Vector representation of genreal sparse Matrix

#include <iostream>
using namespace std;
#define m 4
#define n 4
int r=0;

typedef struct sparsematrix
{
  int row,col,data;
}sm;


int main()
{   
    sm c[r];
    int a[m][n]={{0,0,3,4},
                        {0,5,6,0},
                        {1,0,0,9},
                        {0,0,0,8}};  
for(int i=0;i<m;i++)
  {
    for(int j=0;j<n;j++)
    { 
      if (a[i][j]!=0)
      { 
        c[r].row=i;
        c[r].col=j;
        c[r].data=a[i][j];
        r++;
      }
    }
  }
  cout<<"Row  "<<"Column  "<<"data "<<endl;
  for(int i=0;i<r;i++)
  {     cout<<c[i].row<<"         ";
        cout<<c[i].col<<"             ";
        cout<<c[i].data<<endl;
  }
}