Bit Vector — Sequences

A bit_vector is more like a vector < bool >

Definition

Bit_vector is defined in the standard header <vector>, and in the nonstandard backward-compatibility header <bvector.h>.

Description

A bit_vector is like a vector<bool>. The difference between the two is that a vector requires at least one byte per element, but a bit_vector only requires one bit per element.

Short example of using bit_vector:

bit_vector V(5);
V[0] = true;
V[1] = false;
V[2] = false;
V[3] = true;
V[4] = false;

for (bit_vector::iterator i = V.begin(); i < V.end(); ++i)
  cout << (*i ? '1' : '0');
cout << endl;

If you need a container for a variable number of bits or Boolean values, you can use the class bit_vector, otherwise if you need a bitfield with static size, you should use bitset.

Bit_vector Operations

Operation	Effect
c.flip()	Negates all Boolean elements (complement of all bits)
m[i].flip()	Negates the Boolean element with index i (complement of a single bit)
m[i]=`val`	Assigns val to the Boolean element with index i (assignment to a single bit)
m[i1] = m[i2]	Assigns the value of the element with index i2 to the element with index i1

References

http://www.sgi.com
Nicolai M. Josuttis: "The C++ Standard Library"
http://www.java2s.com
http://msdn.microsoft.com/en-us
Kris Jamsa, Lars Klander: "Jamsa's C/C++ Programmer's Bible"

Example 1

Problem

This example of program illustrates examples of implicit casts done by the ostream class to a variable of bit_vector.

Workings

#include <vector>
#include <iostream>

using namespace std;

int main( )
{
   bit_vector v; 

   v.push_back(false);
   bit_vector::reference ref1=v.at(0);
   cout <<ref1<<endl; // ref1 implicitly cast to bool 

   bool b1; 

   // one form of an explicit cast
   b1=ref1.operator bool( );
   cout <<b1<<endl; 

   // another form of an explicit cast
   b1=bool(ref1);
   cout <<b1<<endl;

   return 0;
}

Solution

Output:

0
0
0

Example 2

Problem

This program illustrates some simple operations using a bit_vector.

Workings

#include <vector>
#include <iostream>

using namespace std;

int main ()
{

    // instantiate a bit_vector to hold 3 elements
    bit_vector v(3);

    // assign 3 elements using the array operator []
    v[0] = true;
    v[1] = true;
    v[2] = false;

    // insert a 4th element using push_back:
    // this will cause the vector to resize the buffer
    v.push_back (true);

    cout <<"The contents of the vector are: "<<endl<<"{";
    for (size_t i = 0; i < v.size(); ++i)
        cout <<v[i]<<' ';
    cout <<"}"<<endl<<endl;

    v.flip();

    cout <<"The flipped contents of the vector are: "<<endl<<"{";
    for (size_t i = 0; i < v.size(); ++i)
        cout <<v[i]<<' ';
    cout <<"}";

    return 0;
}

Solution

Output:

The contents of the vector are:{1 1 0}
The flipped contents of the vector are:{0 1 1}

Example 3

Problem

This program modifies the program used in http://codecogs.izyba.com/reference/computing/containers/sequences/vector.php"Example 1" using a bit_vector instead of vector.

Workings

#include <iostream>
#include <vector>

using namespace std;
const SIZE=4;

int main()
{
  // dynamically allocated vector initially contains no elements
  bit_vector v(4);
  for (int i=0; i<SIZE; i++)
    if (i>1)
      v.push_back(i-2);
    else
      v.push_back(i);
  cout <<"First element is: "<<v.front()<<"."<<endl;
  cout <<"Last element is: "<<v.back()<<"."<<endl;
  cout <<"The vector contains "<<v.size()<<" elements."<<endl;

  cout <<"Erase the last element."<<endl;
  v.erase(v.end()-1); // erase the last element in the vector
  cout <<"The new last element is "<<v.back()<<"."<<endl;
  
  cout <<"Erase the first element."<<endl;
  v.erase(v.begin()); // erase the first element in the vector
  cout <<"The new first element is "<<v.front()<<"."<<endl;

  cout <<"The vector now contains "<<v.size()<<" elements."<<endl; 

  return 0;
}

Solution

Output:

First element is: 0
Last element is: 1
The vector contains 4 elements.
Erase the last element.
The new last element is 0.
Erase the first element.
The new first element is 1.
The vector now contains 2 elements.