Exercise 2.4

Introduce a static local vector to hold the elements of your Pentagonal series. This function returns a const pointer to the vector. It accepts a position by which to grow the vector if the vector is not that size as yet. Implement a second function that, given a position, returns the element at that position. Write a main() function to exercise these functions.

#include <vector> 


#include <iostream> 


using namespace std; 





inline bool check_validity( int pos ) 


    {  return ( pos <= 0 || pos > 64 ) ? false : true; } 





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; 


} 





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; 


} 





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'; 


    if ( pentagonal_elem( 12, elem )) 


         cout << "element 12 is " << elem << '\n'; 





    if ( pentagonal_elem( 64, elem )) 


         cout << "element 64 is " << elem << '\n'; 


 } 

When compiled and executed, this program generates the following:

element 8 is 92 


Sorry. Invalid position: 88 


element 12 is 210 


element 64 is 6112