multiplication of extra large integers
Calculation of the exact value 1000 factorial is nearly impossible by using traditional variables. This code multiplies extra large integers by using strings. 1000! is calculated in 10 seconds
Original Author: zafer ketizmen
Inputs
two integers in string form
Returns
multiplication result as a string
Code
' sample of usage calculation of 1000!
' msgbox may truncate the entire string
' so it is set into clipboard
'Dim Ret As String
'Dim k As Integer
'
'Ret = "1"
'For k = 1 To 1000
' Ret = Multiply(CStr(k), Ret)
'Next
'Clipboard.Clear
'Clipboard.SetText (Ret)
'
'MsgBox Ret
Private Function Multiply(a_num1 As String, a_num2 As String) As String
Dim ls_line() As String
Dim ls1 As String
Dim ls2 As String
Dim ls_mul As String
Dim li_num As Integer
Dim li_mul As Integer
Dim li_elde As Integer
Dim li_sum As Integer
Dim li_maxlen As Integer
Dim li_linecount As Integer
Dim li_up As Integer
Dim k As Long
Dim j As Long
' select larger one
Select Case True
Case Len(a_num1) >= Len(a_num2)
ls1 = a_num1
ls2 = a_num2
Case Len(a_num1) < Len(a_num2)
ls1 = a_num2
ls2 = a_num1
End Select
' start multiplication
li_maxlen = -1
For j = Len(ls2) To 1 Step -1
li_elde = 0
ls_mul = ""
li_num = CInt(Mid(ls2, j, 1)) ' number from right
For k = Len(ls1) To 1 Step -1
li_mul = li_num * CInt(Mid(ls1, k, 1)) + li_elde ' ex : 7 times 7 = 49
If k = 1 Then
ls_mul = CStr(li_mul) + ls_mul
Else
ls_mul = CStr(li_mul Mod 10) + ls_mul ' get 9 from 49
li_elde = (li_mul - (li_mul Mod 10)) / 10 ' remains 4 from 49
End If
Next
' add extra zeros to the Right
For k = 1 To Len(ls2) - j + 1 - 1
ls_mul = ls_mul + "0"
Next
' store result as a one line string
ReDim Preserve ls_line(1 To Len(ls2) - j + 1)
ls_line(Len(ls2) - j + 1) = ls_mul
If Len(ls_mul) > li_maxlen Then li_maxlen = Len(ls_mul)
Next
li_linecount = UBound(ls_line)
' add extra zeros to the Left
For k = 1 To li_linecount
li_up = li_maxlen - Len(ls_line(k))
For j = 1 To li_up
ls_line(k) = "0" + ls_line(k)
Next
Next
' start summation
li_elde = 0
ls_mul = ""
For k = li_maxlen To 1 Step -1
li_sum = 0
For j = 1 To li_linecount
li_sum = li_sum + CInt(Mid(ls_line(j), k, 1))
Next
li_sum = li_sum + li_elde
If k = 1 Then
ls_mul = CStr(li_sum) + ls_mul
Else
ls_mul = CStr(li_sum Mod 10) + ls_mul
li_elde = (li_sum - (li_sum Mod 10)) / 10
End If
Next
Multiply = ls_mul
End Function
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