DenseBase，自动和二进制操作表示数组具有不同的形状 [英] DenseBase, auto, and binary operation says arrays have different shape

问题描述

我编写了一个函数，该函数使用两个 DenseBase 作为参数。

I write a function that takes two DenseBase as arguments.

该函数使用 .derived（）。array（）转换两个 Array 和 Matrix 到 Array 。

The function uses .derived().array() to convert both Array and Matrix to Array.

我厌倦了多次编写派生的并使用自动。

I got tired of writing derived for many times and use auto.

但是自动会导致奇怪的错误。 Eigen抱怨 x2 和 y2 的形状不相同。

But auto leads to strange error. Eigen complains that x2 and y2 don't have same shape.

如果我不想多次写 .derived（）。array（），我该怎么用？

If I don't want to write .derived().array() for many times, what can I use?

本征来自 https://github.com/eigenteam/ eigen-git-mirror.git

#include <Eigen/Eigen>
int main() {
    Eigen::ArrayXf x(3);
    Eigen::ArrayXf y(3);
    x << 1, 2, 3;
    y << 4, 5, 6;
    // x.derived().array() * y.derived().array();
    auto x2 = x.derived().array();
    auto y2 = y.derived().array();
    y2 = x2 * y2; 
}

运行时错误：

CwiseBinaryOp.h:110: ...

Assertion `aLhs.rows() == aRhs.rows() 
           && aLhs.cols() == aRhs.cols()' failed.

推荐答案

您可以使用<$ c $解决运行时问题c> auto x2 = x.array（）。derived（）; ，即：反向数组并派生。但是 auto 在这里不是可取的。这就是为什么。假设您有：

You can fix the runtime issue with auto x2 = x.array().derived();, that is: reverse array and derived. But auto is not desirable here. Here is why. Say you have:

template <typename T> void foo(DenseBase<T> &x);

如果 T 是 Array<> ，然后 x.array（）。derived（）是 Array<> 和 x2 将是 x 的深层副本。在这种情况下，您想使用 auto& x2 = ... 。

If T is an Array<> then x.array().derived() is an Array<> and x2 will be a deep copy of x. In this case you would like to use auto& x2 = ....

如果 T 是别的东西，例如 Matrix<> ，然后 auto x2 = x.array（）。derived（）; 很好，但是不是 auto& x2 = ... 。

If T is something else, e.g., a Matrix<>, then auto x2 = x.array().derived(); is perfectly fine, but not auto& x2 = ....

因此，您真正想要的东西很复杂：

So what you really want is something as complicated as:

internal::ref_selector<std::decay<decltype(x.array().derived())>::type>::non_const_type
  x2 = x.array().derived();

不好：（

一个简单解决方案是不要为创建数组世界中的输入而烦恼并创建 ArrayWrapper ：

A simpler solution is to not bother and create an ArrayWrapper even for inputs that are already in the array world:

ArrayWrapper<T> x2(x.derived());

另一个简单的解决方案是强制调用者在数组世界中传递表达式：

Yet another simple solution is to enforce the caller to pass expressions in the array world:

template <typename T> void foo(ArrayBase<T> &x) {
  T& x2(x.derived());
  ...
}

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