Zichuan Ye, a coworker wrote for fun a factorial calculator. It does both, multiplication and factorials providing a postback and callback implementation in ASP.NET and VB.NET. A great tool to download and learn from it. He was kind enough to provide the source code to share this great tool with all of us. Something that took him an hour to create would take me a few days to figure it out.
Just take a look at the multiply function below.
Private Function DoMultiply(ByVal A() As Short, ByVal B() As Short) As Short()
Dim iCnt As Integer
Dim jCnt As Integer
Dim c() As Short
iCnt = A.Length
jCnt = B.Length
Dim i As Integer
Dim j As Integer
Dim kCnt As Integer
kCnt = iCnt + jCnt - 1
ReDim c(kCnt)
For i = 0 To kCnt
c(i) = 0
Next
For i = iCnt - 1 To 0 Step -1
For j = jCnt - 1 To 0 Step -1
DigitMultiply(i + j + 1, A(i), B(j), c)
Next
Next
Return c
End Function
Public Function DigitMultiply(ByVal kIndex As Integer, _
ByVal iValue As Short, ByVal jValue As Short, ByVal c() As Short) As Integer
Dim c1 As Short
Dim c2 As Short
Dim cm As Short
cm = iValue * jValue
If (cm < 10) Then
DigitalAdd(cm, kIndex, c)
Else
c1 = (cm \ 10)
c2 = (cm Mod 10)
DigitalAdd(c2, kIndex, c)
DigitalAdd(c1, kIndex - 1, c)
End If
End Function
Public Sub DigitalAdd(ByVal iValue As Short, ByVal kIndex As Integer, ByRef c() As Short)
Dim cm As Short
Dim c10 As Short
Dim c0 As Short
cm = c(kIndex) + iValue
If cm < 10 Then
c(kIndex) = cm
Else
c10 = cm \ 10
c0 = (cm Mod 10)
c(kIndex) = c0
DigitalAdd(c10, kIndex - 1, c)
End If
End Sub
Download the code from here
Cheers
Al