How do I convert VB's Double to COBOL's COMP-3? - vb.net

Question

Does anyone here know how to convert a VB Double to Cobol S9(15)V99 Comp-3 data type?

Answer 1

If it doesn't need to be done in the same program, it seems to me it would be easier to find a common format that both VB and COBOL can understand.
That would be text. In other words, the simplest solution may be to write the number out to a file as text "3.14159" and have the COBOL code read it in in that format and MOVE it to the COMP-3 field?
If that's not possible, the COMP-3 is a fairly simple BCD type. I would convert the number to a string anyway then take it two characters at a time into a byte array.
The S9(15)V99 requires 18 nybbles (a nybble being 4 bits, or half an octet) to store:
the integer bit (fifteen nybbles).
the fractional bit (two nybbles).
the sign (one nybble).
No space is needed for the decimal point since a V is an implied decimal, not a real one.
So the number 3.14 would be represented as the bytes:
00 00 00 00 00 00 00 31 4C
The only tricky bit is that final sign nybble (C for positive and D for negative).
Here's a bit of code I whipped up in Excel VBA (I don't have VB installed on this machine unfortunately) that shows you how to do it. The makeComp3() function should be easily transferred into a real VB program.
The macro test program outputs the values 0, 49 and 76 which are hex 00, 31 and 4C respectively (00314C is +3.14).
The first step (after all the declarations) is to make the double an implied decimal by multiplying it by the relevant power of ten then turning it into an integer:
Option Explicit
' makeComp3. '
' inp is the double to convert. '
' sz is the minimum final size (with sign). '
' frac is the number of fractional places. '
Function makeComp3(inp As Double, sz As Integer, frac As Integer) As String
Dim inpshifted As Double
Dim outstr As String
Dim outbcd As String
Dim i As Integer
Dim outval As Integer
Dim zero As Integer
zero = Asc("0")
' Make implied decimal. '
inpshifted = Abs(inp)
While frac > 0
inpshifted = inpshifted * 10
frac = frac - 1
Wend
inpshifted = Int(inpshifted)
Next, we make it into a string of the correct size, to make processing easier:
' Get as string and expand to correct size. '
outstr = CStr(inpshifted)
While Len(outstr) < sz - 1
outstr = "0" & outstr
Wend
If Len(outstr) Mod 2 = 0 Then
outstr = "0" & outstr
End If
Then we process that string two digits at a time and each pair is combined into an output nybble. The final step is to process the last digit along with the sign:
' Process each nybble pair bar the last. '
outbcd = ""
For i = 1 To Len(outstr) - 2 Step 2
outval = (Asc(Mid(outstr, i)) - zero) * 16
outval = outval + Asc(Mid(outstr, i + 1)) - zero
outbcd = outbcd & Chr(outval)
Next i
' Process final nybble including the sign. '
outval = (Asc(Right(outstr, 1)) - zero) * 16 + 12
If inp < 0 Then
outval = outval + 1
End If
makeComp3 = outbcd & Chr(outval)
End Function
And this is just the test harness, though it could probably do with a few more test cases :-)
Sub Macro1()
Dim i As Integer
Dim cobol As String
cobol = makeComp3(3.14159, 6, 2)
For i = 1 To Len(cobol)
MsgBox CStr(Asc(Mid(cobol, i)))
Next i
End Sub