Question about code

Question

I'm designing a program for a beginner VB college course and it's supposed to compile miles driven, gallons used and MPG for both a single leg  of a trip and for the entire trip.  When the button is pressed to calculate the MPG, the miles and gallons are put in their respective boxes below where a running tally is shown and the MPG is shown in it's own box for that single input.  A totals box is below that where, as the inputs are entered, shows a total mileage, gallons, and MPG usage for the entire trip.  The problem I am having is that the miles/gallons/mpgListBox.Items entry (used to compile the list in their respective boxes) is showing as an error in the VB 2010 Express Error List as " 'Items' is not a member of 'System.Windows.Forms.TextBox'.    I'm following the examples in the book and I'm still getting these errors.  Additionally, I'm getting another error for a text entry in the event that 0 is entered for gallons that is supposed to display "Gallons must be greater than zero."

Here is the complete code that I have. 

' MPG Average

Public Class MPGAverage
   ' The mpg average calculated individually and overall based on miles and fuel
   Private Sub calculateMPGButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles calculateMPGButton.Click

      ' User enters mileage
      If milesDrivenTextBox.Text <> String.Empty Then
         ' Add mileage to end of milesListBox
         milesListBox.Items.Add(milesDrivenTextBox.Text)
         milesDrivenTextBox.Clear()  ' Clears milesDrivenTextBox.Text
      End If

      ' User enters gas useage in gallons
      If gallonsUsedTextBox.Text <> String.Empty Then
         ' Add fuel used to end of gallonsListBox
         gallonsListBox.Items.Add(gallonsUsedTextBox.Text)
         gallonsUsedTextBox.Clear()  ' Clears gallonsUsedTextBox.Text
   Else gallonsUsedTextBox.Text = 0 Then gallonsListBox.Text = "Gallons must be greater than 0"

      End If

      Dim miles As Double ' Miles input
      Dim totalMiles As Double ' Total miles driven
      Dim fuel As Double ' Fuel input
      Dim totalFuel As Double ' Total fuel used
      Dim mpg As Double ' MPG calculation
      Dim totalMPG As Double ' Total MPG calculation

      ' Initialization
      miles = 0
      fuel = 0
      totalMiles = 0
      totalFuel = 0

      'Processing
      Do While totalMiles < milesListBox.Items.Count
         miles = milesListBox.Items(totalMiles) ' get next mileage input
         totalMiles += miles
      Loop

      Do While totalFuel < gallonsListBox.Items.Count
         fuel = gallonsListBox.Items(totalFuel) ' get next fuel input
         totalFuel += fuel
      Loop

      ' Termination
    If mpg <> 0 Then mpg = miles / fuel mpgListBox.Text ' Calculating mpg on single entries

      End If

      ' Displaying total miles, fuel and mpg
      totalResultsLabel.Text = "Total miles driven: " & totalMiles & vbCrLf & "Total gallons used: " & totalFuel & vbCrLf & "Total MPG: " & String.Format("{0:F}", totalMPG)

   End Sub

End Class

 

Thank for any help you can give on this.

V/R,

Brian

Answer 1

The first error you are getting indicates that you have added a textbox instead of a listbox when adding milesListBox to the form.

Re the second problem:  an If statement can have the following elements

If some condition Then
    DoSomething
Elseif some other condition
    Do Something Else
Else
    Something to do if neither of the above conditions is met
End If

The ElseIf and Else statements are optional but note that the Else cannot not specify a condition to be met.  You should be using an ElseIf

Answer 2

Thank you Dave, I can't believe that overlooked something as simple as that but that took care of the majority of the problems.  I still don't see how this statement is incorrect though:

 

If mpg <> 0 Then mpg = miles / fuel mpgListBox.Text ' Single entry MPG calculation
      End If

 

The way I understand this is answer of miles / fuel will be placed in the mpgListBox as text for every entry of miles and gallons. 

 

Would you, or anyone else be able to explain it a bit better? 

 

Thanks

Answer 3

If you are putting the statement to be executed on the same line as the If condition Then you don't use the End If

either

If somecondition Then dosomething

or

If somecondition Then
    dosomething
End If

 

Answer 4

Thanks Dave, it makes a lot more sense now.  Since I'm at work, I'll have to give this a try in the morning when I get off.  I appreciate all your help.

 

Brian

Answer 5

I was able to clear out all errors but now, when I enter in the miles and gallons used, it will populate in their respective single entry boxes, but no MPG.  Additionally, only the first entry will populate the bottom box which is supposed to display the total miles, gallons, and mpg.  I've researched the book and some online help forums and can't seem to figure out where I'm screwing up at.  Here is the completed code so far:

' MPG Average

Public Class MPGAverage
   ' The mpg average calculated individually and overall based on miles and fuel
   Private Sub calculateMPGButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles calculateMPGButton.Click

      ' User enters mileage
      If milesDrivenTextBox.Text <> String.Empty Then
         ' Add mileage to end of milesListBox
         milesListBox.Items.Add(milesDrivenTextBox.Text)
         milesDrivenTextBox.Clear()  ' Clears milesDrivenTextBox.Text
      End If

      ' User enters gas useage in gallons
      If gallonsUsedTextBox.Text <> String.Empty Then
         ' Add fuel used to end of gallonsListBox
         gallonsListBox.Items.Add(gallonsUsedTextBox.Text)
         gallonsUsedTextBox.Clear()  ' Clears gallonsUsedTextBox.Text
      End If

      Dim miles As Decimal ' Miles input
      Dim totalMiles As Decimal ' Total miles driven
      Dim fuel As Decimal ' Fuel input
      Dim totalFuel As Decimal ' Total fuel used
      Dim mpg As Decimal ' MPG calculation
      Dim totalMPG As Decimal ' Total MPG calculation

      ' Initialization
      miles = 0
      fuel = 0
      totalMiles = 0
      totalFuel = 0

      'Processing
      Do While totalMiles < milesListBox.Items.Count
         miles = milesListBox.Items(totalMiles) ' get next mileage input
         totalMiles += miles
      Loop

      Do While totalFuel < gallonsListBox.Items.Count
         fuel = gallonsListBox.Items(totalFuel) ' get next fuel input
         totalFuel += fuel
      Loop

      ' Termination
      If mpg <> 0 Then mpg = miles / gallonsListBox.Text ' Single entry MPG calculation


      ' Displaying total miles, fuel and mpg
      totalResultsLabel.Text = "Total miles driven: " & totalMiles & vbCrLf & "Total gallons used: " & totalFuel & vbCrLf & "Total MPG: " & String.Format("{0:F}", totalMPG)

   End Sub
End Class

 

Thanks for any help you can give on this. 

Brian

Answer 6

Well you are displaying totalMPG but you have no code to calculate what it is so the display is correct.

BTW - before you go any further you ought to turn Option Strict On and correct all the errors you then see.

There are also numerous other errors in your code - did this really come from a book?

Answer 7

HI Brian,

 Your if statement is in the correct format but your logic is incorrect. There is two things wrong with the logic in this statement.

1. You are testing the MPG for 0 and it will always be 0 since there is nothing in your application to change this value before making the test. Thus the if statement will never be true. Try using the ( miles <> 0 and CDbl( gallonsListbox.text) <> 0) in the if statement instead of MPG <> 0. This will test both numbers to make sure they are not zero before making the calculation.

2. You are not converting the gallonsListBox.Text to a double so that it can be used in a calculation. Any listbox or textbox is a string even though it contains a number. Strings contain charectors that represent numbers and cannot be used in calculations without first being converted. 

Hope this helps you

Curtis

---

Always Lost in Code, Always mark answers as correct if they answer you question and solve your problem. This way others when searching for similar problems can find the answer faster.

Answer 8

Just to elaborate a little on "numerous other errors"

Have a look at the code in your first loop:

Do While totalMiles < milesListBox.Items.Count
     miles = milesListBox.Items(totalMiles)
     totalMiles += miles
Loop

What you are trying to do here is add together all the mileages you have added to the listbox.

The test for the loop says "do this until the totalmiles is greater than the number of items in the listbox" which is not what you want.

The first time round you set miles equal to the value of the first item in the listbox.

You then add that value to totalmiles which will then almost certainly exceed the number of items in the box so the loop will only execute once. 

What you should have here is something like:

For Count as Integer = 0 to milesListBox.Items.Count - 1
    totalmiles += CDec(milesListBox.Items(Count))
Next

or you could use a For Each loop

For Each Item as String in milesListBox.Items
    totalmiles += cdec(Item)
Next

 

Answer 9

Curtis and Dave,

Since I'm at work right now I don't have access to VB (government computer) I will add the changes to the current code I have here and test it later this morning when I get off work.  I will be sure to post the results.  Thank you both for all your help, I sincerely appreciate it.

Brian

Answer 10

Ok, I made some changes, and this is what I have.  It will now list all items (miles, gallons and MPG) in their respective boxes with the correct math for the MPG.  The bad part is that now in the bottom box where everything should be compiled together, the math that is being done is number of items * current entry for both totalMiles and totalFuel.  The MPG that shows is whatever the value is for the current entry.  I've looked all through the book I have for my class and the examples they show match up with what I have in the code.  Here is the code I currently have:

' MPG Average
Public Class MPGAverage

   ' The mpg average calculated individually and overall based on miles and fuel
   Private Sub calculateMPGButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles calculateMPGButton.Click

      Dim miles As Integer ' Miles input
      Dim totalMiles As Integer ' Total miles driven
      Dim fuel As Integer ' Fuel input
      Dim totalFuel As Integer ' Total fuel used
      Dim mpg As Integer ' MPG calculation
      Dim totalMPG As Integer ' Total MPG calculation

      ' User enters mileage
      If milesDrivenTextBox.Text <> String.Empty Then
         ' Add mileage to end of milesListBox
         milesListBox.Items.Add(milesDrivenTextBox.Text)
         miles = milesDrivenTextBox.Text
         milesDrivenTextBox.Clear()  ' Clears milesDrivenTextBox.Text
      End If

      ' User enters gas useage in gallons
      If gallonsUsedTextBox.Text <> String.Empty Then
         ' Add fuel used to end of gallonsListBox
         gallonsListBox.Items.Add(gallonsUsedTextBox.Text)
         fuel = gallonsUsedTextBox.Text
         gallonsUsedTextBox.Clear()  ' Clears gallonsUsedTextBox.Text
      End If

      'Processing
      For Each Item As String In milesListBox.Items
         totalMiles += CDec(miles)
      Next

      For Each Item As String In gallonsListBox.Items
         totalFuel += CDec(fuel)
      Next

      ' calculate mpg and totalmpg
      mpg = miles / fuel
      totalMPG = totalMiles / totalFuel

      ' Displaying total miles, fuel and mpg
      mpgListBox.Items.Add(mpg) ' displays mpg in the listbox output

      totalResultsLabel.Text = "Total miles driven: " & totalMiles & vbCrLf & "Total gallons used: " & totalFuel & vbCrLf & "Total MPG: " & String.Format("{0:F}", totalMPG)
      ' Now that totalMPG is calculated above it will be output in your ResultsLabel.Text
   End Sub
End Class

 

Thanks for your help.

Answer 11

 

  Private Sub calculateMPGButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles calculateMPGButton.Click

    Dim miles As Integer
    Dim totalMiles As Integer '<== Wrong
    Dim fuel As Integer
    Dim totalFuel As Integer '<== Wrong
    Dim mpg As Integer '<== Wrong
    Dim totalMPG As Integer '<== Wrong

    If milesDrivenTextBox.Text <> String.Empty Then

      milesListBox.Items.Add(milesDrivenTextBox.Text)
      miles = milesDrivenTextBox.Text '<== Wrong
      milesDrivenTextBox.Clear()
    End If

    If gallonsUsedTextBox.Text <> String.Empty Then

      gallonsListBox.Items.Add(gallonsUsedTextBox.Text)
      fuel = gallonsUsedTextBox.Text '<== Wrong
      gallonsUsedTextBox.Clear()
    End If

    For Each Item As String In milesListBox.Items
      totalMiles += CDec(miles) '<== Wrong
    Next

    For Each Item As String In gallonsListBox.Items
      totalFuel += CDec(fuel) '<== Wrong
    Next

    mpg = miles / fuel
    totalMPG = totalMiles / totalFuel
    mpgListBox.Items.Add(mpg)
    totalResultsLabel.Text = "Total miles driven: " & totalMiles & vbCrLf & "Total gallons used: " & totalFuel & vbCrLf & "Total MPG: " & String.Format("{0:F}", totalMPG)

  End Sub

Answer 12

Why have you changed all your variables from Decimal to Integer. 

Why didn't you take my advice to turn Option Strict On.

VB is now doing implicit narrowing conversions for you which you are totally unaware of and which can give you wildly inaccurate results.

Try entering 4.5 as the mileage and 1.5 as the fuel.  Obviously the correct result is 3 mpg but your code will tell you it is 2 mpg.  That is because VB is doing implicit conversions of the numbers you entered from Double to Integer.  As the conversion uses Bankers rounding the 4.5 gets rounded to 4 and the 1.5 gets rounded to 2.  If you had Option Strict On you would have seen this problem before running the program.

I actually gave you the code for summing all the entries in the listbox:

    For Each Item As String In gallonsListBox.Items
        totalFuel += CDec(Item)
    Next

So why did you change it to:

    For Each Item As String In gallonsListBox.Items
        totalFuel += CDec(fuel)
    Next

There's little point offering advice if you are going to ignore it.