runtime error (SIGSEGV)

Question

#include<iostream>
#include<string>
using namespace std;
int main()
{
   char arr[1000][80];
   char output[1000][80];
   int n,i,j;
   int num[1000];
   cin>>n;
   for(i=0;i<n;i++)
   {
    cin>>num[i];
      cin>>arr[i];
   }
   for(i=0;i<n;i++)
   {
      for(j=(num[i]-1);j<(strlen(arr[i])-1);j++)
      {
        arr[i][j]=arr[i][j+1];
      }
      arr[i][j]='\0';
      cout<<"\n"<<(i+1)<<" "<<arr[i];
   }
  return 0;
}

This is the code which while uploading on scoj gives the above error. THe same code runs fine on borland c++.

Answer 1

There's nothing inherently wrong as far as I can see at first glance. However a segfault is certainly very possible when providing inputs your code can't cope with.

There's nothing preventing from writing outside the boundaries of arr on input for example.

Is there a specific input for which this fails?

Answer 2

Depending on the input you pass to this program, the variable n may be more than 1000, cin>>arr[i] may read more than 80 characters, and if num[i] <= 0 || num[i] >= 80 then you will index past the beginning or end of one of your strings. All of these problems exist because this code uses fixed-size arrays and doesn't do any bounds checking.