Car Payment Calculator - PowerPoint

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Car Payment Calculator - PowerPoint Powered By Docstoc
					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
      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
             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>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 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 -1s
Dim 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 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 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

				
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