c++: std::accumulate使用

实现

/**
   *  @brief  Accumulate values in a range with operation.
   *
   *  Accumulates the values in the range [first,last) using the function
   *  object @p __binary_op.  The initial value is @p __init.  The values are
   *  processed in order.
   *
   *  @param  __first  Start of range.
   *  @param  __last  End of range.
   *  @param  __init  Starting value to add other values to.
   *  @param  __binary_op  Function object to accumulate with.
   *  @return  The final sum.
   */
  template<typename _InputIterator, typename _Tp, typename _BinaryOperation>
    inline _Tp
    accumulate(_InputIterator __first, _InputIterator __last, _Tp __init,
	       _BinaryOperation __binary_op)
    {
      // concept requirements
      __glibcxx_function_requires(_InputIteratorConcept<_InputIterator>)
      __glibcxx_requires_valid_range(__first, __last);

      for (; __first != __last; ++__first)
	__init = __binary_op(__init, *__first);
      return __init;
    }

累加

   std::vector<int> vec(100,1);
    int s=0;
    for(auto& i:vec){
        s+=i;
    }
    std::cout<<s<<" "<<vec.size()<<std::endl;
    std::cout<<"sum:"<<std::accumulate(vec.begin(),vec.end(),int{0})<<std::endl;

输出

Hello World!
100 100
sum:100

累乘

 vec.push_back(2);
 vec.push_back(3);
 std::cout<<"multiplies:"<<std::accumulate(vec.begin(),vec.end(),int{1},std::multiplies<int>())<<std::endl;

输出

multiplies:6

自定义

可以使用形如fun(R,T)->R的函数，返回R类型。T为集合的类型，R为自定义类型，如下面的例子，统计字符串中\n的数量，R为int，T为char

 std::string str{"abc\ncde\nfgh\n"};
 std::cout<<"line::"<<std::accumulate(str.begin(),str.end(),0,[](int nCount,char ch){return (ch == '\n')?(nCount+1):nCount;});

输出

line::3