Question

As it’s probably obvious from my code I’m really new to C language. I’m working on a program which is called from a larger Python program via subprocess.PIPE / cin. My intent was to assign an array of a size directed from Python. Now I realized that I can assign an integer to 1202th block of that array independently of a number I pass to my program. What exactly happens here? Is such array safe to use or is it advisable to use some other feature (I was thinking of vector).

int main()
{
  string group_str;
  int group_num;
  getline (cin, group_str);
  stringstream( group_str ) >> group_num;
  cout << "Group number" << group_num <<"\n";

  int group[ group_num ];

  group[ 1202 ] = 233;

  for (int i=0; i < 1203 ; i++)
    {
      cout << group[i] << '\t' <<  i << endl;
    }

  return 0;
}

Answer 1

I’d just use C++ STL’s std::vector instead of raw C arrays.
#include <vector> and then use std::vector‘s proper constructor to create a vector of specified size, e.g.

vector<int> group(group_num);

Then you can access (read and modify) the vector item at a given index using operator[], just like with raw C arrays.

(If you want bounds checking on the vector index you may consider using std::vector::at() method, which throws an exception if the vector index is out of bounds.)