Essential C++练习2.3

将练习2.2的Pentagonal数列求值函数拆分为两个函数，其中之一为inline，用来检验元素个数是否合理。如果合理而且尚未被计算，便执行第二个函数，执行实际求值工作。

#include
#include
using namespace std;

inline bool calc_elems(vector& vec, int pos);
void really_calc_elems(vector& vec, int pos);
void display_elems(vector& vec, const string& title);

int main()
{
	vector pent;
	const string title("Pentagonal Numeric Series");

	if (calc_elems(pent, 0))
	{
		display_elems(pent, title);
	}

	if (calc_elems(pent, 8))
	{
		display_elems(pent, title);
	}

	if (calc_elems(pent, 14))
	{
		display_elems(pent, title);
	}

	if (calc_elems(pent, 128))
	{
		display_elems(pent, title);
	}

	system("pause");
	return EXIT_SUCCESS;
}

inline bool calc_elems(vector& vec, int pos)
{
	if (pos<=0||pos>64)
	{
		cout << endl;
		cerr << "Sorry! Invalid position: " << pos << endl;
		return false;
	}
	if (vec.size()& vec, int pos)
{
	for (unsigned i = vec.size() + 1; i <= pos; i++)
	{
		vec.push_back(i * (3 * i - 1) / 2);
	}
}
void display_elems(vector& vec, const string& title)
{
	cout << endl << title << endl;
	for (unsigned i = 0; i < vec.size(); i++)
	{
		cout << vec[i] << " ";
	}
	cout << endl;
}