Reading bits from bytes

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Hi all.

I'm having a little trouble. I need to read bits from a byte, the idea is that I read a byte from a stream and the I have to read bits from that byte.
A simple example would be I read a byte "A" which would be byte "065". Now I need to read that as "01000001", well actually I need to read it as "0","1","0","0","0","0","0","1" or any other interpretation like True or False it's irrelevant.

Hypothetically, the code would count how many "1"(Trues) are there in a file.

I can live with any encoding as long as I can read any byte in the data as the data I need to read is God only knows what.

I've tried a couple of things but none of them worked as it should.
I just can't get to the bottom of it. I read bytes no problem but just can't seem to get the bits part done.

I'd appreciate any code sample. Thanks.

Posted 10-Oct-10 3:37am

Holc

Updated 17-Oct-21 14:59pm

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2 solutions

Solution 3

If all you want is counting the number of bits set in a large array of bytes, there is no need to perform bit operations at all: all it takes is a magic array that converts all possible byte values to the number of bits set in that byte value.

C#

int[] bitsSet=new int[256] {0,1,1,2, 1,2,2,3, 1,2,2,3, 2,3,3,4 ...};

There are many ways to come up with the correct values for this array:
1. simply counting bits using bit operations;
2. constructing the array dynamically, e.g. using 8 nested for loops, each representing one bit being clear or set;
3. detecting a repetition in the number pattern;
4. using recursion.

Whatever you choose, once the bitsSet array is available, you'll get way more performance out of it, as you are taking care of 8 bits at a time by using a simple array lookup and one addition.

:)

Posted 17-Oct-21 14:59pm

Luc Pattyn

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Richard Deeming 18-Oct-21 5:37am

Hopefully the OP managed to solve their question in the last eleven years. :)

It was dragged back into the "active" list by the repost of solution 1 posted as solution 2.

Luc Pattyn 18-Oct-21 11:14am

Quick Answers it is :D

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DaveAuld · Accepted Answer · 2010-10-10T03:53:00

Have a read of this article on BitWise operations to determine the state of individual bits. That was the original way of doing things.
http://visualbasic.about.com/od/usingvbnet/a/bitops01.htm[^]

We now also have BitArray available e.g.;

VB

Dim x As Byte
x = &H65
Dim ba As New BitArray({x})
MsgBox("Bit Status; " + ba.Get(7).ToString + _
                    " " + ba.Get(6).ToString + _
                    " " + ba.Get(5).ToString + _
                    " " + ba.Get(4).ToString + _
                    " " + ba.Get(3).ToString + _
                    " " + ba.Get(2).ToString + _
                    " " + ba.Get(1).ToString + _
                    " " + ba.Get(0).ToString)