VBA permutations of undetermined number of variables - algorithm

Question

Recently I am trying to get the permutation of undetermined number of variables. For undetermined I mean I was aiming to create an input box for users to put in the number.
Start from simple. Originally I was aiming to get a 4 digits permutations with each digit have different number of variables, i.e. 1st digit can only be A,B,C,D; 2nd digit be E,F; 3rd digit be G, H etc. Code are below:
Sub Permut()
Count = 1
For a = 1 To 4
For b = 1 To 2
For c = 1 To 2
For d = 1 To 2
For e = 1 To 2
'chr(97) is the alphabet "a"
Cells(Count, 1) = Chr(96 + a) & Chr(96 + Len(a) + b) & Chr(96 + Len(a) + Len(b) + c) & _
Chr(96 + Len(a) + Len(b) + Len(c) + d) & Chr(96 + Len(a) + Len(b) + Len(c) + Len(d) + e)
Count = Count + 1
Next
Next
Next
Next
Next
End Sub
This will give you 64 different combinations without repetition.
Just wondering is there a way to generalize this process so that people can choose how many variables in total as well as within each digit?
Thank you.

Answer 1

Here is a solution, where you would pass the Permut function the minimum value for each of the characters (digits) as one string, and the maximum characters also as a string. Both strings should have an equal number of characters of course:
Function Permut(min, max)
Dim str, nxt, count
str = min
count = 1
Do While str < max
Cells(count, 1) = str
count = count + 1
nxt = ""
For i = Len(str) To 1 Step -1
If Mid(str, i, 1) < Mid(max, i, 1) Then
nxt = ChrW(AscW(Mid(str, i, 1))+1) & nxt
Exit For
End If
nxt = Mid(min, i, 1) & nxt
Next
str = Left(str, Len(str) - Len(nxt)) & nxt
Loop
Cells(count, 1) = str
End Sub
You would call it like this:
Permut "abc", "bcf"
That example would produce this list on your sheet:
abc
abd
abe
abf
acc
acd
ace
acf
bbc
bbd
bbe
bbf
bcc
bcd
bce
bcf
How to Execute This with User Input and Button Click
If you want to call this code in response to an event, such as a button click, and want to pass it the contents of two cells where the user would first enter the min and max strings, then follow these steps:
Place an ActiveX command button on your sheet (put it somewhere in D1 to leave room for some other stuff)
Double click it to generate the empty click event handler. If that does not work, go to the code window and select the name of the button from the drop-down at the top of the window, and select Click from the next drop down.
Complete the code of that event handler as follows (I assume the button is called CommandButton1, but don't change the generated name):
Code:
Private Sub CommandButton1_Click()
Permut Range("B1"), Range("C1")
End Sub
This code assumes the user has to enter the min and max digits/characters in cells B1 and C1. The A column is of course reserved for the output of the code.
For a more complete explanation on how to add a command button and attach code to its click event, read "Add a command button (ActiveX control)" in the Office manual.

Answer 2

credit to the answer from trincot above.
I have tried to run the code with a bit modification coz I am not sure how to get set value into cells (0,1). It keeps saying error. But If I change the starting point to Cells(1,1) then I will miss the last permutation. So I just add an additional if statement to get the code work as I want.
Function Permut(min, max)
Dim str, nxt, count
str = min
count = 1
Do While str < max
Cells(count, 1) = str
count = count + 1
nxt = ""
For i = Len(str) To 1 Step -1
If Mid(str, i, 1) < Mid(max, i, 1) Then
'asc("a")=97; chr(97) ="a"
nxt = Chr(AscW(Mid(str, i, 1)) + 1) & nxt
Exit For
End If
nxt = Mid(min, i, 1) & nxt
Next
str = Left(str, Len(str) - Len(nxt)) & nxt
If str = max Then
Cells(count, 1) = str
End If
Loop
End Function