Question

I’ve got a data file that is a single line consisting of a nested series of doubles eg.

[[0.127279,0.763675,0.636396],[0.254558,0.890955,0.636396],
[0.127279,0.636396,0.763675],[0.254558,0.763675,0.763675],
[0.381838,0.890955,0.763675],[0.127279,0.509117,0.890955],
[0.254558,0.636396,0.890955],[0.509117,0.890955,0.890955]]

I’d like to be able to read this into a STL vector<vector<double> > using the stream operator which is templated across the inner type of A:

vector<vector<double> > A;
FIN >> A;

I’ve figured out a way to do this when the vector is not nested, ie. a simple vector<T> as so:

template <class T>
istream& operator>>(istream& s, vector<T> &A){
  T x;
  string token; char blank;

  s >> blank; // Gobble the first '['
  while( getline(s, token, ',') ) {
   istringstream input(token);
   input >> x;
   A.push_back(x);
  }
  s >> blank; // Gobble the last ']'
  return s;
}

But I’m having problem with the istream& operator>>(istream& s, vector<vector<T> >&A) part as I can’t seem to catch the inner ]‘s properly. I’m sure that Boost has a way of doing this, but I’d like to see a solution with the STL for pedagogical purposes.

Note: I’m aware that overloading the stream operator for vector<T> can have far-reaching undesirable consequences and that the implementation should be wrapped up in its own class – I’m using this example above as it stands to clarify the question.

EDIT:
I’d like the method to be robust enough to handle a input array whose size (and inner array) size is not known in advance, but inferred from reading the stream.

Answer 1

Actually, the problem with your code that you want to use the same function for both, when T is:

• vector<double>
• double

But the logic which needs to read the data into vector and double is slightly different. So you cannot do that, at least not with that simple logic:

I would prefer to write two functions, to handle both cases separately. After all, even in your case, the compiler will generate two different functions for each value of T.

template <class T>
istream& operator>>(istream& s, vector<T> &A)
{
  T x;
  string token; char blank;

  s >> blank; // Gobble the first '['
  while( getline(s, token, ',') ) 
  {
   istringstream input(token);
   input >> x;
   A.push_back(x);
  }
// s >> blank; // Gobble the last ']'
  return s;
}

template <class T>
istream& operator>>(istream& s, vector<vector<T>> &A)
{
  vector<T> x;
  string token; 
  char blank;

  s >> blank; // Gobble the first '['
  while( getline(s, token, ']') ) 
  {
   istringstream input(token);
   input >> x;
   s >> blank; //read , after [...]
   A.push_back(x);
   x.clear();
  }
  s >> blank; // Gobble the last ']'
  return s;
}

Test code:

int main() {
        vector<vector<double>> A;       
        cin >> A;
        for(size_t i = 0 ;i < A.size(); ++i)
        {
            for(size_t j = 0 ; j < A[i].size(); ++j)
                 cout << A[i][j] <<"   ";
            cout << endl;
        }
        return 0;
}

Input:

[[1,2,3],[4,5,6],
[7,8,9],[10,11,12],
[13,14,15],[16,17,18],
[19,20,21],[22,23,24]]

Output:

1   2   3   
4   5   6   
7   8   9   
10   11   12   
13   14   15   
16   17   18   
19   20   21   
22   23   24 

Online demo : http://ideone.com/iBbmw