Problem with C++ Vector Size

[ziggi]

Hey guys,

my code which was running fine with 2D arrays until I replaced arrays by 2D vectors and now it's giving segmentation fault. To spot out the problems i tried to find the size of vector but it's "0", i do not understand why it's 0 as it shud b 5x2 or 10???? even when i try to print the capacity of vector A (which being 2D vector)
shud b 5x2 or 10 is coming 5 ????

Plz help !

part of code is

int Nv = 5;
int Nevx = 2;
int MAX = 99999;

vector<vector<int> > A;
  A.reserve(Nv);
  for (int i=0;i<Nv;i++){ 
    A[i].reserve(Nevx);
    for (int j=0;j<Nevx;j++){
      A[i][j] = MAX;
    }
  }

cout <<"A size = " << A.size()<<endl;
cout <<"A capacity = " << A.capacity()<<endl;

[Valmont]

For the outer std::vector you can't use "reserve()" because the std::vector can't know how to align the memory. Realize that the inner std::vector can vary in size per outer element!

Here are a few solutions.

1) use resize():

#include <iostream>
#include <vector>

using namespace std;

int main()
{
  int Nv = 5;
  int Nevx = 2;
  int MAX = 99999;
  
  vector<vector<int> > A;
  
  A.resize(Nv);
  for(unsigned i=0; i<Nv; ++i)
  {
    for (int j=0; j<Nv; ++j)
    {
      A[j].reserve(Nevx);
      A[i][j] = MAX;
      cout<<"A["<<i<< "]["<< j<< "]= "<<A[i][j]<<endl;
    }
  }  
    
  return 0;
}

2) Resize at creation

#include <iostream>
#include <vector>

using namespace std;

int main()
{
  int Nv = 5;
  int Nevx = 2;
  int MAX = 99999;
  
  vector<vector<int> > A(Nv);
  
  for(unsigned i=0; i<Nv; ++i)
  {
    for (int j=0; j<Nv; ++j)
    {
      A[j].reserve(Nevx);
      A[i][j] = MAX;
      cout<<"A["<<i<< "]["<< j<< "]= "<<A[i][j]<<endl;
    }
  }
  
  return 0;
}

TIP 1:
Besides, if you are absolutely sure that your 2D vector is going to hold numbers only, then consider learning the ways of std::valarray as the inner of std::vector.

TIP 2:
Try to use "unsigned" integers for vector indices by heart. If your vector contains more elements then the signed integer can handle then weird things will happen. A vector guarantees access times of O(n) so it is very feasable to assume that vectors will have lots of elements. Be prepared.