Advanced Iteration Control Techniques in Visual Basic

1. Control structures Part 2 iteration control To enable repetition of a statement block

2. Index of projects in this ppt • Summing integers • Factorials • Car payment calculator • Chorelist • Interest/ payments

3. loops • Syntax of the do while…loop is Do While boolean-expression Stmts Loop • While, do and loop are reserved symbols. • Use this loop to continue a processing block

4. Examples of use • Compute sums, quotients, or products of many values. • Process multiple entries in a list of data • Compute compounded interest • Approximate methods of calculus

5. Counting… Dim count as integer Count=0 Do while count<=10 ‘if count<=10 enter block Count+=1 ‘ put other processing code here Loop ‘ goes back up and does it again ‘more code… display answer

6. Adding more functionality: Find… sum of integers 1..10 Dim sum,count as integer Sum=0 Count=0 Do while count<=10 sum +=count Count+=1 Loop ‘more code… display answer

7. Putting code from previous slide into formload event and displaying in a label

8. Modify code above to compute 5! Dim prod, count As Integer prod = 1 Count = 1 Do While count <= 5 prod *= count count += 1 Loop 'more code… display answer lblanswer.Text = "factorial of 5=" & prod

9. Running the form

10. Modify code to compute N! Dim prod,i,N as integer ‘ need to check that input value N is >=0 Prod=1 Count=1 Do while count<=N prod *=count Count++ Loop ‘more code… display answer

11. The complete form

12. Error checking

13. More error checking

14. Button click code for factorial calculations Dim prod, count, N As Integer prod = 1 count = 1 Try N = Integer.Parse(txtinput.Text) If N >= 0 Then Do While count <= N prod *= count count += 1 Loop lblanswer.Text = "factorial of 5=" & prod Else lblanswer.Text = "input error" End If Catch ex As FormatException lblanswer.Text = "input error" End Try

15. Lab exercises: Compute AB and logAB • For B an integer, AB can be computed very similarly to N! except instead of building the product 1*2*3*…*N, we build the product A*A*A*…*A (B many times) • logAB is the inverse of the exponential function and, as you might guess, can be computed inversely to our previous calculation: instead of multiplications, do divisions. The count of divisions until we reach 1 is an approximate value of logAB • (Note that VB already provides these functions but for the lab we will write our own)

16. Sketch of code for one lab: AB dim B as Integer ‘ somehow get user input of A and B, check to make sure B is integer>=0 Answer=1 Do while B>0 Answer *=A B -=1 Loop

17. A Car payment calculator

18. Add listbox to form

19. Error checks and Calculations • Make sure data is entered in all fields

20. Error checks and Calculations • To calculate, you’ll need to get input values as double or decimal. Call them (for example) yrlyinterest, monthlyinterest, nummonths and amount. • Before beginning, be sure to subtract the “down payment” from the price • Divide yearly interest by 12 to get monthly interest • Start year at 2 and build a loop that counts to 5 (years) • Inside the loop call a VB function PMT(monthlyinterest,nummonths,-amount) and print results.

21. Code for button click Private Sub btnCalc_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalc.Click Dim years, months As Integer Dim monpayment, down, interest, moninterest, price As Double years = 2 lstPayments.Items.Clear() If txtRate.Text = "" Or txtDown.Text = "" Or txtAmt.Text = "" Then ‘input error lstPayments.Items.Add("You must enter values in all fields") Else price = Decimal.Parse(txtAmt.Text) interest = Decimal.Parse(txtRate.Text) down = Decimal.Parse(txtDown.Text) price -= down moninterest = interest / 12 lstPayments.Items.Add("Years" & ControlChars.Tab & "Monthly payment") Do While years <= 5 months = years * 12 monpayment = Pmt(moninterest, months, -price) lstPayments.Items.Add("" & years & ControlChars.Tab _ & monpayment.ToString("C")) years += 1 Loop End If End Sub

22. notes • I used _ to mark a continued line • I used ControlChars.Tab to tab output • I used lstPayments.Items.Clear() to clear listbox contents each time the sub is entered • I used lstPayments.Items.Add(…) to print a heading before entering the loop • I used lstPayments.Items.Add("" & years & ControlChars.Tab & monpayment.ToString("C")) to “add” a line to the list box

23. What else could you do? • Use try catch to check for number format error of input values.

24. For… next loop • Has structure For index = start to final step increment VB stmts Next index • The step increment is optional but would allow you to count by 2, 3, -1, etc. • If omitted the increment is 1.

25. For…next loop • The occurrence of the name of the index variable after the next keyword is optional. • Once entered, start, final and increment can’t be modified to alter loop function. • Once entered, it is bad coding practice to change the loop index with an assignment inside the loop.

26. examples • Count by 2s: Dim index as integer For index =1 to 100 step 2 ‘Vb stmts go here Next Count by -1s Dim index as integer For index =100 to 1 step -1 ‘Vb stmts go here Next index

27. Examples: we usually count by 1s Dim index as integer,start,last Start=23 Last =407 For index =start to last ‘Vb stmts go here Next

28. Exercises: how many times do the following loops iterate? ‘Example 1 Dim index,start,last as integer For index =1 to 100 step 2 Vb stmts Next ‘Example 2 For index =100 to 1 step -5 Vb stmts Next ‘Example 3: can you provide an expression involving start and last? For index =start to last Vb stmts Next

29. Exercises: how many times do the following loops iterate? ‘Example 4 For index =1 to 100 step -2 Vb stmts Next ‘Example 5 For index =101 to 100 Vb stmts Next ‘Example 6 For index =1 to 100 Vb stmts Index=2 Next

30. An interest calculator

31. Controls and iteration • This project uses NumericUpDown controls for data entry, a button, some labels and a listbox. • This project uses a for…next loop instead of a do while…

32. NumericUpDown: set minimum, maximum, increment, thousands separator and decimal places

33. The code for btnCalc click event handler Private Sub btnCalc_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalc.Click Dim years, count, rate As Integer Dim amt, principle, dblrate As Double Dim stroutput As String years = Integer.Parse(numyears.Text) ‘get value from numericupdown dblrate = Decimal.Parse(numrate.Text) / 100 principle = Integer.Parse(numPrinciple.Text) For count = 1 To years amt = principle * (dblrate) principle += amt ‘ we can do it without exponentiation stroutput = count.ToString() & " " & principle.ToString("C") AccountBalance.Items.Add(stroutput) Next 'end for..next End Sub

34. Exercise: Changing the interest calculator into a loan calculator

35. Project is left as an exercise • Change button function to: • Calculate this month’s interest • Add interest and subtract loan amount from principle each month • Display output only for the years, not months. (Ie. Every 12th month add to the display the current year and principle remaining) • Keep going until principle is 0. • Clear items from listbox • You might want to change for…next into do while…loop

36. A note about interest calculations • In general, the calculation of interest is (new) principle= principle*(1+cmprate)^cmpinterval • Here, cmprate and cmpinterval are the interest rate for the compounding period and the period itself. So, a principle at 5% per year, compounded monthly, for one year, would be principle=principle*(1+.05/12)^12