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C++: Identity Matrix

Bjarne-stroustrup
 


Build an identity matrix of a size known at runtime.

An identity matrix is a square matrix, of size n × n, where the diagonal elements are all 1s, and the other elements are all 0s.

I_n = \begin{bmatrix} 1 & 0 & 0 & \cdots & 0 \\ 0 & 1 & 0 & \cdots & 0 \\ 0 & 0 & 1 & \cdots & 0 \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & 0 & \cdots & 1 \\ \end{bmatrix}

Library: STL

template<class T>
class matrix
{
public:
    matrix( unsigned int nSize ) : 
      m_oData(nSize * nSize, 0), m_nSize(nSize) {}
 
      inline T& operator()(unsigned int x, unsigned int y)
      {
          return m_oData[x+m_nSize*y];
      }
 
      void identity()
      {
          int nCount = 0;
          int nStride = m_nSize + 1;
          std::generate( m_oData.begin(), m_oData.end(), 
              [&]() { return !(nCount++%nStride); } );
      }
 
      inline unsigned int size() { return m_nSize; }
 
private:
    std::vector<T>    m_oData;
    unsigned int      m_nSize;
};
 
int main()
{
    int nSize;
    std::cout << "Enter matrix size (N): ";
    std::cin >> nSize;
 
    matrix<int> oMatrix( nSize );
 
    oMatrix.identity();
 
    for ( unsigned int y = 0; y < oMatrix.size(); y++ )
    {
        for ( unsigned int x = 0; x < oMatrix.size(); x++ )
        {
            std::cout << oMatrix(x,y) << " ";
        }
        std::cout << std::endl;
    }
    return 0;
}

Library: boost

#include <boost/numeric/ublas/matrix.hpp>
 
int main()
{
    using namespace boost::numeric::ublas;
 
    int nSize;
    std::cout << "Enter matrix size (N): ";
    std::cin >> nSize;
 
    identity_matrix<int> oMatrix( nSize );
 
    for ( unsigned int y = 0; y < oMatrix.size2(); y++ )
    {
        for ( unsigned int x = 0; x < oMatrix.size1(); x++ )
        {
            std::cout << oMatrix(x,y) << " ";
        }
        std::cout << std::endl;
    }
 
    return 0;
}
Output:
Enter matrix size (N): 5
1 0 0 0 0
0 1 0 0 0
0 0 1 0 0
0 0 0 1 0
0 0 0 0 1

SOURCE

Content is available under GNU Free Documentation License 1.2.