C++ Program to Check if Graph is DAG

This is a C++ Program to check whether graph is DAG. In mathematics and computer science, a directed acyclic graph (DAG Listeni/’dæg/), is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of edges that eventually loops back to v again.

Here is source code of the C++ Program to Check Whether Graph is DAG. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.

```
#include<stdio.h>
```
```
#include<iostream>
```
```
#include<conio.h>
```
```
using namespace std;
```
```
int c = 0;
```
```
struct adj_list
```
```
{
```
```
        int dest;
```
```
        adj_list *next;
```

}*np = NULL, *np1 = NULL, *p = NULL, *q = NULL;

```
struct Graph
```
```
{
```
```
        int v;
```
```
        adj_list *ptr;
```
```
} array[6];
```
```
void addReverseEdge(int src, int dest)
```
```
{
```
```
    np1 = new adj_list;
```
```
    np1->dest = src;
```
```
    np1->next = NULL;
```
```
    if (array[dest].ptr == NULL)
```
```
    {
```
```
        array[dest].ptr = np1;
```
```
        q = array[dest].ptr;
```
```
        q->next = NULL;
```
```
    }
```
```
    else
```
```
    {
```
```
        q = array[dest].ptr;
```
```
        while (q->next != NULL)
```
```
        {
```
```
            q = q->next;
```
```
        }
```
```
        q->next = np1;
```
```
    }
```
```
}
```
```
void addEdge(int src, int dest)
```
```
{
```
```
    np = new adj_list;
```
```
    np->dest = dest;
```
```
    np->next = NULL;
```
```
    if (array[src].ptr == NULL)
```
```
    {
```
```
        array[src].ptr = np;
```
```
        p = array[src].ptr;
```
```
        p->next = NULL;
```
```
    }
```
```
    else
```
```
    {
```
```
        p = array[src].ptr;
```
```
        while (p->next != NULL)
```
```
        {
```
```
            p = p->next;
```
```
        }
```
```
        p->next = np;
```
```
    }
```
```
    //addReverseEdge(src, dest);
```
```
}
```
```
void print_graph(int n)
```
```
{
```
```
    for (int i = 0; i < n; i++)
```
```
    {
```

        cout << "Adjacency List of " << array[i].v << ": ";

```
        while (array[i].ptr != NULL)
```
```
        {
```

            cout << (array[i].ptr)->dest << " ";

            array[i].ptr = (array[i].ptr)->next;

```
        }
```
```
        cout << endl;
```
```
    }
```
```
}
```
```
 
```
```
int checkDAG(int n)
```
```
{
```
```
    int count = 0;
```
```
    int size = n - 1;
```
```
    for (int i = 0; i < n; i++)
```
```
    {
```

        //cout << "Adjacency List of " << array[i].v << ": ";

```
        if (count == size)
```
```
        {
```
```
            return 1;
```
```
        }
```
```
        if (array[i].ptr == NULL)
```
```
        {
```
```
            count++;
```
```
            for (int j = 0; j < n; j++)
```
```
            {
```
```
 
```

                while (array[j].ptr != NULL)

```
                {
```

                    if ((array[j].ptr)->dest == (array[i].ptr)->dest)

```
                    {
```

                        (array[j].ptr)->dest = -1;

```
                    }
```

                    array[i].ptr = (array[i].ptr)->next;

```
                }
```
```
            }
```
```
 
```
```
        }
```
```
    }
```
```
    return 0;
```
```
}
```
```
int main()
```
```
{
```
```
    int n = 6;
```

    cout << "Number of vertices: " << n << endl;

```
 
```
```
    for (int i = 0; i < n; i++)
```
```
    {
```
```
        array[i].v = i;
```
```
        array[i].ptr = NULL;
```
```
    }
```
```
    addEdge(0, 1);
```
```
    addEdge(1, 2);
```
```
    addEdge(1, 3);
```
```
    addEdge(3, 4);
```
```
    addEdge(4, 5);
```
```
    addEdge(5, 3);
```
```
    addEdge(5, 2);
```
```
    print_graph(n);
```

    cout << "The given graph is 'Directed Acyclic Graph' :";

```
    if (checkDAG(n) == 1)
```
```
        cout << " True";
```
```
    else
```
```
        cout << " False";
```
```
}
```

Output:

$ g++ CheckDAG.cpp
$ a.out
 
Number of vertices: 6
Adjacency List of 0: 1 
Adjacency List of 1: 2 3 
Adjacency List of 2: 
Adjacency List of 3: 4 
Adjacency List of 4: 5 
Adjacency List of 5: 3 2 
The given graph is 'Directed Acyclic Graph' : True
------------------
(program exited with code: 0)
Press return to continue

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