How to get the 9th Bit from a short

Question

Im writing a VB.NET app that reads in bytes. I have a short which contains 2 Bytes received from an outside source. I now need to get "Bit 8" from the high byte, i cant work out how to do this i can get "bit 1" back as true to tell me if the source is switched on but cant get "bit 8" which i assume is in the second byte.

i've tried

Dim bit8 = (p_Value And (1 >> 512 - 1)) <> 0

this works for bit 1

Dim bit1 = (p_Value And (1 << 1 - 1)) <> 0

Document of device gives me

low byte

bit 0 through to bit7

High byte

bit 8 *the one i want through to bit 15

I've searched but everything seems to be for single bytes.

Phil

could you please rewrite/edit your question? Be clear, help us to help you — Leandro Bardelli, Apr 22 '15 at 11:47
Your bit shifting looks somewhat suspect in the first line. I'd also recommend potentially using a [`Flags` enum](http://stackoverflow.com/questions/8447/what-does-the-flags-enum-attribute-mean-in-c). — James Thorpe, Apr 22 '15 at 11:51
If you're reading bytes, how or why do you end up with int16/short? — hometoast, Apr 22 '15 at 12:02

score 1 · Answer 1 · answered Apr 22 '15 at 11:57

1

You should be able to use this function and check the result which will be True if the bit is set (1) or False if the bit is not set (0):

Function IsBitSet(value As Integer, bit As Integer) As Boolean
    Return ((value And CInt(2 ^ bit)) > 0)
End Function

answered Apr 22 '15 at 11:57

Matt Wilko

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Curious: would casting a short to an int alter the way this one works? – hometoast Apr 22 '15 at 12:01
@hometoast - no it would just return False for the top order bits – Matt Wilko Apr 22 '15 at 12:03
Exponentiation seems like overkill. – dbasnett Apr 22 '15 at 12:05

score 1 · Accepted Answer · answered Apr 22 '15 at 12:02

1

If you are working with shorts you can use a left shift

    Dim shrt As Short = 256S ' bit 8 on

    If (shrt And 1 << 8) <> 0 Then
        Stop
    End If

answered Apr 22 '15 at 12:02

dbasnett

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score 0 · Answer 3 · answered Apr 22 '15 at 13:51

An alternative is to use the BitArray class. You would then have an array of booleans, one for each bit:

Public Sub Main()
    Dim value As Short = 123
    Dim ba As New BitArray(BitConverter.GetBytes(value))

    'Get 9th bit.  Bit indexing starts at 0
    Dim bit9 = ba(8)
End Sub

How to get the 9th Bit from a short

3 Answers3