Bit manipulation question.

[Angahran]

Hi,
I've been trying to do something that seems like ist should be simple but I'm not having any luck.

I have an integer field with values from 0 to 65535 which is 16 flags.
What I want to do is test for each of these flags individually and display if they are true or false.

However, when I try ANDing the field with 1, 2, 4, etc to get the bits, it always returns -1 (unless the field is 0 then it returns 0).

Thought of manually making a lookup table but the thought of typing 65535 rows of yes/no made my hands start to ache.

Sounds simple, but I'm pulling my hair out trying to get it to work.

Any suggestions much appreciated.

[FishVal]

Hi, Angahran.

It looks like the operation is performed as boolean (True=-1, False=0). I couldn't reproduce your situation. I get bitwise operators AND, OR, XOR working as expected. Could you post an exact code you have?

Regards,
Fish

[Angahran]

Here's the dummy code I've been trying out.

classes is the field in the table and bit1 is my output field.
Eventually I'd like to be able to check all 16 bits, but getting 1 to work would be a good start :P

Private Sub Command6_Click()
On Error GoTo Err_Command6_Click

bit1.Value = classes.Value & 1

Exit_Command6_Click:
Exit Sub

Err_Command6_Click:
MsgBox Err.Description
Resume Exit_Command6_Click

End Sub

[FishVal]

:)

VBA syntax differs form C one.
"&" in VBA is "AND"
"|" in VBA is "OR"
"^" in VBA is "XOR"

The following simple function checks specified bit in Integer(word).

Public Function CheckBit(ByVal intInput As Integer, _
                         ByVal intBitNumber As Integer) As Boolean
 
    CheckBit = intInput And 2 ^ intBitNumber
 
End Function
 

[Angahran]

Thanks :)
Was getting overflow on anything above 32767 so changed to Double.

Public Function DblCheckBit(ByVal dblInput As Double, _
ByVal dblBitNumber As Double) As Boolean
DblCheckBit = dblInput And 2 ^ dblBitNumber
End Function

Now going to dummy up a form with 'Classes' and 16 True/Falses and see how that looks.

Thanks again.

[FishVal]

Better change it to Long, though storing 16 bit values you don't need it as soon as Integer values above 32767 are represented as negative (VBA doesn't have unsighned types).

Regards,
Fish

[ADezii]

Hi,
I've been trying to do something that seems like ist should be simple but I'm not having any luck.

I have an integer field with values from 0 to 65535 which is 16 flags.
What I want to do is test for each of these flags individually and display if they are true or false.

However, when I try ANDing the field with 1, 2, 4, etc to get the bits, it always returns -1 (unless the field is 0 then it returns 0).

Thought of manually making a lookup table but the thought of typing 65535 rows of yes/no made my hands start to ache.

Sounds simple, but I'm pulling my hair out trying to get it to work.

Any suggestions much appreciated.

I have an integer field with values from 0 to 65535 which is 16 flags

The maximum allowable value for an Integer is 32767 (a Bit is assigned to indicate whether or not the Number is Positive or Negative).

Here is a little something I threw together for you. Take a look at it and see if it is what you need. Simply pass to the Function a Long Integer and let it do the work for you:

Public Function fConvertToBinary(ByVal lngValueToConvert As Long) As String
'Converts a Long Integer into its Binary equivalent and returns each Bit
'position indicating whether it is set or not
Dim strBinary As String, strReturn As String, intCounter As Integer
Dim intBits As Integer, intBitPosition As Integer
 
intBits = 24
intCounter = (Len(str$(lngValueToConvert)) - 1) * 4
 
'Create a String that is large enough to hold the Bits so that we can use the
'Mid Statement to replace the zeros with the real Bit Values
strBinary = String(intCounter, "0")
 
Do While lngValueToConvert
  'The remainder is the Bit Value,so set this in the String
  Mid$(strBinary, intCounter, 1) = CStr(lngValueToConvert Mod 2)
  lngValueToConvert = lngValueToConvert \ 2
  'Decrement intCounter, so we'll know which Bit to set in strBinary
  intCounter = intCounter - 1
Loop
 
For intCounter = 1 To (intBits \ 4)
  strReturn = strReturn & Left$(strBinary, 4)
  strBinary = Mid$(strBinary, 5)
Next intCounter
 
Debug.Print "*******************************"
Debug.Print strReturn
Debug.Print "*******************************"
For intCounter = Len(strReturn) To 1 Step -1
  intBitPosition = intCounter - (Len(strReturn) + 1)
  Debug.Print "Bit #" & Format$(intBitPosition, "00") & " - " & _
              IIf(Mid$(strReturn, intCounter, 1) = 1, "True", "False")
Next
End Function

'Function Call
Call fConvertToBinary(65535)

OUTPUT:

*******************************
00001111111111111111
*******************************
Bit #-01 - True
Bit #-02 - True
Bit #-03 - True
Bit #-04 - True
Bit #-05 - True
Bit #-06 - True
Bit #-07 - True
Bit #-08 - True
Bit #-09 - True
Bit #-10 - True
Bit #-11 - True
Bit #-12 - True
Bit #-13 - True
Bit #-14 - True
Bit #-15 - True
Bit #-16 - True
Bit #-17 - False
Bit #-18 - False
Bit #-19 - False
Bit #-20 - False

[Angahran]

Thanks for all the help.

Slowly beating this form into shape, but seems like there's a new problem hiding behind every line of code.

Apparently, I have hit some maximum number of fields limit on my form when I tried linking the lookup tables I had created.

So, I thought I'd try working without the lookup tables and check the bits as part of the main query.

Here's a dummy part showing what I was trying to do.

SELECT classes FROM qryItems WHERE (CheckBit(classes,0) = 'True');

Unfortunately, no matter how I specify the 'True' part I get a type mismatch error and Access '07 crashes :(
I've tried single quotes, double quotes, no quotes, all with the same type mismatch error.