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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