: 1+(1+2)+(1+2+3)+ ............ +(1+2+3+4+......+19+20) 의 합을 구하시오..

STL을 써서, for 루프를 쓰지 않고 구현하는 방법입니다.

// Illustrating the generic partial_sum algorithm
#include <iostream>
#pragma hdrstop
#include <algorithm>
#include <numeric>
#include <vector>
#include <iterator>

using namespace std;

int main()
{
  cout << "Illustrating the generic iota and partial_sum algorithm."
       << endl;
  const int N = 20;
  vector<int> x1(N), x2(N);

  // ANSI C++ 표준에는 없습니다: 빌더 5이하에서는 STLport를 설치하세요.
  iota(x1.begin(), x1.end(), 1);
 
  copy(x1.begin(), x1.end(), ostream_iterator<int>(cout, " "));
  cout << endl;

  // Compute the partial sums of 1, 2, 3, ..., N
  // putting the result in x2:
  partial_sum(x1.begin(), x1.end(), x2.begin());

  copy(x2.begin(), x2.end(), ostream_iterator<int>(cout, " "));
  cout << endl;

  return 0;
}