Finding a palindromic number

[Karl.Green]

As part of Project Euler I have to find the largest palindrome made from the product of two 3-digit numbers.

I have created this code but when i3 gets to 909090 there is an error which states largest "Index was outside the bounds of the array." on the line

If i3(0) = i3(5) And i3(1) = i3(4) And i3(2) = i3(3) Then

Here is my full code, I'm not sure exactly what is wrong?
Also, i3 is a string because I couldn't work out how to go through an integer digit by digit?

Public Class Form1

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Dim i As Integer = 999
        Dim i2 As Integer = 999
        Dim i3 As String
        Dim found As Boolean = False

        Do Until found = True
            If i2 > 1 Then
                i2 = i2 - 1
            Else : i2 = 999
                i = i - 1
            End If

            i3 = i * i2
            If i3.Length = 5 Then
                If i3(0) = i3(5) And i3(1) = i3(4) And i3(2) = i3(3) Then
                    found = True
                End If
            ElseIf i3.Length = 4 Then
                If i3(0) = i3(4) And i3(1) = i3(3) Then
                    found = True
                End If
            End If
        Loop

        MsgBox(i3)

    End Sub
End Class

[dunfiddlin]

if i3.Length = 5 Then there ain't no such thing as i3(5)!

[MattP]

Rather than going through and checking based on length you could use SequenceEqual.

productString.SequenceEqual(productString.Reverse)

I would also recommend using meaningful variable names. i, i2 & i3 are terrible names for variables (especially i3 which is a string). You'll thank yourself later when you come back to a project and can immediately tell what a variable is for.

[dbasnett]

For reference this is Problem 4.

Solution based on this:

        Dim palindrome As New List(Of String)
        Dim stpw As New Stopwatch
        stpw.Reset()
        For num1 As Long = 999 To 110 Step -1
            If num1 Mod 11 = 0 Then
                For num2 As Long = 999 To 100 Step -1
                    Dim num3 As String = (num2 * num1).ToString
                    If num3.Length = 6 Then
                        Dim m As String = num3.Substring(0, 3)
                        Dim n As String
                        n = StrReverse(num3.Substring(3, 3))
                        If m = n Then palindrome.Add(num3 & "." & m.ToString)
                    End If
                Next
            End If
        Next
        stpw.Stop()
        palindrome.Sort()
        palindrome.Reverse()
        Debug.WriteLine(palindrome(0))
        Stop

[Karl.Green]

Thank you I will change the code to use that.

Also I know they are terrible variable names and I usually use self-identifying names but for programs this size I tend not to bother.

i3 was originally an integer but then I changed it to a string because as I said earlier, I couldn't work out how to go through an integer digit by digit and I didn't bother changing it from i3 after.

[dbasnett]

refined:

        Dim palindrome As New List(Of String)
        Dim stpw As New Stopwatch
        stpw.Reset()
        For num1 As Long = 990 To 110 Step -11
            If num1 Mod 11 = 0 Then
                For num2 As Long = 999 To 101 Step -1
                    Dim num3 As Long = num2 * num1
                    If num3 >= 101101 Then
                        Dim s As String = num3.ToString
                        Dim m As String = s.Substring(0, 3)
                        Dim n As String = s.Substring(3, 3)
                        If m = StrReverse(n) Then palindrome.Add(num3 & "." & num1.ToString & "." & num2.ToString)
                    Else
                        Exit For
                    End If
                Next
            End If
        Next
        palindrome.Sort()
        palindrome.Reverse()
        stpw.Stop()
        Debug.WriteLine(stpw.ElapsedTicks.ToString)
        Debug.WriteLine(palindrome(0))
        Stop