Could someone please explain this (VB) - vb.net

Question

I'm new to coding and have recently been curious about creating multiple permutations of a selection of characters. My solution had a lot of nested For Next loops which was clunky, so I searched other solutions and found the one below, however I cannot fully understand it.
Dim chars() As Char = "1234567890abcdefghijklmnopqrstuvwxyz".ToCharArray
Dim csize As Integer = chars.Length - 1
Dim upto As String
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
upto = " "
Dim max_length As Integer = 25
For i = 1 To max_length
bf_recursion(0, i)
Update()
Next
End Sub
Private Sub bf_recursion(ByVal index As Integer, ByVal depth As Integer)
Dim current() As Char = upto.ToCharArray()
For i = 0 To csize
current(index) = chars(i)
upto = CStr(current)
TextBox1.Text = (CStr(current))
TextBox1.Refresh()
Me.Refresh()
'\\lblOutput.Text = CStr(current)
If index <> (depth - 1) Then
bf_recursion(index + 1, depth)
End If
Next
End Sub
I do not understand the section where Current(Index) = Chars(i) since from my understanding it is making the Current(index) value stored in that index equal to the value in the characters, however somehow in the next line is creates a string from the Current(index) value that produces the correct result.
Help would be greatly appreciated, thank you.

Answer 1

Ok, I had to copy your code to see the result as I wasn't sure what you were trying to do, but let's get it explaining.
For the first cycle we have:
csize = 35
index = 0
depth = 1
The loop is easy enough: you loop from 0 to 35 (35 included)
For i = 0 To csize
At Index 0of context, you store the character at position i
current(index) = chars(i)
So when looping, this will give this result:
current(0) = "1" 'i = 1
current(0) = "2" 'i = 2
...
current(0) = "a" 'i = 11
current(0) = "b" 'i = 12
the CStr function changes the array current to a string by placing all elements one after each other:
upto = CStr(current)
Label1.Text = (CStr(current))
Label1.Refresh()
Me.Refresh()
Then comes the recursive part, but for the first loop, we don't need it...
If index <> (depth - 1) Then
bf_recursion(index + 1, depth)
End If
Next
Ok, now we finished the first cycle, let's go the the 2nd one:
csize = 35
index = 0
depth = 2
What's the difference? Nothing, until we reach this code:
If index <> (depth - 1) Then
bf_recursion(index + 1, depth)
End If
In the code above, index isn't the same as depth - 1 anymore, so we go recursive, and ->important<- instead of passing index = 0, we pass index + 1! What difference does this make?
This time, we don't store our character at current(0) but at current(1):
current(index) = chars(i)
which gives:
current(1) = "1" 'i = 1
current(1) = "2" 'i = 2
...
current(1) = "a" 'i = 11
current(1) = "b" 'i = 12
But, as the line before we did this:
Dim current() As Char = upto.ToCharArray()
current(0) is already filled in! Because upto contains the first character. That's what this line is for:
upto = CStr(current)
In short:
-> You store the characters you calculated in the upto string.
-> Each time you enter bf_recursion, you recover the characters you already have
-> index is 1 larger each time, so you change the NEXT character
One important note
With your application, you create a very very long loop that the user can't terminate without killing the process. You might want to look into that.