insert in a vector of vector

[mickey]

hi I'm trying to do this:

vector < vector <double> > temp;
	temp.resize(_set->size());	
	vector < vector <double> >::iterator iit=temp.begin();
	int i=0;
	vector < vector <double> >::iterator itra = _set->begin();	
	srand ((unsigned)time(NULL));
	while ( ! _set->empty() )  {
		do {
			i = rand() % (int) _set->size();
			cout << " i " << i << endl; 
			if (i == 0 &&  _set->size() == 1) break;
		} while (i >= (int) _set->size() || i == 0);
		temp.insert (temp.begin(), itra+i, itra+i); 	//here it is filling 	nothing
		int dist = (int) distance (_set->begin(), _set->begin()+i);
		_set->erase(itra+dist);
		++iit;
	}
	 *_set = temp;

vector < vector <double> > temp;
	temp.resize(_set->size());	
	vector < vector <double> >::iterator iit=temp.begin();
	int i=0;
	vector < vector <double> >::iterator itra = _set->begin();	
	srand ((unsigned)time(NULL));
	while ( ! _set->empty() )  {
		do {
			i = rand() % (int) _set->size();
			cout << " i " << i << endl; 
			if (i == 0 &&  _set->size() == 1) break;
		} while (i >= (int) _set->size() || i == 0);
		temp.insert (temp.begin(), itra+i, itra+i); 	//here it is filling 	nothing
		int dist = (int) distance (_set->begin(), _set->begin()+i);
		_set->erase(itra+dist);
		++iit;
	}
	 *_set = temp;

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my aim is take my vec of vec and shuffle its elements (its vectors).
Before chose a vector of _set at random, put it at begin of temp and erase it from _set; I've been trying this way and it seems me more difficult...
the calcolus of 'i' is ok. but insert doesn't work.
Can anyone help me?
thanks