push_back doesn't work as I aspected

Question

i got a strange things with vector.push_back ... i wrote inside a loop

 while(size = getSectionSize(inputFile) ){ 


  data->at(k).resize(size*3) ;
  sizes.push_back(size) ;

  cout << "sizes at k = "<< k << " = " << sizes.at(k) << endl;

  double val;      
  int j=0; 
  while(getline(inputFile,row) && j < size){
        istringstream elem(row);
        for(int i=0; i < 3; i++)
              //elem >> data->at(k).at(j).at(i); 
              elem >> val ;
              data->at(k).at(j).push_back(val); 
        elem >> tmp ;                    // 4th columns to skip
        j++;
  }

but i got

terminate called after throwing an instance of 'std::out_of_range'

... instead if i write inside a loop

for(int i=0; i<size; i++)                // resize vector ! 
          data->at(k).at(i).resize(3) ;

and then reads the the data from file with push_back the program goes fine but if i try to write out the data i got a lot of 0 0 0 (this should be related to the fact that with resize the vector increase 2*size ... and not just size)

so I have to use the orrible :

elem >> data->at(k).at(j).at(i); 

In this way everythings goes fine ! but i would understand why push_back doesn't works without a previously resize!!