6.3 Indirection with lists of strips

You can also do indirection with a container of one-dimensional strips. This is useful when you want to manipulate some arbitrarily-shaped, well-connected subdomain of an array. By representing the subdomain as a list of strips, you allow Blitz++ to operate on vectors, rather than scattered points; this is much more efficient.

Strips are represented by objects of type RectDomain<N>, where N is the dimensionality of the array. The RectDomain<N> class can be used to represent any rectangular subdomain, but for indirection it is only used to represent strips.

You create a strip by using this function:

RectDomain<N> strip(TinyVector<int,N> start,
                    int stripDimension, int ubound);

The start parameter is where the strip starts; stripDimension is the dimension in which the strip runs; ubound is the last index value for the strip. For example, to create a 2-dimensional strip from (2,5) to (2,9), one would write:

TinyVector<int,2> start(2,5);
RectDomain<2> myStrip = strip(start,secondDim,9);

Here is a more substantial example which creates a list of strips representing a circle subset of an array:

  const int N = 7;
  Array<int,2> A(N,N), B(N,N);
  typedef TinyVector<int,2> coord;

  A = 0;
  B = 1;

  double centre_i = (N-1)/2.0;
  double centre_j = (N-1)/2.0;
  double radius = 0.8 * N/2.0;

  // circle will contain a list of strips which represent a circular
  // subdomain.

  list<RectDomain<2> > circle;
  for (int i=0; i < N; ++i)
  {
    double jdist2 = pow2(radius) - pow2(i-centre_i);
    if (jdist2 < 0.0)
      continue;

    int jdist = int(sqrt(jdist2));
    coord startPos(i, int(centre_j - jdist));
    circle.push_back(strip(startPos, secondDim, int(centre_j + jdist)));
  }

  // Set only those points in the circle subdomain to 1
  A[circle] = B;

After this code, the A array contains:

  0  0  0  0  0  0  0
  0  0  1  1  1  0  0
  0  1  1  1  1  1  0
  0  1  1  1  1  1  0
  0  1  1  1  1  1  0
  0  0  1  1  1  0  0
  0  0  0  0  0  0  0