Memory allocation

There are several options for allocating memory for matrices and higher-dimensional arrays. Memory can be allocated statically or dynamically and contiguously or not contiguously. Library routines typically need array elements to be contiguous in memory.

• Static contiguous:
  double foo[numcol*numrow]; 
  foo[col+row*numcol]=0.0;
• Static not contiguous
  double foo[numrow][numcol] 
  foo[row][col]=0.0;
• Dynamic contiguous:
  double* foo=new double[numcol*numrow]; 
  foo[col+row*numcol]=0.0; 
  delete [] foo;
• Dynamic not-contiguous:
  double** foo=new (double*)[numrow]; 
  for(int i=0;i<numrow;i++){ 
     foo[i]=new double[numcol]; 
  } 
  foo[row][col]=0.0; 
  for(int i=0;i<numrow;i++){ 
     delete [] foo[i]; 
  delete [] foo;

Notice that for contiguous arrays, you need to calculate the location of the element of interest. There is a trick for getting around this:

double* tmp=new double[numcol*numrow]; 
double** foo=new (double*)[numrow]; 
for(int i=0;i<numrow;i++){ 
   foo[i]=&(tmp[i*numcol]); 
} 
// now foo[i] is a pointer to the start of each row, and we can use row,col indexing 
foo[row][col]=0.0; 
delete [] tmp; // delete with a single call 
// plus, we can pass foo to routines requiring a contiguous array