Iterator Filters

Filtering adaptors for Aleph iterators, including graph arc/node filters. More...

Files
file	filter_iterator.H
	Generic filter iterator wrapper for Aleph containers.

Detailed Description

Filtering adaptors for Aleph iterators, including graph arc/node filters.

This module documents iterator-level filtering in Aleph-w.

Aleph-w provides filtering in two complementary ways:

1. Generic filtered iterators using filter_iterator.H (Aleph::Filter_Iterator)
2. Graph-specific filtered iterators (nodes/arcs) in tpl_graph.H, built on top of Aleph::Filter_Iterator

1) Generic <tt>Filter_Iterator</tt>

Aleph::Filter_Iterator wraps an Aleph iterator and hides elements that do not satisfy a predicate.

Key points:

• The predicate type is the template parameter Show_Item.
• The wrapper preserves the usual Aleph iterator interface (has_curr(), get_curr(), next() / next_ne(), etc.).
• You can attach context via set_cookie() / get_cookie() if needed.

#include <filter_iterator.H>
#include <tpl_dynDlist.H>
 
using namespace Aleph;
 
struct ShowEven {
  bool operator()(int x) const { return x % 2 == 0; }
};
 
int main()
{
  DynDlist<int> xs;
  for (int i = 1; i <= 10; ++i)
    xs.append(i);
 
  Filter_Iterator<DynDlist<int>, DynDlist<int>::Iterator, ShowEven> it(xs);
 
  for (; it.has_curr(); it.next())
    {
      auto x = it.get_curr();
      (void) x;
    }
}

2) Graph arc/node filters (practical example)

Graph traversal in Aleph-w often exposes a filter hook via a functor parameter (commonly called Show_Arc or Show_Node). This is used throughout tpl_graph.H and shortest-path algorithms like Dijkstra.H.

Example: exclude "highways" from shortest-path

If your arc payload indicates whether an arc represents a highway, you can provide a Show_Arc filter so that Dijkstra ignores those arcs.

#include <tpl_graph.H>
#include <Dijkstra.H>
 
using namespace Aleph;
 
struct RoadInfo {
  double distance;
  bool is_highway;
};
 
using G = List_Graph<Graph_Node<int>, Graph_Arc<RoadInfo>>;
 
struct RoadDistance {
  using Distance_Type = double;
  double operator()(G::Arc * a) const noexcept { return a->get_info().distance; }
};
 
struct NoHighways {
  bool operator()(G::Arc * a) const noexcept { return !a->get_info().is_highway; }
};
 
int main()
{
  G g;
  auto * s = g.insert_node(0);
  auto * t = g.insert_node(1);
 
  g.insert_arc(s, t, RoadInfo{10.0, true});   // highway
  g.insert_arc(s, t, RoadInfo{12.0, false});  // regular road
 
  Dijkstra_Min_Paths<G, RoadDistance, Node_Arc_Iterator, NoHighways> dij(RoadDistance{}, NoHighways{});
 
  Path<G> path(g);
  auto d = dij(g, s, t, path);
  (void) d;
}

Notes:

• The filter functor (NoHighways) is the SA template parameter in Dijkstra_Min_Paths.
• The distance functor (RoadDistance) defines how arc weights are read.