19.2 BitArray

A BitArray is similar to an ArrayList , except that it is used to store only bits represented as boolean values. A 1 bit is true , while a bit is false . BitArray objects are extremely useful for mathematical operations such as those involving long encryption keys. The class has the following public methods which are useful for operations on the bits it encapsulates: Xor , Not , Or , And .

The program below demonstrates how a BitArray can be used. The output is interspersed with the code to show the outputs of the different sections.

 1: using System;  2: using System.Collections;  3:  4: public class TestClass  {  5:    public static void Main()  {  6:  7:       // By default, all bits are false if not specified  8:       BitArray ba1 = new BitArray(4);  9:       Console.WriteLine("ba1"); 10:       Console.WriteLine("Length:" + ba1.Length); 11:       Console.WriteLine("Values:"); 12:       PrintCollection(ba1); 13: 

Output:

 ba1 Length:4 Values: False,False,False,False, 

Line 8 creates a new BitArray object containing four bits.

 14:       // Use of  Set  method 15:       Console.WriteLine("Setting elements 0 and 2"); 16:       ba1.Set(0,true); 17:       ba1.Set(2,true); 18:       PrintCollection(ba1); 19: 

Output:

 Setting elements 0 and 2 True,False,True,False, 

 20:       // Accessing a BitArray like an index 21:       Console.WriteLine("ba1[0] : " + ba1[0]); 22:       Console.WriteLine("ba1[1] : " + ba1[1]); 23:       Console.WriteLine("ba1[2] : " + ba1[2]); 24:       Console.WriteLine("ba1[3] : " + ba1[3]); 25: 

Output:

 ba1[0] : True ba1[1] : False ba1[2] : True ba1[3] : False 

A BitArray object can be used like an index ^[1] as demonstrated in these lines.

^[1] Apparently a public indexer is implemented within this class. For more information about C# indexers, see Chapter 21.

 26:       // Create a BitArray containing true bits 27:       BitArray ba2 = new BitArray(4, true); 28:       Console.WriteLine("ba2"); 29:       Console.WriteLine("Length:" + ba2.Length ); 30:       Console.WriteLine("Values:"); 31:       PrintCollection(ba2); 32: 

Output:

 ba2 Length:4 Values: True,True,True,True, 

In this case, a new BitArray object is created with an overloaded constructor which takes in a boolean value too. A BitArray object encapsulating four true bits is created.

 33:       // Passing in an array of byte values 34:       byte[] myBytes = new byte[4] {1, 2, 3, 4}; 35:       BitArray ba3 = new BitArray(myBytes); 36:       Console.WriteLine("ba3"); 37:       Console.WriteLine("Length:" + ba3.Length); 38:       Console.WriteLine("Values:"); 39:       PrintCollection(ba3); 40: 

Output:

 ba3 Length:32 Values: True,False,False,False,False,False,False,False, False,True,False,False,False,False,False,False, True,True,False,False,False,False,False,False, False,False,True,False,False,False,False,False, 

A byte takes up eight bits in C#. On line 35, a new BitArray object is created with a byte array containing four byte elements. In all, the BitArray object will have a size of 32 bits. Within the BitArray , the bits are ordered with the least significant bit first. So, decimal 1 is translated into 10000000 ₂ , 2 is translated into 01000000 ₂ and are saved as bits in the BitArray . Note that the BitArray object does not keep track of the byte boundaries “ as far as it is concerned , it simply stores a consecutive meaningless series of 32 bits.

 41:       // Passing in an array of int values 42:       int[]  myInts  = new int[4] {1, 2, 3, 4}; 43:       BitArray ba4 = new BitArray(myInts); 44:       Console.WriteLine("ba4"); 45:       Console.WriteLine("Length:" + ba4.Length); 46:       Console.WriteLine("Values:"); 47:       PrintCollection(ba4); 48: 

Output:

 ba4 Length:128 Values: True,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,True,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, True,True,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,True,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, False,False,False,False,False,False,False,False, 

In line 43, a new BitArray instance is created with an int array containing four int s. Each int is 16 bits long in C#. Hence decimal 1 is translated into 1000000000000000 ₂ (least significant bit first).

 49:       // Passing in an array of boolean values 50:       bool[] myBools = 51:         new bool[4] {true, false, true, true}; 52:       BitArray ba5 = new BitArray(myBools); 53:       Console.WriteLine("ba5"); 54:       Console.WriteLine("Length:" + ba5.Length ); 55:       Console.WriteLine("Values:"); 56:       PrintCollection(ba5); 57:    } 58: 

Output:

 ba5 Length:4 Values: True,False,True,True, 

The BitArray constructor is also overloaded to take in an array of bool s.

 59:   /* Print out the contents of the BitArray object 60:    * Only 8 elements are printed on each line for 61:    * easy reading. 62:    */ 63:   public static void PrintCollection(IEnumerable bArray){ 64:     IEnumerator enumerator = bArray.GetEnumerator(); 65:     int i = 0; 66:     while (enumerator.MoveNext()){ 67:       if (i>=8){ 68:         i = 0; 69:         Console.WriteLine(); // new line 70:       } 71:       i++; 72:       Console.Write(enumerator.Current + ","); 73:     } 74:     Console.WriteLine(); 75:   } 76: }