STL之partial_sum

需要的头文件：

numeric

源码：

//版本1
template <class _InputIterator, class _OutputIterator, class _Tp>
_OutputIterator
__partial_sum(_InputIterator __first, _InputIterator __last,
_OutputIterator __result, _Tp*)
{
_Tp __value = *__first;
while (++__first != __last) {
__value = __value + *__first;
*++__result = __value;
}
return ++__result;
}

template <class _InputIterator, class _OutputIterator>
_OutputIterator
partial_sum(_InputIterator __first, _InputIterator __last,
_OutputIterator __result)
{
if (__first == __last) return __result;
*__result = *__first;
return __partial_sum(__first, __last, __result, __VALUE_TYPE(__first));
}
//版本2
template <class _InputIterator, class _OutputIterator, class _Tp,
class _BinaryOperation>
_OutputIterator
__partial_sum(_InputIterator __first, _InputIterator __last,
_OutputIterator __result, _Tp*, _BinaryOperation __binary_op)
{
_Tp __value = *__first;
while (++__first != __last) {
__value = __binary_op(__value, *__first);
*++__result = __value;
}
return ++__result;
}

template <class _InputIterator, class _OutputIterator, class _BinaryOperation>
_OutputIterator
partial_sum(_InputIterator __first, _InputIterator __last,
_OutputIterator __result, _BinaryOperation __binary_op)
{
if (__first == __last) return __result;
*__result = *__first;
return __partial_sum(__first, __last, __result, __VALUE_TYPE(__first),
__binary_op);
}

作用：

计算局部总和，存储第一元素的值，然后计算第n个元素与前n-1个元素的总和。

通过二元仿函数我们可以取代 operator+

例子：

#include <numeric>
#include <functional>
#include <vector>
#include <iterator>
#include <iostream>
#include <algorithm>

using namespace std;

int main()
{
int ia[5] = { 1,2,3,4,5 };
vector<int> iv(ia, ia + 5);

//作为输出
ostream_iterator<int> oiter(cout, " ");

//计算局部总和
partial_sum(begin(iv), end(iv), oiter);
//1 3 6 10 15 ( 第n个新元素是前n个旧元素的相加总计 )

cout << endl;
//计算局部阶乘
partial_sum(begin(iv), end(iv), oiter,[](int a, int b)
{
return a*b;
});
// 1 2 6 24 120
cout << endl;

//计算斐波那契数列
vector<int> fac = {1,1};
int loop = 20;
cout << fac[0] << endl;
while (loop-- > 0)
{
partial_sum(begin(fac), end(fac), begin(fac), [&](int a, int b)
{
swap( fac[0], fac[1] );
return a+b;
});
cout <<  fac[0] << endl;
}
//fac:  1  1
//fac:  1  2    swap->  2 1
//fac:  2  3    swap->  3 2
//fac:  3  5    swap->  5 3
//fac:  5  8    swap->  8 5
//fac:  8  13   swap->  13 8
//fac:  ......
return 0;
}

注意：

如果减法和加法的定义一如常规定义，那么partial_sum与之前介绍过的adjacent_difference互为逆运算。这里的意思是，如果对区间1,2,3,4,5做partial_sum，获得结果1,3,6,10,15，再对此做adjacent_different，便会获得原始区间值1,2,3,4,5