Run time error in graph

Question

I am implementing Graph for the first time and for that I took this problem from SPOJ.

Took help of geeksforgeeks, applied union find algorithm to find out whether or not graph contains a cycle but I get run time error (SIGSEGV).

Can you please help why is it so?

#include<iostream>
#include<stdlib.h>
#include<string.h>
using namespace std;

struct Edge{
 int s,d;
};
struct Graph{
 int v,e;
 struct Edge* edge;
};
struct Graph* create(int v, int e){
 struct Graph* graph=(struct Graph*)malloc(sizeof (struct Graph));
 graph->v=v;
 graph->e=e;
 graph->edge=(struct Edge*)malloc(sizeof (struct Edge));
 return graph;
};
int Find(int p[],int i)
{
 if (p[i] == -1)
    return i;
return Find(p, p[i]);
}
void Union(int p[],int i, int j)
{
 p[j]=i;
}
bool CheckCycle(struct Graph* graph)
{
 int *p=(int*)malloc(graph->v* sizeof (int));
 memset(p,-1,graph->v * sizeof (int));
  /*for(int i=0;i<graph->v;i++)
      cout<<"p"<<i<<"="<<p[i];
  cout<<"\n";*/
 for(int i=0;i<graph->e;i++)
 {
      /*cout<<"edge"<<i<<" src="<<graph->edge[i].s<<"\n";
      cout<<"edge"<<i<<" dest="<<graph->edge[i].d<<"\n";*/
      int x=Find(p,graph->edge[i].s);
      int y=Find(p,graph->edge[i].d);
      /*cout<<"x="<<x<<" "<<"y="<<y<<"\n";*/
      if(x==y)
           return true;
      Union(p,x,y);
 }
 return false;
}
int main()
{
 ios_base::sync_with_stdio(false);
 int N,M,v1,v2;
 cin>>N>>M;
 if(M!=(N-1))
      cout<<"NO\n";
 else{
      struct Graph* graph=create(N,M);
      for(int i=0;i<M;i++)
      {
           cin>>v1;
           graph->edge[i].s=v1-1;
           cin>>v2;
           graph->edge[i].d=v2-1;
      }
      if(CheckCycle(graph))
           cout<<"NO\n";
      else
           cout<<"YES\n";
 }
}

Time to try your debugger. Also, using the stl vector would allow you to write real c++ code and to make the access to your arrays (e.g. p[i]) safer — vib, May 28 '15 at 18:20
`Can you please help why is it so?` Programming is more than taking code, compiling it, and running it. It also involves *debugging*. Have you debugged your code? And it seems to me that an overwhelming number of these "SPOJ" posts and other "contest web sites" posts show little to no debugging effort. — PaulMcKenzie, May 28 '15 at 18:23
@PaulMcKenzie Yes, I tried. I found a bug that the input I was taking earlier was causing the error as I am considering vertices numbered from 0 but question demands numbering from 1. I thought that it was the possible reason for error as Find function was trying to access a member that didn't exist. So, fixed that. I don't find any other flaws. — unixia, May 28 '15 at 18:26

score 0 · Accepted Answer · answered May 28 '15 at 18:40

One issue is this in your main program:

graph->edge[i].s=v1-1;

You created a single edge. If i is greater than 0, then this is an out-of-bounds access.

Look how you created edge in the create function:

 graph->edge=(struct Edge*)malloc(sizeof (struct Edge));

That allocation holds a single edge, not multiple edges. Given how you coded the rest of your program in a C-like fashion, what you probably wanted is this:

 graph->edge=(struct Edge*)malloc(sizeof(Edge) * e);

Also, you should strive to not use single-letter variable names. It is hard to read code with e, v, etc. as member variable names. Name those items m_edge, m_vertex or something that is more descriptive.

Thank you so much! That was the only error. Allocation was wrong. I'll keep in mind to use descriptive variables from next time. Thanks a lot! — unixia, May 28 '15 at 18:51

Run time error in graph

1 Answers1