essional 2.2 练习中的查了查_elems中的int ix=vec.size()

#include <vector>
#include<string>
#include<iostream>
using namespace std;
bool calc_elements(vector<int> &vec, int pos);
void display_elems(vector<int> &vec, const string &title,ostream &os=cout);
int main()
{
    vector<int> pent;
    const string title("Pentagonal Numeric Series");
    if (calc_elements(pent, 0))
        display_elems(pent, title);
    if (calc_elements(pent, 8))
        display_elems(pent, title);
    if (calc_elements(pent, 14))
        display_elems(pent, title);
    if (calc_elements(pent, 138))
        display_elems(pent, title);
}
bool calc_elements(vector<int> &vec, int pos)
{
    if (pos <= 0 || pos > 64) {
        cerr << "......" << pos << endl;
        return false;
    }
        for (int ix = vec.size();ix < pos;++ix)
            vec.push_back((ix * (3 * ix - 1)) / 2);
            return true;

}
void display_elems(vector<int> &vec, const string &title,ostream &os)
{
    cout << '\n' << title << "\n\t";
    for (int ix = 0;ix <vec.size();++ix)
        cout<< vec[ix] << ' ';
    cout<< endl;
}

中

bool calc_elements(vector<int> &vec, int pos)
{
    if (pos <= 0 || pos > 64) {
        cerr << "......" << pos << endl;
        return false;
    }
        for (int ix = vec.size();ix < pos;++ix)
            vec.push_back((ix * (3 * ix - 1)) / 2);
            return true;

}

int ix 为什么要等于vec.size()

http://blog.sina.com.cn/s/blog_a9fd746b01018ijp.html