program Flow Control: Repetition Do/Loop For Loopindex = InitialValue to TestValue [Step Increment] (Body of loop) Next [LoopIndex] Pretest: Do {While | Until} Condition (Body of loop) Loop Posttest: Do (Body of loop) Loop {While | Until} Condition For/Next

When do you use each kind of loop? For/Next If you know how many times you want to go through the loop Do/Loop posttest If you don’t know how many times you want to go through the loop AND you know you want to go through the loop at least once Do/Loop pretest – If you don’t know how many times you want to go through the loop AND you want the option of skipping the loop altogether

Remember: Software Development Process Analyze the problem Analyze the problem determine inputs and outputs Design the solution Design the solution Identify task, sub-tasks develop algorithms for each task and subtask Program the solution Program the solution VB steps: create form, set properties, write code Test and correct the solution Test and correct the solution

Sample Problem: Gathering Data Cheap Mart Inc., a local discount store, would like to be able to gather and analyze their sales data. The store is open daily from noon and again from pm. Write a VB program that allows the user to enter the data for any number of days, and display out the daily and grand totals.

Step 1: Analyze the problem Determine inputs: number of days sales data for each day, morning and afternoon Determine outputs: sales data for each day daily and grand totals

Step 2: Design the solution Task Cheap Market Inc. Sales Report Subtasks: Display table headings Read in number of days Read in sales input Display Totals yes Start Display Table headings Read in #days Read in Sales input Display totals Stop Finished? no

Step 2: Design the solution Task: Cheap Market Inc. Sales Report Pseudocode: Initialize totals to zero Display table headings Read in the number of sales days For each day For each time period in a day Read in sales data Display sales data Add to daily total End For Display daily total Add daily total to grand total End For Display grand total How to validate?

Step 2: Design the solution Task: Read in Sales Data Pseudocode: Do Prompt a message Read Input Data If the data entered is not numeric then display a message indicating the error to the user Else If the data entered is a number less or equal than zero display a message indicating the error to the user Until input data is correct

Step 3: Write the application Steps in creating a VB application: 1. Create the form(s) Place the controls on the form 2. Set properties for each the controls Use the naming conventions! 3. Write the code Make code readable - naming conventions, comments, spacing and indentation, etc. Use proper scope (most should be local) Use forms to implement main tasks Use sub-procedures and functions to program subtasks.

More Controls

To display a large number of choices use a combo box or a list box The options are specified in the Items property at design time Combo Box and List Box Combo Box A combo box displays a list of options Simple Dropdown The currently selected value is displayed. When a combo box is clicked on, all of its possible values are displayed and can be selected from. List Box A list box displays a list of options The currently selected value is highlighted. a scroll bar will appear on the right side of the list box allowing the user to display the remainder of the list.

Combo Boxes and List Boxes Items can be specified at design time (Item property) during run time with code Code can access items add items to list of items delete items from list of items

ListBox/Combo Box Items Every item in the Items collection is identified by an Index The first item in the Items collection: 0 The second item : 1 And so on First Virginia Bank Suntrust Bank Citibank Farmers & Merchant Bank List Box /Combo Box Items Property SelectIndex SelectIndex property contains the index of the currently selected item SelectItem SelectItem property contains the currently selected item Items property is the collection of items in the combo box/List box Note: List/Combo Boxes start with 0!

Manipulate List with Code Set first item to be selected in Form Load lbBank.selectedIndex = 3 cbBank.selectedIndex=3 Remove an item specified by its index on the Item List lbBank.Items.removeAt(itemIndex) cbBank.Items.removeAt(itemIndex) Add items to list Append the item to the list cbBank.Items.add(“Chase Manhattan”) lbBank.Items.add(“Chase Manhattan”) To retrieve the current selectedItem strBank=cbBank.SelectedItem strBank=lbBank.SelectedItem

For Next Time: Do Loops Activity Do Lab 9 Answer the Worksheet