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Articles Home » C++ » C++ Program to Find Independent Sets in a Graph by Graph Coloring

C++ Program to Find Independent Sets in a Graph by Graph Coloring

This is a C++ Program to find largest independent set by graph coloring. In graph theory, an independent set or stable set is a set of vertices in a graph, no two of which are adjacent. That is, it is a set I of vertices such that for every two vertices in I, there is no edge connecting the two. Equivalently, each edge in the graph has at most one endpoint in I. The size of an independent set is the number of vertices it contains. Independent sets have also been called internally stable sets. A maximal independent set is either an independent set such that adding any other vertex to the set forces the set to contain an edge or the set of all vertices of the empty graph.

Here is source code of the C++ Program to Find Independent Sets in a Graph by Graph Coloring. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include <iostream>
```
```
#include <fstream>
```
```
#include <string>
```
```
#include <vector>
```
```
using namespace std;
```
```
 
```

bool removable(vector<int> neighbor, vector<int> cover);

int max_removable(vector<vector<int> > neighbors, vector<int> cover);

vector<int> procedure_1(vector<vector<int> > neighbors, vector<int> cover);

vector<int> procedure_2(vector<vector<int> > neighbors, vector<int> cover,

```
        int k);
```
```
int cover_size(vector<int> cover);
```
```
ifstream infile("graph.txt");
```
```
ofstream outfile("sets.txt");
```
```
 
```
```
int main()
```
```
{
```
```
    //Read Graph
```

    cout << "Independent Set Algorithm." << endl;

    int n, i, j, k, K, p, q, r, s, min, edge, counter = 0;

```
    infile >> n;
```
```
    vector<vector<int> > graph;
```
```
    for (i = 0; i < n; i++)
```
```
    {
```
```
        vector<int> row;
```
```
        for (j = 0; j < n; j++)
```
```
        {
```
```
            infile >> edge;
```
```
            row.push_back(edge);
```
```
        }
```
```
        graph.push_back(row);
```
```
    }
```
```
    //Find Neighbors
```
```
    vector<vector<int> > neighbors;
```
```
    for (i = 0; i < graph.size(); i++)
```
```
    {
```
```
        vector<int> neighbor;
```

        for (j = 0; j < graph[i].size(); j++)

```
            if (graph[i][j] == 1)
```
```
                neighbor.push_back(j);
```
```
        neighbors.push_back(neighbor);
```
```
    }
```

    cout << "Graph has n = " << n << " vertices." << endl;

    //Read maximum size of Independent Set wanted

    cout << "Find an Independent Set of size at least k = ";

```
    cin >> K;
```
```
    k = n - K;
```
```
    //Find Independent Sets
```
```
    bool found = false;
```

    cout << "Finding Independent Sets..." << endl;

```
    min = n + 1;
```
```
    vector<vector<int> > covers;
```
```
    vector<int> allcover;
```
```
    for (i = 0; i < graph.size(); i++)
```
```
        allcover.push_back(1);
```

    for (i = 0; i < allcover.size(); i++)

```
    {
```
```
        if (found)
```
```
            break;
```
```
        counter++;
```
```
        cout << counter << ". ";
```
```
        outfile << counter << ". ";
```
```
        vector<int> cover = allcover;
```
```
        cover[i] = 0;
```

        cover = procedure_1(neighbors, cover);

```
        s = cover_size(cover);
```
```
        if (s < min)
```
```
            min = s;
```
```
        if (s <= k)
```
```
        {
```

            outfile << "Independent Set (" << n - s << "): ";

            for (j = 0; j < cover.size(); j++)

```
                if (cover[j] == 0)
```

                    outfile << j + 1 << " ";

```
            outfile << endl;
```

            cout << "Independent Set Size: " << n - s << endl;

```
            covers.push_back(cover);
```
```
            found = true;
```
```
            break;
```
```
        }
```
```
        for (j = 0; j < n - k; j++)
```

            cover = procedure_2(neighbors, cover, j);

```
        s = cover_size(cover);
```
```
        if (s < min)
```
```
            min = s;
```

        outfile << "Independent Set (" << n - s << "): ";

        for (j = 0; j < cover.size(); j++)

```
            if (cover[j] == 0)
```

                outfile << j + 1 << " ";

```
        outfile << endl;
```

        cout << "Independent Set Size: " << n - s << endl;

```
        covers.push_back(cover);
```
```
        if (s <= k)
```
```
        {
```
```
            found = true;
```
```
            break;
```
```
        }
```
```
    }
```
```
    //Pairwise Intersections
```
```
    for (p = 0; p < covers.size(); p++)
```
```
    {
```
```
        if (found)
```
```
            break;
```

        for (q = p + 1; q < covers.size(); q++)

```
        {
```
```
            if (found)
```
```
                break;
```
```
            counter++;
```
```
            cout << counter << ". ";
```
```
            outfile << counter << ". ";
```

            vector<int> cover = allcover;

            for (r = 0; r < cover.size(); r++)

                if (covers[p][r] == 0 && covers[q][r] == 0)

```
                    cover[r] = 0;
```

            cover = procedure_1(neighbors, cover);

```
            s = cover_size(cover);
```
```
            if (s < min)
```
```
                min = s;
```
```
            if (s <= k)
```
```
            {
```

                outfile << "Independent Set (" << n - s << "): ";

                for (j = 0; j < cover.size(); j++)

```
                    if (cover[j] == 0)
```

                        outfile << j + 1 << " ";

```
                outfile << endl;
```

                cout << "Independent Set Size: " << n - s << endl;

```
                found = true;
```
```
                break;
```
```
            }
```
```
            for (j = 0; j < k; j++)
```

                cover = procedure_2(neighbors, cover, j);

```
            s = cover_size(cover);
```
```
            if (s < min)
```
```
                min = s;
```

            outfile << "Independent Set (" << n - s << "): ";

            for (j = 0; j < cover.size(); j++)

```
                if (cover[j] == 0)
```

                    outfile << j + 1 << " ";

```
            outfile << endl;
```

            cout << "Independent Set Size: " << n - s << endl;

```
            if (s <= k)
```
```
            {
```
```
                found = true;
```
```
                break;
```
```
            }
```
```
        }
```
```
    }
```
```
    if (found)
```

        cout << "Found Independent Set of size at least " << K << "." << endl;

```
    else
```

        cout << "Could not find Independent Set of size at least " << K << "."

                << endl << "Maximum Independent Set size found is " << n - min

```
                << "." << endl;
```

    cout << "See sets.txt for results." << endl;

```
    system("PAUSE");
```
```
    return 0;
```
```
}
```
```
 
```

bool removable(vector<int> neighbor, vector<int> cover)

```
{
```
```
    bool check = true;
```

    for (int i = 0; i < neighbor.size(); i++)

```
        if (cover[neighbor[i]] == 0)
```
```
        {
```
```
            check = false;
```
```
            break;
```
```
        }
```
```
    return check;
```
```
}
```
```
 
```

int max_removable(vector<vector<int> > neighbors, vector<int> cover)

```
{
```
```
    int r = -1, max = -1;
```

    for (int i = 0; i < cover.size(); i++)

```
    {
```

        if (cover[i] == 1 && removable(neighbors[i], cover) == true)

```
        {
```

            vector<int> temp_cover = cover;

```
            temp_cover[i] = 0;
```
```
            int sum = 0;
```

            for (int j = 0; j < temp_cover.size(); j++)

                if (temp_cover[j] == 1 && removable(neighbors[j], temp_cover)

```
                        == true)
```
```
                    sum++;
```
```
            if (sum > max)
```
```
            {
```
```
                max = sum;
```
```
                r = i;
```
```
            }
```
```
        }
```
```
    }
```
```
    return r;
```
```
}
```
```
 
```

vector<int> procedure_1(vector<vector<int> > neighbors, vector<int> cover)

```
{
```
```
    vector<int> temp_cover = cover;
```
```
    int r = 0;
```
```
    while (r != -1)
```
```
    {
```

        r = max_removable(neighbors, temp_cover);

```
        if (r != -1)
```
```
            temp_cover[r] = 0;
```
```
    }
```
```
    return temp_cover;
```
```
}
```
```
 
```

vector<int> procedure_2(vector<vector<int> > neighbors, vector<int> cover,

```
        int k)
```
```
{
```
```
    int count = 0;
```
```
    vector<int> temp_cover = cover;
```
```
    int i = 0;
```

    for (int i = 0; i < temp_cover.size(); i++)

```
    {
```
```
        if (temp_cover[i] == 1)
```
```
        {
```
```
            int sum = 0, index;
```

            for (int j = 0; j < neighbors[i].size(); j++)

                if (temp_cover[neighbors[i][j]] == 0)

```
                {
```
```
                    index = j;
```
```
                    sum++;
```
```
                }
```

            if (sum == 1 && cover[neighbors[i][index]] == 0)

```
            {
```

                temp_cover[neighbors[i][index]] = 1;

```
                temp_cover[i] = 0;
```

                temp_cover = procedure_1(neighbors, temp_cover);

```
                count++;
```
```
            }
```
```
            if (count > k)
```
```
                break;
```
```
        }
```
```
    }
```
```
    return temp_cover;
```
```
}
```
```
 
```
```
int cover_size(vector<int> cover)
```
```
{
```
```
    int count = 0;
```

    for (int i = 0; i < cover.size(); i++)

```
        if (cover[i] == 1)
```
```
            count++;
```
```
    return count;
```
```
}
```

Output:

$ g++ LargestIndependentSetGraphColoring.cpp
$ a.out
 
graph.txt:
4
0 1 1 1
1 0 1 1
1 1 0 1
1 1 1 0
 
set.txt:
Independent Set ( 1 ): 2
------------------
(program exited with code: 0)
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C++ Program to Find Independent Sets in a Graph by Graph Coloring

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