| // -*- C++ -*- |
| |
| // Copyright (C) 2005, 2006 Free Software Foundation, Inc. |
| // |
| // This file is part of the GNU ISO C++ Library. This library is free |
| // software; you can redistribute it and/or modify it under the terms |
| // of the GNU General Public License as published by the Free Software |
| // Foundation; either version 2, or (at your option) any later |
| // version. |
| |
| // This library is distributed in the hope that it will be useful, but |
| // WITHOUT ANY WARRANTY; without even the implied warranty of |
| // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| // General Public License for more details. |
| |
| // You should have received a copy of the GNU General Public License |
| // along with this library; see the file COPYING. If not, write to |
| // the Free Software Foundation, 59 Temple Place - Suite 330, Boston, |
| // MA 02111-1307, USA. |
| |
| // As a special exception, you may use this file as part of a free |
| // software library without restriction. Specifically, if other files |
| // instantiate templates or use macros or inline functions from this |
| // file, or you compile this file and link it with other files to |
| // produce an executable, this file does not by itself cause the |
| // resulting executable to be covered by the GNU General Public |
| // License. This exception does not however invalidate any other |
| // reasons why the executable file might be covered by the GNU General |
| // Public License. |
| |
| // Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL. |
| |
| // Permission to use, copy, modify, sell, and distribute this software |
| // is hereby granted without fee, provided that the above copyright |
| // notice appears in all copies, and that both that copyright notice |
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| // warranty. |
| |
| /** |
| * @file priority_queue_dijkstra_example.cpp |
| * A basic example showing how to cross reference a vector and a |
| * priority-queue for modify. |
| */ |
| |
| /** |
| * This example shows how to cross-reference priority queues |
| * and a vector. I.e., using a vector to |
| * map keys to entries in a priority queue, and using the priority |
| * queue to map entries to the vector. The combination |
| * can be used for fast modification of keys. |
| * |
| * As an example, a very simple form of Diskstra's algorithm is used. The graph |
| * is represented by an adjacency matrix. Nodes and vertices are size_ts, and |
| * it is assumed that the minimal path between any two nodes is less than 1000. |
| */ |
| |
| |
| |
| #include <vector> |
| #include <iostream> |
| #include <ext/pb_ds/priority_queue.hpp> |
| |
| using namespace std; |
| using namespace pb_ds; |
| using namespace pb_ds; |
| |
| // The value type of the priority queue. |
| // The first entry is the node's id, and the second is the distance. |
| typedef std::pair<size_t, size_t> pq_value; |
| |
| // Comparison functor used to compare priority-queue value types. |
| struct pq_value_cmp : public binary_function<pq_value, pq_value, bool> |
| { |
| inline bool |
| operator()(const pq_value& r_lhs, const pq_value& r_rhs) const |
| { |
| // Note that a value is considered smaller than a different value |
| // if its distance is* larger*. This is because by STL |
| // conventions, "larger" entries are nearer the top of the |
| // priority queue. |
| return r_rhs.second < r_lhs.second; |
| } |
| }; |
| |
| int main() |
| { |
| enum |
| { |
| // Number of vertices is hard-coded in this example. |
| num_vertices = 5, |
| // "Infinity". |
| graph_inf = 1000 |
| }; |
| |
| // The edge-distance matrix. |
| // For example, the distance from node 0 to node 1 is 5, and the |
| // distance from node 1 to node 0 is 2. |
| const size_t a_a_edge_legnth[num_vertices][num_vertices] = |
| { |
| {0, 5, 3, 7, 6}, |
| {2, 0, 2, 8, 9}, |
| {2, 1, 0, 8, 0}, |
| {1, 8, 3, 0, 2}, |
| {2, 3, 4, 2, 0} |
| }; |
| |
| // The priority queue type. |
| typedef pb_ds::priority_queue< pq_value, pq_value_cmp> pq_t; |
| |
| // The priority queue object. |
| pq_t p; |
| |
| // This vector contains for each node, a find-iterator into the |
| // priority queue. |
| vector<pq_t::point_iterator> a_it; |
| |
| // First we initialize the data structures. |
| |
| // For each node, we push into the priority queue a value |
| // identifying it with a distance of infinity. |
| for (size_t i = 0; i < num_vertices; ++i) |
| a_it.push_back(p.push(pq_value(i, graph_inf))); |
| |
| // Now we take the initial node, in this case 0, and modify its |
| // distance to 0. |
| p.modify(a_it[0], pq_value(0, 0)); |
| |
| // The priority queue contains all vertices whose final distance has |
| // not been determined, so to finish the algorithm, we must loop |
| // until it is empty. |
| while (!p.empty()) |
| { |
| // First we find the node whose distance is smallest. |
| const pq_value& r_v = p.top(); |
| const size_t node_id = r_v.first; |
| const size_t dist = r_v.second; |
| |
| // This is the node's final distance, so we can print it out. |
| cout << "The distance from 0 to " << node_id |
| << " is " << dist << endl; |
| |
| // Now we go over the node's neighbors and "relax" the |
| // distances, if applicable. |
| for (size_t neighbor_i = 0; neighbor_i < num_vertices; ++neighbor_i) |
| { |
| // Potentially, the distance to the neighbor is the distance |
| // to the currently-considered node + the distance from this |
| // node to the neighbor. |
| const size_t pot_dist = dist + a_a_edge_legnth[node_id][neighbor_i]; |
| |
| // "Relax" the distance (if appropriate) through modify. |
| if (pot_dist < a_it[neighbor_i]->second) |
| p.modify(a_it[neighbor_i], pq_value(neighbor_i, pot_dist)); |
| } |
| |
| // Done with the node, so we pop it. |
| p.pop(); |
| } |
| |
| return 0; |
| } |