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Posted 12 Jun 2006

My BitArray Class

, 12 Jun 2006
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A BitArray class with more function


There's a BitArray class in namespace System.Collections.BitArray. But there's a big problem in it.

if you define two bytes as:

byte[] bits = new byte[2];
bits[0] = 1;
bits[1] = 3;

And use them in constructor of BitArray class, the bits must be "0000000100000011" because the byte[0] is "00000001"(binary view of 1) and the byte[1] is "00000011"(binary view of 3). Constructor first must use the first byte and then use the second byte.

But it won't !

The bits in the BitArray class will be "100000001100000". as you can see, the constructor first inverse each byte and then use it. and if you inverse all bits you gain "0000001100000001" wich that's not what we want.

Why ?

As i understood the problem come from CPU ! i wrote a thread in MSDN forum <!-- Article Starts - DO NOT ADD HTML/BODY START TAGS--> i understood most of our cpu works little-endian (thank's nobugz in MSDN forum) it means they inverse bits.

for example if you send "00000001" to cpu it will store it in ram as "10000000" and when you read it cpu inverse it again and show you "00000001".

so our problem occur when we send "00000001" to cpu, cpu store it in ram as "10000000" and when we want the first bit the cpu return "1".

i think this problem occur in the constructor of System.Collections.BitArray. BitArray class must define a pointer to the first bit of array and start reading bits from there.

The program below show the problem. first it store 1 and 3 in two bytes. then use byte array for constructor. and show you a messagebox contain bits. it's not bad to change byte to int and 16 to 64 to see how problem will change. the constructor will inverse every 32 bits of data (int length)

byte[] bits = new byte[2];
bits[0] = 1;
bits[1] = 3;
BitArray BA = new BitArray(bits);
string st = string.Empty;
for(int i=0; i< 16; i++)
  st += Convert.ToInt16(BA.Get(i)).ToString();
MessageBox.Show(st); //source don't contain this test program

My BitArray Class

So i started to write my own bit array class. not even solve this problem also add more functions to it.

Inheritance Problem

First i want to write class that inherits from System.Collections.BitArray. but i don't know why microsoft defined BitArray class as a Sealed( notInheritable) class.

So i declared a ArrayList and store my bits in it. i have tried to my class be similar to BitArray class.

How it work's

I show an example of how my constructor work's for a byte array as an argument:

for each byte in array use Convert.ToString(TheByte,2). this function convert the byte to binary string. if our first byte be 43 this function return "101011". as you can see it's only 6 bit length. for store in our array a byte must be 8 bits length. so we call FixLength function to solve length problem. FixLength add two zero at the begining. and return "00101011"

Now we can add "00101011" to our array. we do this by calling AddBits function. this function add "false" to arrayList for each 0 and add "true" to arrayList for each 1 

public JIBitArray(byte[] bits)
  string st;
  foreach(byte b in bits)
    st = FixLength(Convert.ToString(b,2),8);
private string FixLength(string num,int length)
  while(num.Length < length)
    num = num.Insert(0,"0");
  return num;
}<FONT size=2>
private void AddBits(string bits)
  foreach(char ch in bits)
    if(ch == '0')
    else if(ch == '1')
      throw(new ArgumentException("bits Contain none 0 1 character"));
}<FONT size=2>

we do the same for other constructors.


i have defined Get, Set, SetAll, Count, Or, Xor, And, Not functions to do the same as Microsoft BitArray class do

And add more functions:

1. ShiftLeft, ShiftRight for shifting bits to left or right

To shift left simply remove bits from left and then add 0 bits to the end

2. GetLong, GetInt, GetByte, GetShort

This functions use to convert our bits to Long Array, Int Array Byte Array and Short Array

I explain one of them:


we need to calculate return array length. To do this:

int</FONT> ArrayBound = (int)Math.Ceiling((double)this._Bits.Count/8);

Then declare array for returning:

byte[] Bits = new byte[ArrayBound];

if out array lenght be 9 bits. we need to return byte array with length of 2. so the array bound will be 2 and it means the lenght of array must be 16 bits. for fixing the length of our array we declare Temp JIBitArray and Fix it length to ArrayBound * ByteLength(which is 8)

JIBitArray Temp = new JIBitArray();
Temp._Bits = this._Bits;
Temp = FixLength(Temp,ArrayBound * 8);

Convert each 8 bits block to a byte by using "Convert.ToByte(string,base)" function.

<FONT color=#0000ff size=2><P>for</FONT><FONT size=2>(</FONT><FONT color=#0000ff size=2>int</FONT><FONT size=2> i=0; i< Temp._Bits.Count; i += 8)</P><P>{</P><P>  Bits[i/8] = Convert.ToByte(Temp.SubJIBitArray(i,8).ToString(),2);</P><P>}</P></FONT>

And then return bits.

3. RemoveBeginingZeros

if you have JIBitArray contain "00001110101" RemoveBeginingZeros return "110101"

4. operators

i have declared operator functions for this class

& And, | Or, ^ Xor, >> ShiftRight, << ShiftLeft

5. SubJIBitArray

it works like SubString function for string class. SubJIBitArray(2,3) for "0110110101" return "101"


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About the Author

Hamed J.I
Web Developer
Iran (Islamic Republic of) Iran (Islamic Republic of)
I began programming with C/C++ when i was 15. Then try to learn VC++ but at the middle of my reading .NET came.

I began to read C# and VB.NET and also began designing basic websites by FrontPage and developed some websites for our school and some other companis.

Later learn Microcontroller and design some digital circuits with PIC microcontrollers for a industrial controller company.

As I learned SQL and ASP.NET developed some website such as news portals that are active now.

Now i'm a software student and teach programming in computer institues. And have my own job by getting projects from companies.

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Comments and Discussions

GeneralProblem with And method Pin
Member 187509017-Mar-08 3:00
memberMember 187509017-Mar-08 3:00 
Generalslow performance Pin
ZronZ7-Jan-07 22:08
memberZronZ7-Jan-07 22:08 
GeneralRe: slow performance [modified] Pin
yianfang28-Aug-07 15:51
memberyianfang28-Aug-07 15:51 
NewsThis guy saved my job!!!!! Pin
Peace2u20-Jun-06 20:44
memberPeace2u20-Jun-06 20:44 
GeneralRe: This guy saved my job!!!!! Pin
Hamed_ji31-Jul-06 12:24
memberHamed_ji31-Jul-06 12:24 
GeneralPretty redundant data type Pin
Alexey A. Popov13-Jun-06 0:47
memberAlexey A. Popov13-Jun-06 0:47 
GeneralRe: Pretty redundant data type Pin
Hamed_ji13-Jun-06 5:18
memberHamed_ji13-Jun-06 5:18 
GeneralRe: Pretty redundant data type Pin
reflex@codeproject13-Jun-06 6:10
memberreflex@codeproject13-Jun-06 6:10 
GeneralRe: Pretty redundant data type Pin
Hamed_ji13-Jun-06 7:51
memberHamed_ji13-Jun-06 7:51 
GeneralRe: Pretty redundant data type Pin
Alexey A. Popov13-Jun-06 7:33
memberAlexey A. Popov13-Jun-06 7:33 
GeneralRe: Pretty redundant data type Pin
Hamed_ji13-Jun-06 8:40
memberHamed_ji13-Jun-06 8:40 
AnswerRe: Pretty redundant data type Pin
Drazen Dotlic14-Jun-06 0:34
memberDrazen Dotlic14-Jun-06 0:34 
AnswerRe: Pretty redundant data type Pin
Hamed_ji31-Jul-06 12:22
memberHamed_ji31-Jul-06 12:22 
AnswerRe: Pretty redundant data type Pin
Drazen Dotlic31-Jul-06 21:20
memberDrazen Dotlic31-Jul-06 21:20 
GeneralRe: Pretty redundant data type Pin
Hamed_ji31-Jul-06 23:22
memberHamed_ji31-Jul-06 23:22 
GeneralRe: Pretty redundant data type Pin
Alexey A. Popov14-Jun-06 6:25
memberAlexey A. Popov14-Jun-06 6:25 
OK, let me answer step by step.
First, when you say that a class cannot provide everything you need, I agree. If we had a class that provided everything we would be out of this busyness Smile | :) However, a class is either a unit (like the System.String) or a template (being it a generic, abstract or just a class with virtual functions). The unit cannot and should not be extended/changed/tampered with, only used. That's why strings (and bit arrays) are sealed.
Second, when you say that BitArray cannot be converted to and from any kind of array, you're wrong. Just take a look at the documentation, CopyTo method and three constructors do just that. However, you are correct when saying that the BitArray does not support bit shifting. This is bad. But BitArray is not a kind of integer (and shfts are applicable to binary integers), it's just a bit map, an array of flags. And BitArray works perfectly for it's purpose.
Third, as far as I remember, endianness does not affect order of bits in a byte, there's no htob/btoh functions and IPAddredd declares HostToNetworkOrder/NetworkToHostOrder only for shorts/ints/longs. So sending BitArrays over the wire as an array of bytes is not an issue. Why am I talking about it then? Because it the only place where you need to bypass the BitArray's interface and get a hand on the raw data.
Fourth, you're correct that there's no SubArray method for the BitArray. Frankly, I've never needed one, but on this I agree, it's better to have this method. Again, you can easyly emulate this method either by copying a range of bits bit by bit or by converting the BitArray to an array of booleans, extracting the required range with Array.Copy and creating a new BitArray. In .Net 3.0 you'll be able to create a kinda mixin with that functionality for the BitArray.
Fifth, when working with the BitArray you don't have to worry about the order of bits, use indexer and forget about pointers (I shudder) because we are in the world of CLR, far away from C/C++, and there's Smalltalk shining above the horizon.
Sixth, when you work with encription, you do need big integers, but not only for logical operators and bit shifts but also for calculations. The BitArray cannot perform arithmetics, that's the fact of life. There are plenty of implementations for big integers, you can find at least two here, on the CodeProject, and one in the CLR extensions for Java/J#.
And seventh, boxing/unboxing of booleans might be good or acceptable in test applications. In a real-world applicaton heavy boxing would hit GC hard eliminating all advantages of ArrayList.

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