# Example: graphs_dijkstra_example

This example demonstrates how to use MRPT C++ implementation of the Dijkstra algorithm for searching optimal paths in a directed, weighted graph. See also the docs for the class mrpt::graphs::CDijkstra

C++ example source code:

```/* +------------------------------------------------------------------------+
|                     Mobile Robot Programming Toolkit (MRPT)            |
|                          https://www.mrpt.org/                         |
|                                                                        |
| Copyright (c) 2005-2022, Individual contributors, see AUTHORS file     |
| Released under BSD License. See: https://www.mrpt.org/License          |
+------------------------------------------------------------------------+ */

#include <mrpt/graphs/CNetworkOfPoses.h>
#include <mrpt/graphs/dijkstra.h>
#include <mrpt/gui/CDisplayWindowPlots.h>
#include <mrpt/random.h>
#include <mrpt/system/CTicTac.h>

#include <iostream>
#include <map>

using namespace mrpt;
using namespace mrpt::graphs;
using namespace mrpt::poses;
using namespace mrpt::math;
using namespace mrpt::gui;
using namespace mrpt::random;
using namespace mrpt::system;
using namespace std;

// The type of my Dijkstra problem:
// (See other options in mrpt::poses::CNetworkOfPoses<>)
using CMyDijkstra = mrpt::graphs::CDijkstra<CNetworkOfPoses2D>;

// adds a new edge to the graph. The edge is annotated with the relative
// position of the two nodes
TNodeID from, TNodeID to, const std::map<TNodeID, CPose2D>& real_poses,
{
CPose2D p = real_poses.find(to)->second - real_poses.find(from)->second;
}

// weight is the distance between two nodes.
double myDijkstraWeight(
const CMyDijkstra::graph_t& g, const TNodeID from, const TNodeID to,
const CMyDijkstra::edge_t& edge)
{
//  return 1;                   // Topological distance
return edge.norm();  // Metric distance
}

// ------------------------------------------------------
//              TestDijkstra
// ------------------------------------------------------
void TestDijkstra()
{
CTicTac tictac;
CNetworkOfPoses2D::global_poses_t optimal_poses, optimal_poses_dijkstra;
std::map<TNodeID, CPose2D> real_poses;

getRandomGenerator().randomize(10);

// ----------------------------
// Create a random graph:
// ----------------------------
const size_t N_VERTEX = 20;
const double DIST_THRES = 10;
const double NODES_XY_MAX = 15;

vector<double> xs, ys;

for (size_t j = 0; j < N_VERTEX; j++)
{
CPose2D p(
getRandomGenerator().drawUniform(-NODES_XY_MAX, NODES_XY_MAX),
getRandomGenerator().drawUniform(-NODES_XY_MAX, NODES_XY_MAX),
getRandomGenerator().drawUniform(-M_PI, M_PI));
real_poses[j] = p;

// for the figure:
xs.push_back(p.x());
ys.push_back(p.y());
}

// Add some edges
for (size_t i = 0; i < N_VERTEX; i++)
{
for (size_t j = 0; j < N_VERTEX; j++)
{
if (i == j) continue;
if (real_poses[i].distanceTo(real_poses[j]) < DIST_THRES)
}
}

// ----------------------------
//  Dijkstra
// ----------------------------
tictac.Tic();
const size_t SOURCE_NODE = 0;
#if 0
const size_t maxTopoDistance = 35;  // according to myDijkstraWeight()
#else
const size_t maxTopoDistance = std::numeric_limits<size_t>::max();
#endif

CMyDijkstra myDijkstra(
graph_links, SOURCE_NODE, &myDijkstraWeight, {}, maxTopoDistance);

cout << "Dijkstra took " << tictac.Tac() * 1e3 << " ms for "
<< graph_links.edges.size() << " edges." << endl;

// Demo of getting the tree representation of
//  the graph & visit its nodes:
// ---------------------------------------------------------
const CMyDijkstra::tree_graph_t graphAsTree = myDijkstra.getTreeGraph();

// Text representation of the tree:
cout << "TREE:\n" << graphAsTree.getAsTextDescription() << endl;

auto lmb = [&]([[maybe_unused]] const TNodeID parent,
const CMyDijkstra::tree_graph_t::TEdgeInfo& edge_to_child,
const size_t depth_level) {
cout << string(depth_level * 3, ' ');
cout << edge_to_child.id << endl;
};

cout << "Depth-first traverse of graph:\n";
cout << SOURCE_NODE << endl;
graphAsTree.visitDepthFirst(SOURCE_NODE, lmb);

cout << endl << "Breadth-first traverse of graph:\n";
cout << SOURCE_NODE << endl;

// ----------------------------
// Display results graphically:
// ----------------------------
CDisplayWindowPlots win("Dijkstra example");

win.hold_on();
win.axis_equal();

for (TNodeID i = 0; i < N_VERTEX && win.isOpen(); i++)
{
if (i == SOURCE_NODE) continue;

const auto dist = myDijkstra.getNodeDistanceToRoot(i);
if (!dist.has_value())
{
cout << "to " << i << "-> no path found.\n";
continue;
}

const CMyDijkstra::edge_list_t path = myDijkstra.getShortestPathTo(i);

cout << "to " << i << " -> #steps= " << path.size()
<< " distance=" << dist.value() << endl;

win.setWindowTitle(format(
"Dijkstra path %u->%u", static_cast<unsigned int>(SOURCE_NODE),
static_cast<unsigned int>(i)));

win.clf();

// plot all edges:
for (CNetworkOfPoses2D::iterator e = graph_links.begin();
e != graph_links.end(); ++e)
{
const CPose2D& p1 = real_poses[e->first.first];
const CPose2D& p2 = real_poses[e->first.second];

vector<double> X(2);
vector<double> Y(2);
X[0] = p1.x();
Y[0] = p1.y();
X[1] = p2.x();
Y[1] = p2.y();
win.plot(X, Y, "k1");
}

// Draw the shortest path:
for (CMyDijkstra::edge_list_t::const_iterator a = path.begin();
a != path.end(); ++a)
{
const CPose2D& p1 = real_poses[a->first];
const CPose2D& p2 = real_poses[a->second];

vector<double> X(2);
vector<double> Y(2);
X[0] = p1.x();
Y[0] = p1.y();
X[1] = p2.x();
Y[1] = p2.y();
win.plot(X, Y, "g3");
}

// Draw All nodes:
win.plot(xs, ys, ".b7");
win.axis_fit(true);

cout << "Press any key to show next shortest path, close window to "
"end...\n";
win.waitForKey();
}

win.clear();
}

int main()
{
try
{
TestDijkstra();
return 0;
}
catch (exception& e)
{
cout << "MRPT exception caught: " << e.what() << endl;
return -1;
}
catch (...)
{
printf("Another exception!!");
return -1;
}
}```