graph - Segmentation Fault C++ -

the input in following format

5 1 2  9.0 1 3 12.0 2 4 18.0 2 3  6.0 2 5 20.0 3 5 15.0 0 1 5 

the first number number of vertexes in graph. next lines 0 edges of graph. first , second numbers being vertexes , third being how far edge between them. trying read in data , store edges there locations in list adjacency vertex. example make graph 5 vertexes edges 1 2&3. 2 4&3&1 etc.

i getting segmentation fault after entering 4 numbers. fault happening on line mygraph.vertexinfo[p1].adjacency -> vertex=p2; starts trying store information. why getting fault?

#include <cstdio> using namespace std;  struct listcell {    listcell* next;    int vertex;    double weight;     listcell(int v, double w, listcell* nxt)    {       vertex = v;       weight = w;       next = nxt;    } };  typedef listcell* list;  struct vertex {    bool signaled;    long distance;    list adjacency;     };  struct graph {    int     numvertices;    vertex* vertexinfo;     graph(int n)    {       numvertices = n;       vertexinfo  = new vertex[n+1];       for(int = 1; <= n; i++)       {          vertexinfo[i].signaled = false;       }    } };  //============================================================== //                   readin //============================================================== //  //==============================================================  void readin() {    int g;    int p1;    int p2;    float edge;    scanf("%i ", &g);     graph mygraph(g);    scanf("%i", &p1);    while(p1 != 0)    {       scanf("%i", &p2);       scanf("%f", &edge);                mygraph.vertexinfo[p1].adjacency -> vertex=p2;        mygraph.vertexinfo[p2].adjacency -> vertex=p1;       mygraph.vertexinfo[p1].adjacency -> weight=edge;        mygraph.vertexinfo[p2].adjacency -> weight=edge;       scanf("%i", &p1);    } } //============================================================== //                   main //==============================================================  int main(int argc, char** argv) {    readin();    return 0; } 

let's @ adjacency is:

struct vertex {    bool signaled;    long distance;    list adjacency;     }; 

and let's @ list is:

typedef listcell* list; 

so, adjacency pointer listcell. that's fine, can work that. before can, must make sure adjacency points something. question becomes adjacency point in code?

the answer is: knows? never make point anything, it's pointing somewhere randomly. , happens when try write random area of memory? well, if you're lucky, crash.

so, how solve problem? simple: initialize , manipulate adjacency correctly. may wish dynamic memory allocation in c++ , standard library provides rather nifty , working implementation of linked list.

sidenote: seem treat adjacency linked list. however, ought note adjacency pointer node in linked list. again, great time learn std::list.