get combinations with repetition - vb.net

Question

How can I write all possible combinations to the console? For example, if user enters abc, then it will write aaa, aab, aac, abb, abc, acc, bbb, bbc, ccc. Please help me.
Here's some code:
Dim abc() As String = {"a", "b", "c"} '
Sub Main()
Console.WriteLine("Enter the amount of characters")
Dim count As Integer = Console.ReadLine
outputStrings("", count)
Console.ReadLine()
End Sub
Private Sub outputStrings(ByVal startString As String, ByVal letterCount As Integer)
For i = 0 To abc.Length - 1
Dim temp As String = startString
temp += abc(i)
If temp.Length = letterCount Then
Console.WriteLine(temp)
If i = abc.Length - 1 Then
Console.WriteLine("----")
End If
Else
outputStrings(temp, letterCount)
End If
Next
End Sub
Something has to be done after the dashed lines to remove unwanted permutation to leave out only valid combinations.

Answer 1

You can restrict the letters used to ones at or to the right of abc(i) with an additional parameter abcIndex, and start the for loop from there. Only strings which have their letters in alphabetical order will be written, which prevents duplicates.
Private Sub outputStrings(ByVal startString As String, ByVal letterCount As Integer, ByVal abcIndex As Integer)
For i = abcIndex To abc.Length - 1
Dim temp As String = startString
temp += abc(i)
If temp.Length = letterCount Then
Console.WriteLine(temp)
Else
outputStrings(temp, letterCount, i)
End If
Next
End Sub
Call with:
outputStrings("", 3, 0)

Answer 2

def go(chars,thusfar):
if len(thusfar) = len(chars):
print thusfar
for char in chars:
go(chars,thusfar+char);
This should be easy enough to translate to VB (read: I don't know VB)

Answer 3

You just need to make a recursive call there.
Dim abc() As String = {"a", "b", "c"} '
Sub Main()
Console.WriteLine("Enter the amount of characters")
Dim count As Integer = Console.ReadLine
outputStrings("", count)
Console.ReadLine()
End Sub
Private Sub outputStrings(ByVal startString As String, ByVal letterCount As Integer)
For i = 0 To abc.Count - 1
Dim temp As String = startString
temp += abc(i)
If temp.Length = letterCount Then
Console.WriteLine(temp)
Else
outputStrings(temp, letterCount)
End If
Next
End Sub
Do note that if someone enters a negative number that your code will run forever. I'll leave fixing that as an easy exercise.

Answer 4

Amazing code from here:
Private Shared Function PermutationsWithRepetition(Of T)(list As IEnumerable(Of T), length As Integer) As IEnumerable(Of IEnumerable(Of T))
If length = 1 Then
Return list.[Select](Function(x) New T() {x})
End If
Return PermutationsWithRepetition(list, length - 1).SelectMany(Function(x) list, Function(t1, t2) t1.Concat(New T() {t2}))
End Function
Can be used with Integer, Char, Double etc.
Example of use:
Dim myarray(1) As Integer
myarray(0) = 1
myarray(1) = 2
Dim k As Integer = 2 'number of "slots" to do the permutations
mypermutations = PermutationsWithRepetition(myarray,k)
For Each row As IEnumerable(Of Integer) In mypermutations
Console.WriteLine("")
For Each col As IntegerIn row
Console.Write(col.toString())
Next
Next
Output:
11
12
21
22