2.4.cpp

#include <vector>
#include <iostream>

using namespace std;

inline bool check_validity (int pos)
	{ return (pos <= 0 || pos >= 64)? false : true; }
	// local static 
const vector<int>* pentagonal_series(int pos)
{
	static vector<int> _elems;
	if (check_validity (pos) && (pos > _elems.size()))
		for (int ix = _elems.size()+1; ix <= pos; ++ix)
			_elems.push_back( (ix*(3*ix-1))/2 );
	return &_elems;
}

bool pentagonal_elem(int pos, int &elem)
{
	if (!check_validity(pos)){
		cout << "sorry, Invalid position:" << pos << endl;
		elem = 0;
		return false;
	}
	const vector<int>* pent = pentagonal_series(pos);
	elem = (*pent)[pos-1];
	return true;
}

int main()
{
	int elem;
	if (pentagonal_elem (8, elem))
		cout << "element 8 is " << elem << '\n';
	if (pentagonal_elem(88,elem))
		cout << "element 88 is " << elem << '\n';
}