Control structures Part 2 iteration control To enable

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

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

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

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

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

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

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

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

Running the form

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

The complete form

Error checking

More error checking

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 += 1 Loop lblanswer. Text = "factorial of 5=" & prod Else lblanswer. Text = "input error" End If Catch ex As Format. Exception lblanswer. Text = "input error" End Try

Lab exercises: Compute AB and log. AB • 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) • log. AB 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 log. AB • (Note that VB already provides these functions but for the lab we will write our own)

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

A Car payment calculator

Add listbox to form

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

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.

Code for button click Private Sub btn. Calc_Click(By. Val sender As System. Object, By. Val e As System. Event. Args) Handles btn. Calc. Click Dim years, months As Integer Dim monpayment, down, interest, moninterest, price As Double years = 2 lst. Payments. Items. Clear() If txt. Rate. Text = "" Or txt. Down. Text = "" Or txt. Amt. Text = "" Then ‘input error lst. Payments. Items. Add("You must enter values in all fields") Else price = Decimal. Parse(txt. Amt. Text) interest = Decimal. Parse(txt. Rate. Text) down = Decimal. Parse(txt. Down. Text) price -= down moninterest = interest / 12 lst. Payments. Items. Add("Years" & Control. Chars. Tab & "Monthly payment") Do While years <= 5 months = years * 12 monpayment = Pmt(moninterest, months, -price) lst. Payments. Items. Add("" & years & Control. Chars. Tab _ & monpayment. To. String("C")) years += 1 Loop End If End Sub

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

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

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.

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.

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

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

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

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

An interest calculator

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

Numeric. Up. Down: set minimum, maximum, increment, thousands separator and decimal places

The code for btn. Calc click event handler Private Sub btn. Calc_Click(By. Val sender As System. Object, By. Val e As System. Event. Args) Handles btn. Calc. 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(num. Principle. Text) For count = 1 To years amt = principle * (dblrate) principle += amt ‘ we can do it without exponentiation stroutput = count. To. String() & " " & principle. To. String("C") Account. Balance. Items. Add(stroutput) Next 'end for. . next End Sub

Exercise: Changing the interest calculator into a loan calculator

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 12 th 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

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