Segmentation fault when appending a symbol!

[jeevsmyd]

for(int i=0;i<=c;i++)
                {
                        l=0;
                        if(strcmp(findedge(tst[i],dfa[0][j]),"null")!=0)
                        {
                            strcpy(t1,findedge(tst[i],dfa[0][j]));

                            //l=strlen(temp);
                            strcat(temp,",");
                            strcat(temp,t1);
                            //temp[l+1]='\0';
                            //temp[l]=',';

                        }
                }

When I try to append comma or some other symbol to temp, a segmentation fault occurs. If that step is omitted everything works perfectly. This is code is from my program to convert NFA to DFA.
You can see that I even tried to manually append the comma without using strcat from those commented lines. Somebody please help me

[jeevsmyd]

Here is the find edge function char * findedge(char st[],char sym[])

{
    strcpy(edg,"");
 for(int i=1;i<=m;i++)
    {
      if(strcmp((nfa[i][0]),st)==0)
      {
          for(int j=1;j<=n;j++)
            if(strcmp(nfa[0][j],sym)==0)
            return nfa[i][j];
      }
    }
    return edg;
}

[zeroliken]

Did you check if variable temp has enough memory to store those?

[Ancient Dragon]

Also, was temp initialized before calling strcat() the first time? eg. char temp[50] = {0};

[jeevsmyd]

Yes, I checked both. Here is the complete code. Somebody please help me

#include<iostream>
#include<stdio.h>
#include<conio.h>
#include<string.h>
using namespace std;
char nfa[50][50][50];
char dfa[20][20][20];
int m,n,md;
char temps[30];
char edg[30];
char t1[50]={0},temp[100]={0},tst[20][20];
int f=-1,r=-1;
struct queue
{
        char state[30];
        char s;
};
queue q[50];
void insert(char in[])
{

    if(f==-1)
    {
        f=r=0;
        strcpy(q[r].state,in);
        q[r].s='A';
    }
    else
    {
        // searching to see if state already present in queue
        for(int i=0;i<=r;i++)
        {
            if(strcmp(q[i].state,in)==0)
            {
                return;
            }
        }
        r++;
        strcpy(q[r].state,in);
        q[r].s=q[r-1].s+1;
        }
}
   char* del()
   {
       return (q[f++].state);
   }
void showdfa()
{
for(int i=0;i<=md;i++)
    {
            for(int j=0;j<=n;j++)
            {
                cout<<dfa[i][j]<<"\t";
            }
            cout<<"\n";
    }
}
void showq()
{
    for(int i=0;i<=r;i++)
    cout<<"\n"<<q[i].state<<"\t"<<q[i].s;
}
char * findedge(char st[],char sym[])
{
    strcpy(edg,"\0");
 for(int i=1;i<=m;i++)
    {
      if(strcmp((nfa[i][0]),st)==0)
      {
          for(int j=1;j<=n;j++)
            if(strcmp(nfa[0][j],sym)==0)
            {
                cout<<"\nreturning"<<nfa[i][j]<<"\n";
                return nfa[i][j];
            }
      }
    }
    return edg;
}
int main()
{
    int i,j;
    cout<<"Enter the no of states and input symbols\n";
    cin>>m>>n;
    strcpy(nfa[0][0],"States");
    cout<<"Enter the input symbols\n";
    for(j=1;j<=n;j++)
        cin>>nfa[0][j];
        cout<<"Enter the values\n";
        for(j=0;j<=n;j++)
        cout<<nfa[0][j]<<"\t";
        cout<<"\n";
    for( i=1;i<=m;i++)
            for(j=0;j<=n;j++)
                cin>>nfa[i][j];
    for(j=0;j<=n;j++)
        {
            strcpy(dfa[0][j],nfa[0][j]);
            strcpy(dfa[1][j],nfa[1][j]);
            insert(nfa[1][j]);
        }
        f++;
        md=2;
    int ar[10];
    int c=0;
    int p=0;
    ar[0]=0;
    cout<<"\n";
    while(f<=r)
    {
        strcpy(temp,"\0");
        for(int i=0;i<=c;i++)
            strcpy(tst[i],"\0");
        c=0;
        p=0;
        strcpy(temps,del());
        strcpy(dfa[md][0],temps);
        for(int k=0;temps[k]!='\0';k++)
        {
            if(temps[k]!=',')
            tst[c][p++]=temps[k];
            else
            {
                tst[c++][p]='\0';
                p=0;
            }
        }
        tst[c][p]='\0';
        int l;
        //strcat(temp,findedge(split(m,ar[c]))
            for(int j=1;j<=n;j++)
            {
                strcpy(temp,"\0");
                for(int i=0;i<=c;i++)
                {
                        l=0;
                        if(strcmp(findedge(tst[i],dfa[0][j]),"null")!=0)
                        {
                            strcpy(t1,findedge(tst[i],dfa[0][j]));

                            //l=strlen(temp);
                            strcat(t1,",");
                            strcat(temp,t1);
                            strcpy(t1,"\0");
                            //temp[l+1]='\0';
                            //temp[l]=',';

                        }
                }
                if(temp[0]!='\0')
                {
                  //l=strlen(temp);
                  //temp[l-1]='\0';
                  insert(temp);
                }
                strcpy(dfa[md][j],temp);
            }
        md++;
    }
    cout<<"\n";
  showq();
  // replace state with new symbols
    /* for(int i=1;i<=md;i++)
    {
            for(j=0;j<=n;j++)
            {
                for(int k=0;k<=c;k++)
                if(strcmp(dfa[i][j],queue[k].state)==0)
                {
                    strcpy(dfa[i][j],queue[k].s);
                }
            }
    }
    */

  cout<<"\n";
 showdfa();
    /*cout<<"\nNFA\n";
    for(int i=1;i<=m;i++)
    {
            for(j=0;j<=n;j++)
            {
                cout<<nfa[i][j]<<"\t";
            }
            cout<<"\n";
    }*/

      //cout<<"\nS2--b--->\t"<<findedge("s2","b");
return 0;
}

[Ancient Dragon]

>> for(j=1;j<=n;j++)
lines 88, 94 and 97

should by j<n to avoid array out-of-bounds errors -- same with other lines that contain similar loops.

[jeevsmyd]

Has that got to do anything with this segmentation fault which occurs only when I append something?

[Ancient Dragon]

It could have everything to do with it when your program corrupts memory anything can happen at the most unexpected times. Fix the loop problems then retest.

[jeevsmyd]

So, I'll have to increment value of n and check <n, right? or check <n+1 has the same effect, right?

[Ancient Dragon]

No. The value of n is the number of elements to check. So the program needs to count from 0 to n-1. Count them on your fingers if you have to get this concept straight in your head. When the value of n == 10, count 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 -- that's 10 items

[jeevsmyd]

n is not the number of elements to check. It is the number of symbols, but number of symbols+1 rows are to be checked because the first column of states is also included.
so, n=0 is not part of the symbol , but is a state. Hence n+1 columns are needed and that is why it check upto <=n

[Ancient Dragon]

It doesn't really matter what n means, when n == 50, q[n] is an invalid value because there are not 51 elements in q. Same problem with nfa becauswe nfa[0][n] is invalid.