[RESOLVED] Convert a floating-point no. to its true/complete numeric value

[anhn]

There are many ways to multiply BIG integers (fast or slow).
We had discussed about this a year ago on these forums (one of the threads are between me and dbasnett).
Search "BigInteger" and/or "LargeInteger" to find more.

This is a way to multiply 2 "digit string", it is not very fast because it use a "primary school method" to multiply.
It will be quicker if chop "number strings" into smaller parts of 4 digits (use with Long) or 7 digits (use with Currency) or 14 digits (use with Decimal Variant).
In this function, it uses 1-digit parts with Byte data type.

Sub BigTest()
    Dim sNum1 As String
    Dim sNum2 As String
    Dim sNum3 As String
    
    sNum1 = "123456789123456789123456789123456789123456789123456789123456" & _
            "789123456789123456789123456789123456789123456789"
    sNum2 = "987654321987654321987654321987654321987654321987654321987654" & _
            "321987654321987654321987654321987654321987654321"

    sNum3 = BigMultiply(sNum1, sNum2)
    
    Debug.Print sNum3

'-- sNum3 = "121932631356500531591068431825636332060204232294772132529340" & _
            "032763907932998475833233043733467611633702179533692882171458" & _
            "314271223746370989178470754610570520042670285474770050906869" & _
            "816338969581771069347203169112635269"
End Sub

Function BigMultiply(ByVal sNum1 As String, sNum2 As String) As String
    Dim d1 As Byte, d2 As Byte, r As Byte, d As Byte
    Dim n1 As Long, n2 As Long, n3 As Long
    Dim i As Long, j As Long, k As Long, n As Long
    Dim bn1() As Byte, bn2() As Byte
    Dim Matrix() As Byte
    Dim sResult As String
    
    bn1 = StrConv(sNum1, vbFromUnicode)
    bn2 = StrConv(sNum2, vbFromUnicode)
    For i = 0 To UBound(bn1): bn1(i) = bn1(i) - 48: Next
    For i = 0 To UBound(bn2): bn2(i) = bn2(i) - 48: Next
    n1 = UBound(bn1) + 1
    n2 = UBound(bn2) + 1
    n3 = n1 + n2
    
    ReDim Matrix(1 To n2, 1 To n3)
    
    sResult = String$(n3, "0")
    
    For i = n2 To 1 Step -1
        r = 0
        For j = n1 To 1 Step -1
            n = bn2(i - 1) * bn1(j - 1) + r '<<-- updated
            d = n Mod 10
            r = n \ 10
            k = i + j
            Matrix(i, k) = d
        Next
        If r > 0 Then Matrix(i, k - 1) = r
    Next
    
    n = 0
    For k = n3 To 1 Step -1
        For i = n2 To 1 Step -1
            n = n + Matrix(i, k)
        Next
        Mid$(sResult, k, 1) = n Mod 10
        n = n \ 10
    Next
    
    If n > 0 Then             '-- there was a mistake in these 2 lines:  
        sResult = n & sResult '-- n must be used instead of r
    Else
        Do While Left$(sResult, 1) = "0"
            sResult = Mid$(sResult, 2)
        Loop
    End If
    
    BigMultiply = sResult

End Function