使用std :: accumulate [英] Using std::accumulate

问题描述

我总是尽可能地尝试结合STL算法，而不是编写手动循环.但是，我很难理解 std :: accumulate 通常如何有用.每当我需要计算总和或平均值时，我几乎总是会最终求助于手动循环，因为我很难让 std :: accumulate 来完成我需要的工作.

I always try to incorporate STL algorithms wherever I can, rather than writing manual loops. However, I'm having difficulty understanding how std::accumulate is generally useful. Whenever I need to calculate sums or averages, I almost always end up resorting to manual loops, because I have difficulty getting std::accumulate to do what I need.

问题是我很少有一个简单的整数向量需要求和.通常，我想使用特定的成员变量求和一个对象数组.是的，我知道有一个采用BinaryFunction的 std :: accumulate 版本，但是我看到的问题是该函数需要采用两个 T 类型的值，其中， T 是 sum 的类型，而不是 operands 的类型.我在理解它的用处时遇到了麻烦.

The problem is that I rarely ever have a simple vector of integers that need to be summed. Usually, I want to sum an array of objects using a particular member variable. Yes, I know there is a version of std::accumulate that takes a BinaryFunction, but the problem I see is that this function needs to take two values of type T, where T is the type of the sum, rather than the type of the operands. I'm having trouble understanding how this is useful.

考虑一个我认为很普遍的情况.我有以下课程:

Consider a case which I assume is pretty common. I have the following class:

struct Foo
{
    Foo(int cost_, int id_) : cost(cost_), id(id_)
    { }

    int cost;
    int id;
};

现在，说我想使用 Foo :: cost 计算 Foo 对象数组的总和.

Now, say I want to calculate the sum of an array of Foo objects, using Foo::cost.

我想说:

std::vector<Foo> vec;
// fill vector with values
int total_cost = std::accumulate(vec.begin(), vec.end(), 0, sum_cost);

sum_cost 定义为:

int sum_cost(const Foo& f1, const Foo& f2)
{
    return f1.cost + f2.cost;
}

问题是，这不起作用，因为 std :: accumulate 期望BinaryFunction接受两个结果总和类型的实例-在这种情况下是只是 int .但是，这对我有什么用呢?如果我的BinaryFunction接受两个 int ，则不能指定我想对 cost 字段求和.

The problem is, this doesn't work because std::accumulate expects a BinaryFunction which takes in two instances of the resulting sum type - which in this case is just int. But how is that useful to me? If my BinaryFunction takes in two ints, I can't specify that I want to sum the cost field.

那么，为什么 std :: accumulate 这样设计?我只是在这里看不到明显的东西吗?

So, why is std::accumulate designed this way? Am I just not seeing something obvious here?

推荐答案

关于将两个相同类型的运算符累加，您是错误的.仅在您愿意时这样做.具体来说，使用运算符是 sum = op(sum，* iter).因此您的代码:

You're wrong about accumulate operator taking two of the same type. It does that only if you want to. The use the operator is specifically sum = op(sum, *iter). Thus your code:

int count = std::accumulate(stuff.begin(), stuff.end(), 0, [](int current_sum, stuff_value_t const& value) { return current_sum + value.member; });

如果您不能使用lambda，那么您当然可以使用标准的活页夹或boost :: bind.

If you can't use lambda then of course you use the standard binders or boost::bind.

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