accumulate

accumulate？就是sum up a range of elements。呵呵。这个挺简单的。以下是这个算法的简单介绍：

Syntax:
#include <numeric>//呵呵，使用这个算法这个头文件是必需要包含进来滴！
TYPE accumulate( input_iterator start, input_iterator end, TYPE val );
TYPE accumulate( input_iterator start, input_iterator end, TYPE val, BinaryFunction f );

The accumulate function computes the sum of val and all of the elements in the range [start,end).
If the binary function f is specified, it is used instead of the + operator to perform the summation.
The accumulate function runs in linear time. //我看了非常久，linear time 应该说的这个算法的复杂度是O（n）吧，呵呵~~~百度之貌似没有结果。

嗯，废话少说，以下来看它的应用。用这个算法来计算1到100的和。

#include <iostream>
#include <vector>
#include <numeric>
using namespace std;
int main()
{
vector<int> v;
const int START = 1, END = 100;
for( int i = START; i <= END; ++i )
v.push_back(i);//把1到100压入vector容器中！
int sum = accumulate( v.begin(), v.end(), 0 );//就是这个算法，非常奇妙吧。
cout << "sum from " << START << " to " << END << " is " << sum << '/n';
return 0;
}

毫无疑问，它的执行结果是"sum from 1 to 100 is 5050"值得注意的是，TYPE accumulate( input_iterator start, input_iterator end, TYPE val );val也是要加进去滴！上面是0，肯定等于没加！

当然，这个程序我们一般用个for循环解决就是，干嘛还要这么大费周折呢。呵呵，事实上 accumulate 可爱的地方不只在于对于数字运算支持，对于非数值运算也是支持的！

The accumulate function can also be used on non-numerical types. The following example uses accumulate to concatenate all of the strings in a vector into a single string:

#include <iostream>
#include <vector>
#include <string>
#include <numeric>
using namespace std;
int main ()
{
string str = "Hello World!";
vector<string> vec(10,str); // vec = ["Hello World!", "Hello World!", ...]
string a = accumulate( vec.begin(), vec.end(), string("HaHaHa----") );
cout << a << endl; // displays "HaHaHa----Hello World!Hello World!Hello..."
return 0;
}

呵呵，第一个STL C++ Algorithms accumulate 算法介绍完成！