NullReferenceException was unhandled VB.net Structure with Array - arrays

Public Class Form1
Structure Crap
Dim CrapA As Integer
Dim CrapB As Single
Dim CrapC() As Long
Private Sub Initialize()
ReDim CrapC(0 To 100)
End Sub
End Structure
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim Stuff(0 To 2) As Crap
Stuff(0).CrapA = 16
Stuff(1).CrapA = 32
Stuff(2).CrapA = 64
'This is the line where the NullReferenceException is thrown
Stuff(0).CrapC.Initialize()
For x = 0 To 100
Stuff(0).CrapC(x) = x ^ 2
Next x
MsgBox(Stuff(0).CrapA)
MsgBox(Stuff(1).CrapA)
MsgBox(Stuff(2).CrapA)
For x = 0 To 100
MsgBox(Stuff(0).CrapC(x))
Next x
End Sub
End Class
So this is a pretty simple program with a baffling error. All I want is an array in my user defined type. I'm moving from VB6 to .net and everything I've read, including What is a NullReferenceException, and how do I fix it? which is a hearty read, doesn't help. Seems I'm stuck in 1999, lol. I understand that the array CrapC is null, it's making it not null that's the problem.

The error happens here:
Stuff(0).CrapC.Initialize()
because the Crap() contains three instances of Crap but you never initialize the CrapC field which is a Long(). So you can call Initialize on Nothing(null). In most cases you don't need this method anyway: "This method is designed to help compilers support value-type arrays; most users do not need this method."
Instead use this array initializer to get a Long() with 100 longs:
Stuff(0).CrapC = New Long(99) {}

Related

Populating two one-dimensional arrays with text file

For my Visual Basic final, my program is required to read data from a text file into two different arrays, each being one-dimensional. The following is my code for doing so:
Option Explicit On
Option Infer Off
Option Strict On
Public Class frmMain
'Constant for filename and a dirty flag variable
Const INVENTORY_FILENAME As String = "inventory.txt"
Dim noFile As Boolean = False
Private Sub frmMain_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'Populates DVD listbox with text file data upon load
'Variable for reading the file
Dim myFile As IO.StreamReader
'Declaring arrays for DVD names and prices
Dim arrayDVD() As String
Dim arrayPrice() As Double
'Variables for populating arrays with respective data
Dim dvdName As String
Dim dvdPrice As Double
Dim i As Integer = 0
'Checking that file exists then reading data to each array
If IO.File.Exists(INVENTORY_FILENAME) Then
myFile = IO.File.OpenText(INVENTORY_FILENAME)
'Read data to arrays
Do Until myFile.Peek = -1
dvdName = myFile.ReadLine()
dvdPrice = Double.Parse(myFile.ReadLine())
arrayDVD = dvdName
arrayPrice = dvdPrice
'Using arrays to populate multicolumn listbox
lstDVD.Items.Add(arrayDVD(i) & arrayPrice(i))
i += 1
Loop
'Closing the file
myFile.Close()
End If
End Sub
End Class
The text file alternates names and prices of DVDs to be read as individual lines, making the arrays parallel:
Pulp Fiction
9.99
Jumanji
13.99
And so on...
I'm receiving a value type error code stating that I cannot convert 'String' to 'String()' or convert 'Double' to 'Double()' when setting the arrays' values equal to their respective variables. Is there a way to correct this? Thanks in advance!
These lines are wrong:
arrayDVD = dvdName
arrayPrice = dvdPrice
arrayDVD and arrayPrice are arrays. You need to assign to a specific element in each of those arrays:
arrayDVD(i) = dvdName
arrayPrice(i) = dvdPrice
Don't forget to make sure the arrays actually have enough elements for this.
Hint: ReDim Preserve is pretty much the least efficient way possible to make sure an array is big enough. Each use will allocate a brand new array, copy the elements one at a time, assign the new array to the old reference, and then release the old array. It does not preserve in-place. Nevertheless, if this is a 100-level course it might be what you are expected to do at this point.
Finally, you should never use Double when working with money (use Decimal instead).
Separate from the question, here is how I might approach this without the weird array limitation:
Private Iterator Function ReadInventoryFile(filePath As String) As IEnumerable(Of (String, Decimal))
Using rdr As New StreamReader(filePath)
Dim DVDName As String = Nothing
While (DVDName = rdr.ReadLine()) IsNot Nothing
Yield (DVDName, Decimal.Parse(rdr.ReadLine()))
End While
End Using
End Function
Const INVENTORY_FILENAME As String = "inventory.txt"
Private data As List(Of (String, Decimal))
Private Sub frmMain_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Try 'Replaces the File.Exists() check
data = ReadInventoryFile(INVENTORY_FILENAME).ToList()
For Each item As (String, Decimal) In data
lstDVD.Items.Add($"{item.Item1}{vbTab}{item.Item2:C}")
Next
Catch
' Actually do something here. Empty catch blocks are rarely correct.
' Note I catch at this level, rather than in the ReadFile() method.
End Try
End Sub

Visual Basic code works but is inelegant - any suggestions?

I am trying to stay ahead of my Year 12 Software class. Starting to work with records and arrays. I have answered the question, but the solution feels very clunky. I am hoping someone has suggestions/links for completing this task in a more efficient way.
The task: read in lines from a text file and into a structure, and then loop through that, populating four list boxes if an animal hasn't been vaccinated.
Here's the code:
Imports System.IO
Public Class Form1
'Set up the variables - customer record, total pets not vaccinated, total records in the file, and a streamreader for the file.
Structure PetsRecord
Dim custName As String
Dim address As String
Dim petType As String
Dim vacced As String
End Structure
Dim totNotVac As Integer
Dim totalRecCount As Integer
Dim PetFile As IO.StreamReader
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
End Sub
Private Sub btnLoad_Click(sender As Object, e As EventArgs) Handles btnLoad.Click
'set an array of records to store each record as it comes in. Limitation: you need to know how many records in the file. Set the array at 15 to allow for adding more in later.
Dim PetArray(15) As PetsRecord
'variables that let me read in a line and split it into sections.
Dim lineoftext As String
Dim i As Integer
Dim arytextfile() As String
'tell them what text file to read
PetFile = New IO.StreamReader("patients.txt")
totNotVac = 0
Try
totalRecCount = 0
' read each line in and split the lines into fields for the records. Then assign the fields from the array. Finally, reset the array and loop.
Do Until PetFile.Peek = -1
'read in a line of text
lineoftext = PetFile.ReadLine()
'split that line into bits separated by commas. these will go into the array.
arytextfile = lineoftext.Split(",")
'dunno whether this is the best way to do it, but stick the array bits into the record, and then clear the array to start again.
PetArray(totalRecCount).custName = arytextfile(0)
PetArray(totalRecCount).address = arytextfile(1)
PetArray(totalRecCount).petType = arytextfile(2)
PetArray(totalRecCount).vacced = arytextfile(3)
totalRecCount += 1
Array.Clear(arytextfile, 0, arytextfile.Length)
Loop
For i = 0 To PetArray.GetUpperBound(0)
If PetArray(i).vacced = "No" Then
lstVaccinated.Items.Add(PetArray(i).vacced)
lstCustomer.Items.Add(PetArray(i).custName)
lstAddress.Items.Add(PetArray(i).address)
lstPetType.Items.Add(PetArray(i).petType)
totNotVac += 1
lblVacTotal.Text = "The total number of unvaccinated animals is " & CStr(totNotVac)
End If
Next
Catch ex As Exception
MsgBox("Something went wrong with the file")
End Try
PetFile.Close()
End Sub
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click
Close()
End Sub
End Class
And one line from the patient.txt file:
Richard Gere,16 Sunset Blvd,Gerbil,No
I hope this isn't out of place.
Regards,
Damian
If you want to use Streams be aware that they need to be disposed.
File.ReadAllLines returns an array of lines. Since the array is initialized where it is declared we don't need to specify a size.
If you use List(Of T) you do not have to know in advance the number of elements in the list. Avoids the limitation of array.
Using a For Each avoids having to use the size. The small c following "," tells the compiler that this is a Char which is what .Split is expecting. If you had Option Strict On, which you always should, you would have seen an error. You add items to the list by creating a New PetsRecord. The parametrized Sub New receives the values and sets the properties.
Don't change the display in the label on each iteration. Use an interpolated string (preceded by a $) which allows embedded variables surrounded by braces { }. It is not necessary to change the number to a string as it is implied by the interpolator. (Available in VS2015 and later)
Public Structure PetsRecord
Public Property custName As String
Public Property address As String
Public Property petType As String
Public Property vacced As String
Public Sub New(name As String, add As String, pType As String, vac As String)
custName = name
address = add
petType = pType
vacced = vac
End Sub
End Structure
Private Sub btnLoad_Click(sender As Object, e As EventArgs) Handles btnLoad.Click
Dim lines = File.ReadAllLines("patients.txt")
Dim lstPet As New List(Of PetsRecord)
For Each line In lines
Dim splits = line.Split(","c)
lstPet.Add(New PetsRecord(splits(0), splits(1), splits(2), splits(3)))
Next
Dim totNotVac As Integer
For Each pet In lstPet
If pet.vacced = "No" Then
lstVaccinated.Items.Add(pet.vacced)
lstCustomer.Items.Add(pet.custName)
lstAddress.Items.Add(pet.address)
lstPetType.Items.Add(pet.petType)
totNotVac += 1
End If
Next
lblVacTotal.Text = $"The total number of unvaccinated animals is {totNotVac}"
End Sub
If you don't need the 'PetsRecord' array to store data, take a look at the following code:
Dim totNotVac As Integer
Private Sub btnLoad_Click(sender As Object, e As EventArgs) Handles btnLoad.Click
File.ReadAllLines("your text file path").ToList().ForEach(Sub(x)
lstVaccinated.Items.Add(x.Split(","c)(0))
lstCustomer.Items.Add(x.Split(","c)(1))
lstAddress.Items.Add(x.Split(","c)(2))
lstPetType.Items.Add(x.Split(","c)(3))
totNotVac += 1
End Sub)
lblVacTotal.Text = "The total number of unvaccinated animals is " & CStr(totNotVac)
End Sub

How to randomly select a string from an array

I am very new to coding so please forgive me.
I have saved some example strings in an array:
Dim intArray(0 To 2) As String
intArray(0) = "I will be on time for class"
intArray(1) = "I will be prepared for class"
intArray(2) = "I will listen to the teacher and follow instructions"
lblTestSWPB.Text = intArray(0)
I know that it works when I click the button to generate for (0) - obviously.Is there a way to add something to make the label display a string at random from the array?
You may use something like: Random class
Dim rnd As New Random 'Make sure that you declare it as New,
'otherwise, it thorws an Exception(NullReferenceException)
Dim intArray(0 To 2) As String
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
intArray(0) = "I will be on time for class"
intArray(1) = "I will be prepared for class"
intArray(2) = "I will listen to the teacher and follow instructions"
End Sub
Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
lblTestSWPB.Text = intArray(rnd.Next(0, 3))
End Sub
Note that rnd.Next is a Function that returns a random Integer from the minimum is the inclusive lower bound and the maximum is the exclusive upper bound.(Thanks to #Enigmativity for the correction.) Click this link if this answer is still unclear for you

Displaying an array with unknown values into a lable box

I am new to coding with visual basic.
Recently, I was tasked by my professor to write a programme that allows the user to enter five words. The words then should be sorted and displayed in alphabetical order.
To do this I decided the best approach would be to use an array.
My thinking was that if I created a counter at the start, I can create a different value for each column of the array when a button is clicked.
If the array exceeds five I have a message box pop-up that resets the code (although I realise I will also have to clear the contents of the array).
My problem arises in displaying the array. I have looked for solutions online, and none have helped me as of yet.
I need to sort the array into alphabetical order and then display it in a label box (lbl_DisplayArray). As I do not know the values of the array, this has proved tricky.
My code is below:
Public Class Form1
Dim i As Integer = 0
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim Array(4) As String
Array(i) = txt_UserWords.Text
End Sub
Private Sub btn_Next_Click(sender As Object, e As EventArgs) Handles btn_Next.Click
i += 1
If i >= 5 Then
i = 0
MsgBox("Array Limit Exceeded. Code Reset")
txt_UserWords.Text = ""
End If
End Sub
Private Sub btn_Sort_Click(sender As Object, e As EventArgs) Handles btn_Sort.Click
lbl_DisplayArray.Text =
End Sub
End Class
You'd be better off using
private myList as new List(of String).
Then to sort them you just call the .Sort() method. Just call .Add(txt_userWords.Text) to add the new string and use .Count to see how many of them you have.
When you're adding them to the label you can use
lbl_DisplayArray.Text = String.Join(vbCrLf, myList)
You'll need the list of to be a member of the class instead of a local variable (as you have declared Array). This will keep it alive and allow you to access it in other methods.
---------- edit ----------
Public Class Form1
private myList as new List(of String)
Private Sub btn_Next_Click(sender As Object, e As EventArgs) Handles btn_Next.Click
If myList.Count >= 5 Then
myList.Clear
Else
myList.add(txt_UserWords.Text)
End If
txt_UserWords.Text = ""
End Sub
Private Sub btn_Sort_Click(sender As Object, e As EventArgs) Handles btn_Sort.Click
myList.Sort()
lbl_DisplayArray.Text = String.Join(vbcrlf, myList)
End Sub
End Class

Deleting Lines from Array

I have an array of lines and I want at some point to erase some of them.
Here's a sample of the code:
Dim canvas As New Microsoft.VisualBasic.PowerPacks.ShapeContainer
Dim lines(20) As PowerPacks.LineShape
Dim it As Integer = 0
Private Sub GoldenSpi_Load(sender As Object, e As EventArgs) Handles MyBase.Load
canvas.Parent = Me
lines.Initialize()
iter.Text = 0
End Sub
Private Sub iter_TextChanged(sender As Object, e As EventArgs) Handles iter.TextChanged
If (it > iter.Text And iter.Text <> 0) Then
ReDim Preserve lines(iter.Text - 1)
End If
If (it <> iter.Text) Then
it = iter.Text
End If
For i As Integer = 1 To iter.Text
lines(i - 1) = New PowerPacks.LineShape(canvas)
lines(i - 1).StartPoint = pA(i)
lines(i - 1).EndPoint = pB(i)
lines(i - 1).BringToFront()
Next
End Sub
After I execute the program, the lines are created. But when I give a value to my textbox that is smaller than the variable 'it', it justs delete the last line and not the rest. Also I saw while debugging that the size of array is reduced. So that means that the contents beyond the size are still kept? Why is that?. Any help is appreciated. Thanks.
EDIT: I tried to create the List like this:
Dim lines As New Generic.List(Of PowerPacks.LineShape)
Private Sub iter_ValueChanged(blabla) Handles iter.ValueChanged
If (it > iter.Value And iter.Value <> 0) Then
lines.RemoveRange(iter.Value - 1, lines.Count - iter.Value)
End If
For i As Integer = 1 To iter.Value
InitPoints()
If i - 1 = lines.Count Then
Dim line As New PowerPacks.LineShape
With line
.StartPoint = pA(i)
.EndPoint = pB(i)
.BringToFront()
.Parent = canvas
End With
lines.Add(line)
End If
Next
End Sub
But still the lines are visible in the form. I debugged it and saw that the list size decreased. The same problem when I had an array. What is going?...
I recommend changing iter.Text to cint(iter.Text), as there is a chance it's comparing both values as text (which is compared differently).
I'd also recommend changing Dim lines(20) As PowerPacks.LineShape to Dim lines As new generic.list(of PowerPacks.LineShape)
That way you don't have to worry about ReDim Preserve (which can be slow when you do it in a loop), and you can easily insert items into any index if you whish
You should use Option Strict On in your project, in order to avoid implicit conversion between types which can give you errors or, worse, unexpected behaviors.
On the other hand, you should not have a TextBox to store numbers unless there is a need. Use a NumericUpDown, for example. Take a look at the MSDN Documentation.
And now, for the array, I recommend using a List, which has all the methods implemented that you need to handle the elements, and has a .ToArray() method that will give you the array if needed.
Try something like this:
Dim it As Integer = 0
Dim lines As New List(Of PowerPacks.LineShape)()
Sub iter_TextChanged(sender As Object, e As EventArgs) Handles iter.TextChanged
Dim iTxt As Integer
Try
iTxt = Integer.Parse(iter.Text)
If it > iTxt AndAlso iTxt <> 0 Then
End If
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Sub
I was going to write to you an example, but I realized that I don't know exactly what you're trying to do. Could you explain?

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