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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 isDo 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 integerCount=0Do while count<=10 ‘if count<=10 enter blockCount+=1‘ put other processing code hereLoop ‘ goes back up and does it again‘more code… display answer
Adding more functionality: Find… sum of integers 1..10
Dim sum,count as integerSum=0Count=0Do while count<=10 sum +=countCount+=1Loop‘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 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 >=0Prod=1Count=1Do while count<=N prod *=countCount++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 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
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)
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>0Answer *=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 clickPrivate 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
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
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 2s:Dim index as integer
For index =1 to 100 step 2
‘Vb stmts go here
Next
Count by -1sDim index as integer
For index =100 to 1 step -1
‘Vb stmts go here
Next index
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
Exercises: how many times do the following loops iterate?
‘Example 1Dim index,start,last as integerFor index =1 to 100 step 2Vb stmtsNext‘Example 2For index =100 to 1 step -5Vb stmtsNext ‘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 4For index =1 to 100 step -2Vb stmtsNext‘Example 5For index =101 to 100Vb stmtsNext ‘Example 6For index =1 to 100
Vb stmtsIndex=2
Next
An interest calculator
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…
NumericUpDown: set minimum, maximum, increment, thousands separator and decimal places
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
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 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
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