Question

I am new to graphs and I am trying to write very simple programs in graph. I have written two functions, one of which creates an empty graph with as many vertices as inputted by the user. The other one, adds a directed edge between two of the vertices.

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The former executes successfully, but the later does not. The program stops running, however the code compiles successfully.

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#include <stdio.h>
#include <stdlib.h>
#define MAX 1000

struct node
{
    int data;
    struct node *next;
};

struct node *arr[MAX];

void createEmptyGraph(int n)
{
    // n is the number of vertices
    int i;

    for(i=0;i<n;i++)
    {
       arr[i]=NULL;
    }

    printf("\nAn empty graph with %d vertices has been created",n);
    printf("\nNo edge is connected yet");
}

void addNode(int startVertex,int endVertex)
{
    // For directed edges
    struct node *n;
    n=(struct node *)malloc(sizeof(struct node));
    n->next=arr[startVertex];
    arr[startVertex]->next=n;

    printf("\nAn edge between directed from %d to %d has been   added",startVertex,endVertex);
}

int main(void)
{
    int num;

    printf("Enter the number of vertices in the graph: ");
    scanf("%d",&num);

    createEmptyGraph(num);
    addNode(0,1);

    return 0;
}

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I am using adjacency list representation of graphs. Please point out the errror(s).

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One more thing, in the createEmptyGraph() method, why can’t I do something like this

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for(i=0;i<n;i++)
{
    arr[i]->data=d;
    arr[i]->next=NULL;
}

Answer 1

You declare an array of pointers:

struct node *arr[MAX];

In your createEmptyGraph() method, you need to allocate each structure in the array first – instead you are setting simply the pointer to null..

for(i=0;i<n;i++)
{
    arr[i]=NULL;
}

The following won’t work because you’ve not allocated each entry:

for(i=0;i<n;i++)
{
    arr[i]->data=d;
    arr[i]->next=NULL;
}

Allocate first (malloc()) and then you can set as above…

So, because of the fact that you’ve not allocated, the following will not work:

n->next=arr[startVertex]; // this is okay, you've set it to NULL
arr[startVertex]->next=n; // ERROR: you are accessing a NULL pointer!