[英]sum of non-integer elements in std::vector
I was reading following question: How to sum up elements of a C++ vector? 我正在阅读以下问题: 如何总结C ++向量的元素? , and I wanted to use second method (
sum_of_elems =std::accumulate(vector.begin(),vector.end(),0);//#include <numeric>
). ,我想使用第二种方法(
sum_of_elems =std::accumulate(vector.begin(),vector.end(),0);//#include <numeric>
)。
However, I don't have std::vector<int>
, but std::vector<struct packet>
. 但是,我没有
std::vector<int>
,而是std::vector<struct packet>
。 The packet
is defined as following: packet
定义如下:
struct packet {
/// ...
int length() const;
///
}
and I want sum of packet lengths. 我想要数据包长度的总和。
This is what I tried: 这是我试过的:
std::vector<packet> packets;
...
std::accumulate(packets.begin(), packets.end(), 0, [](const packet& a, const packet& b){return a.length() + b.length();});
but it doesn't work. 但它不起作用。 In C#, I'd write something like
在C#中,我会写一些类似的东西
packet[] p;
p.Select(x => p.length()).Sum();
Is it possible to do something like that in C++? 是否有可能在C ++中做类似的事情? I can write method for iterating through the vector and doing it on my own, but I prefer the functional approach where possible.
我可以编写迭代矢量并自己完成的方法,但我更喜欢功能方法。
I would note that the C# implementation is slightly different, in essence. 我注意到C#实现本质上略有不同。
In C++ you are trying to add int
and packet
whilst in C# you first provide a transformation step from packet
to int
and then add int
s. 在C ++中,您尝试添加
int
和packet
而在C#中,您首先提供从packet
到int
的转换步骤,然后添加int
。
The equivalent C++, without adaptation: 等效的C ++,没有适应:
std::vector<size_t> lengths; // a length cannot be negative!
std::transform(packets.begin(),
packets.end(),
backward_inserter(lengths),
[](packet const& p) { return p.length(); });
auto const sum = std::accumulate(lengths.begin(), lengths.end(), 0ul);
Of course, it is wasteful to store the intermediate lengths, however it does work out of the box. 当然,存储中间长度是浪费的,但它确实可以开箱即用。
But because we are cool, let us have look at Boost.Range
, and more precisely: 但是因为我们很酷,让我们看一下
Boost.Range
,更确切地说:
Which have a bit of coolness like Linq: 像Linq一样有点酷:
#include <boost/range/numeric.hpp> // accumulate
#include <boost/range/adaptor/transformed.hpp>
size_t total_size(std::vector<packet> const& packets) {
return boost::accumulate(
packets | boost::transformed([](packet const& p) { return p.length(); }),
0ul);
}
You are accumulating via a binary operation. 您正在通过二进制操作进行累积。 Your accumulated value starts with
0
(an int
), so the left hand side of your binary operation has to be convertible-from 0
-- otherwise, how does it start adding? 您的累计值以
0
(一个int
)开头,因此二进制操作的左侧必须是可转换的 - 从0
- 否则,它是如何开始添加的?
Try this: 尝试这个:
std::accumulate(
packets.begin(),
packets.end(),
0,
[](int a, const packet& b){
return a + b.length();
}
);
you can also do this via a simple loop: 你也可以通过一个简单的循环来做到这一点:
int acc = 0;
for( const packet& p : packets ) {
acc += packets.length();
}
The first parameter of the accumulate operation is the running total. 累积操作的第一个参数是运行总计。 In your case, this is an integer, not a packet, so your lambda should be
在你的情况下,这是一个整数,而不是一个数据包,所以你的lambda应该是
[](int a, const packet& b) {return a + b.length();}
The problem is your accumulate function. 问题是你的累积功能。 Its first parameter has to be of the type you're trying to accumulate (
int
in this case) and add a value on top of that. 它的第一个参数必须是你试图积累的类型(在这种情况下为
int
)并在其上添加一个值。
Your lambda function should look like this: [](int currTotal, const packet& b){return currTotal + b.length();}
你的lambda函数应如下所示:
[](int currTotal, const packet& b){return currTotal + b.length();}
Apart from lamba, you can change it to 除了lamba,您可以将其更改为
std::accumulate(packets.begin(), packets.end(), 0, packet());
Where you can define functor as: 您可以在哪里定义仿函数:
int operator() (int result, const packet& obj)
{
return result+ obj.length();
}
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