6 Indirection

Indirection is the ability to modify or access an array at a set of selected index values. Blitz++ provides several forms of indirection:

• Using a list of array positions: this approach is useful if you need to modify an array at a set of scattered points.
• Cartesian-product indirection: as an example, for a two-dimensional array you might have a list I of rows and a list J of columns, and you want to modify the array at all (i,j) positions where i is in I and j is in J. This is a cartesian product of the index sets I and J.
• Over a set of strips: for efficiency, you can represent an arbitrarily-shaped subset of an array as a list of one-dimensional strips. This is a useful way of handling Regions Of Interest (ROIs).

In all cases, Blitz++ expects a Standard Template Library container. Some useful STL containers are list<>, vector<>, deque<> and set<>. Documentation of these classes is often provided with your compiler, or see also the good documentation at http://www.sgi.com/Technology/STL/. STL containers are used because they are widely available and provide easier manipulation of “sets” than Blitz++ arrays. For example, you can easily expand and merge sets which are stored in STL containers; doing this is not so easy with Blitz++ arrays, which are designed for numerical work.

STL containers are generally included by writing

#include <list>   // for list<>
#include <vector> // for vector<>
#include <deque>  // for deque<>
#include <set>    // for set<>

The [] operator is overloaded on arrays so that the syntax array[container] provides an indirect view of the array. So far, this indirect view may only be used as an lvalue (i.e. on the left-hand side of an assignment statement).

The examples in the next sections are available in the Blitz++ distribution in <examples/indirect.cpp>.