[RESOLVED] Binary Search of Arrays/Lists

[rosleenXOXO]

my binary search isnt working, i think it has to do with the fact that im using a list and its searching thru an array idk could you guys please help me out

List Code:

Option Strict On

Public Class Form1

    Dim J As Integer = 0
    Dim MyArray As New List(Of Integer)


Private Sub btnPutNumberIntoArray_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnPutNumberIntoArray.Click
        Dim number As Integer
        Integer.TryParse(txtNumbers.Text, number)
        If number < 1 Then
            MessageBox.Show("Please enter a number greater then zero.")
        Else
            MyArray.Add(number)
            txtNumbers.Clear()

        End If
    End Sub

Private Sub btnDisplayNumbersInArray_Click_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisplayNumbersInArray.Click
        

        txtDisplayNumbers.Clear()
        txtDisplayNumbers.Focus()

        For J As Integer = 0 To MyArray.Count - 1
            txtDisplayNumbers.Text &= MyArray(J).ToString &                      
             Environment.NewLine
        Next
    End Sub

Binary Search :

Private Sub btnBinarySearch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnBinarySearch.Click

        Dim Key As Integer
        Dim Low As Integer
        Dim High As Integer
        Dim Mid As Integer




        txtBinaryKey.Text = CStr(Key)
        Low = 0
        High = 4

        While Low <= High
            Mid = (High + Low) \ 2
            If Key = MyArray(Mid) Then
                lblBinarySearchLocation.BackColor = Color.Silver
                lblBinarySearch.Text = "Search Key found"
                lblBinarySearchLocation.Text = "Array(" & Str(Mid) & ")"

                Low = High + 1

            ElseIf Key < MyArray(Mid) Then
                High = Mid - 1
            ElseIf Key > MyArray(Mid) Then
                Low = Mid + 1
            End If
        End While

        If Key <> MyArray(Mid) Then
            lblBinarySearch.Text = "Search Key not found."
            lblBinarySearchLocation.BackColor = Color.Red
            lblBinarySearchLocation.Text = ""
        End If

    End Sub

[Shaggy Hiker]

There appear to be a few issues right off the top:

        txtBinaryKey.Text = CStr(Key)
        Low = 0
        High = 4

Since you never put a value into key, all this line does would be to put 0 into the textbox. That surely isn't what you are trying to do. The key is what you are searching for isn't it? Is it supposed to have been entered into the textbox by the user? If so, then you have that first line backwards, because you are putting 0 into the textbox rather than putting the textbox contents into Key. For that, you could use CInt rather than CStr, but CInt will throw an exception if the user doesn't enter a valid integer into the textbox, so it is safer to use Integer.TryParse.

As for the next two lines, I would agree that Low is 0, as that would be the bottom of the list, but setting High = 4 is going to be pretty bad. That would guarantee that the search will only find a match in the first five items of the collection. Perhaps you will only have five items in the collection, but that's still not a good reason to write it that way. Instead, set High = MyArray.Count-1.

Finally, I'd suggest coming up with a different name for Mid. It should work in your code, but Mid is also a legacy method, so the compiler might confuse the variable for the function in some cases and produce weird behavior.

[rosleenXOXO]

There appear to be a few issues right off the top:

        txtBinaryKey.Text = CStr(Key)
        Low = 0
        High = 4

Since you never put a value into key, all this line does would be to put 0 into the textbox. That surely isn't what you are trying to do. The key is what you are searching for isn't it? Is it supposed to have been entered into the textbox by the user? If so, then you have that first line backwards, because you are putting 0 into the textbox rather than putting the textbox contents into Key. For that, you could use CInt rather than CStr, but CInt will throw an exception if the user doesn't enter a valid integer into the textbox, so it is safer to use Integer.TryParse.

As for the next two lines, I would agree that Low is 0, as that would be the bottom of the list, but setting High = 4 is going to be pretty bad. That would guarantee that the search will only find a match in the first five items of the collection. Perhaps you will only have five items in the collection, but that's still not a good reason to write it that way. Instead, set High = MyArray.Count-1.

Finally, I'd suggest coming up with a different name for Mid. It should work in your code, but Mid is also a legacy method, so the compiler might confuse the variable for the function in some cases and produce weird behavior.

thank you for your help
so i changed what you said but i still get an error for some reason
and i had the

txtbinary.text = cstr(Key)

because i get an error if id it like

 Key = Cint(txtBinary.Text)

but i changed the code arround to use integer.Parse

 Private Sub btnBinarySearch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnBinarySearch.Click

        Dim Key As Integer
        Dim Low As Integer
        Dim High As Integer
        Dim Mid1 As Integer




        Key = Integer.Parse(txtBinaryKey.Text)
        Low = 0
        High = MyArray.Count - 1

        While Low <= High
            Mid1 = (High + Low) \ 2
            If Key = MyArray(Mid1) Then
                lblBinarySearchLocation.BackColor = Color.Silver
                lblBinarySearch.Text = "Search Key found"
                lblBinarySearchLocation.Text = "Array(" & Str(Mid1) & ")"

                Low = High + 1

            ElseIf Key < MyArray(Mid1) Then
                High = Mid1 - 1
            ElseIf Key > MyArray(Mid1) Then
                Low = Mid1 + 1
            End If
        End While

        If Key <> MyArray(Mid1) Then
            lblBinarySearch.Text = "Search Key not found."
            lblBinarySearchLocation.BackColor = Color.Red
            lblBinarySearchLocation.Text = ""
        End If

    End Sub

I still a get an error:
FormatExcpetion was unhandled.
Input string was not in a correct format.

[Shaggy Hiker]

Integer.Parse is not the same as Integer.TryParse, which is what I recommended. It is used differently, too:

If Integer.TryParse(txtBinary.Text, Key) Then
 'It was an integer, and is now in Key.
Else
 'It was not an integer, so do whatever you want here.
End If

[rosleenXOXO]

Thank you for your help and patience, but any number i enter it wont even go thru the first if statement, its saying that the any/all numbers i enter are not integers ?

 
Private Sub btnBinarySearch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnBinarySearch.Click

        Dim Key As Integer
        Dim Low As Integer
        Dim High As Integer
        Dim Mid1 As Integer

        Low = 0
        High = MyArray.Count - 1

        If Integer.TryParse(txtBinaryKey.Text, Key) Then
            While Low <= High
                Mid1 = (High + Low) \ 2
                If Key = MyArray(Mid1) Then
                    lblBinarySearchLocation.BackColor = Color.Silver
                    lblBinarySearch.Text = "Search Key found"
                    lblBinarySearchLocation.Text = "Array(" & Str(Mid1) & ")"

                    Low = High + 1

                ElseIf Key < MyArray(Mid1) Then
                    High = Mid1 - 1
                ElseIf Key > MyArray(Mid1) Then
                    Low = Mid1 + 1
                End If
            End While

            If Key <> MyArray(Mid1) Then
                lblBinarySearch.Text = "Search Key not found."
                lblBinarySearchLocation.BackColor = Color.Red
                lblBinarySearchLocation.Text = ""
            End If

        Else
            MessageBox.Show("Please enter a valid number.")
        End If
     
    End Sub

[rosleenXOXO]

Yea something is definitely wrong, i cant enter ANY number(negative, decimal, integer) its just to the end of the loop and gives me the message

  
        Else
            MessageBox.Show("Please enter a valid number.")
        End If

[Shaggy Hiker]

What are you entering? Is there something funny with your textbox? You aren't adding spaces, symbols, or something like that are you?

[rosleenXOXO]

Im entering : 1,2,4,then i tired negative numbers -2,-4,-9. Im not adding any spaces either... i don't think there anything wrong with the textbox, ill re-make the textbox see if that changes things...

[rosleenXOXO]

hmm that's really weird, i made a different textbox and now it works.
once again THANK YOU FOR ALL YOUR HELP!

[Shaggy Hiker]

That IS really weird. The only thing I can think of is that you had a space as default text in the original textbox. Then, when you went to add more text, there would be a space at the beginning of anything else you entered. You probably wouldn't notice a space, as it would be a thin and invisible character, but you might never have gotten rid of it, either, and it would mess up the TryParse. Just a guess, though.