Graph Construction

This is sample code that is to implement the graph explained in Algorithm : Graph Construction. Refer to the page to understand the concept,

#include <malloc.h>

#include <stdio.h>

// A structure to represent a node in the adjacency list.

typedef struct NODE

{

    int data;

    struct NODE *next;

} node;

// A structure to represent list of vertexes(Node)

typedef struct VERTEXLIST

{

    node *vlisthead;

} vertexlist;

// A structure to store the graph vertexes. This will store an Array

// and each element of the array will point to the head of a linked list

typedef struct GRAPH

{

    int v;

    vertexlist *vl;

} Graph;

// A function to declare the graph. This will create an array that will store the head of each

// linked list. Each of the linked list will store the nodes that are connected to a specific node

Graph* CreateGraph(int n)

{

    int i;

    Graph *vlist;

    vlist = (Graph*) malloc(sizeof(Graph));

    // store the number of nodes given by the user    

    vlist->v = n;

    // allocate a block of memory location for the array that will store the head

    // for each of the linked list

    vlist->vl = (vertexlist*) malloc(sizeof(vertexlist)*(n+1));

    // Initialize the head to NULL.

    for(i = 0; i < n+1; i++)

    {

        vlist->vl[i].vlisthead = NULL;

    }

    return vlist;

}

// A  function to add the edge into the graph.

// This will add the node at the beginning of the linked list for the node v1.

void AddNode(Graph *G, int v1, int v2)

{

    node *newnode1;

    node *newnode2;

    node *temp;

    newnode1 = (node*) malloc(sizeof(node));

    newnode1->data = v1;

    newnode1->next = NULL;

    newnode2 = (node*) malloc(sizeof(node));

    newnode2->data = v2;

    newnode2->next = NULL;

    // Connection  v1 to v2.

    if(G->vl[v1].vlisthead == NULL)

    {

        // If the head is null, insert the newnode2(v2) to the head of the linked list

        // that stores all the nodes that are connected to the node v1.

        G->vl[v1].vlisthead = newnode2;

    }

    else

    {

        // Otherwise, add the node at the beginning of the linkedlist.

        newnode2->next = G->vl[v1].vlisthead;

        G->vl[v1].vlisthead = newnode2;

    }

}

int main()

{

    int i, v = 4;

    Graph *G = CreateGraph(v);

    AddNode(G, 1, 3);

    AddNode(G, 3, 2);

    AddNode(G, 4, 1);

    AddNode(G, 4, 3);

    printf("\n\nLinkedList representaion of the Incidence Matrix for a graph: ");

    for(i = 0; i < v; i++)

    {

        printf("\n\tV(%d) -> {",i+1);

        while(G->vl[i+1].vlisthead != NULL)

        {

            printf(" %d",G->vl[i+1].vlisthead->data);

            G->vl[i+1].vlisthead = G->vl[i+1].vlisthead->next;

        }

        printf(" }");

    }

}

Result :------------------------------------------------------

LinkedList representaion of the Incidence Matrix for a graph:

        V(1) -> { 3 }

        V(2) -> { }

        V(3) -> { 2 }

        V(4) -> { 3 1 }

Now let me visualize what each of the code does.

Graph *G = CreateGraph(v);

AddNode(G, 1, 3);

AddNode(G, 3, 2);

AddNode(G, 4, 1);

AddNode(G, 4, 3);