特征索引，更新特定行的所有列 [英] Eigen indexing, update all column of a specific row

问题描述

假设我有一个 ArrayXXf (或 MatrixXf ) m .在for循环的每次迭代中，我都想用 VectorXf 逐行填充 m .

Let say I have an ArrayXXf (or MatrixXf) m. In each iteration of a for loop, I want to fill m row-wise with a VectorXf.

Eigen::ArrayXXf m(5, 5);

for (int i = 0; i < 5; i++)
{
    Eigen::VectorXf vec(5);
    vec << i, i + 1, i + 2, i+3, i+4;

    //fill m row wise
    // in matlab I will do something like m(i,:) = vec; 
    // in numpy this will looks like m[i:] = vec;
    // that means when i is 0 m looks like 
    //          [ 0 1 2 3 4 5
    //           -  - - - - -
    //           -  - - - - -
    //           -  - - - - -
    //           -  - - - - -]
}

如何在Eigen中实现这一目标?

How can I achieve that in Eigen?

推荐答案

使用block()函数.

Use block() function.

#include <iostream>
#include <Eigen/Dense>

using namespace std;

int main()
{
    Eigen::ArrayXXf m(5, 5);

    for (int i = 0; i < 5; i++) {
        Eigen::VectorXf vec(5);
        vec << i, i + 1, i + 2, i+3, i+4;

        m.block(i, 0, 1, 5) << vec.transpose();
    }

    std::cout << m << std::endl;
    return 0;
}

输出:

0 1 2 3 4
1 2 3 4 5
2 3 4 5 6
3 4 5 6 7
4 5 6 7 8

还有一种更简单的选择:row()函数.

There is one simpler alternative also: row() function.

#include <iostream>
#include <Eigen/Dense>

using namespace std;

int main()
{
    Eigen::ArrayXXf m(5, 5);

    for (int i = 0; i < 5; i++) {
        Eigen::VectorXf vec(5);
        vec << i, i + 1, i + 2, i+3, i+4;

        m.row(i) = vec.transpose();
    }

    std::cout << m << std::endl;
    return 0;
}

输出:

0 1 2 3 4
1 2 3 4 5
2 3 4 5 6
3 4 5 6 7
4 5 6 7 8

P.S.transpose()是必需的，因为默认情况下Eigen :: VectorXf是列向量，不是行向量.

P.S. transpose() is required because Eigen::VectorXf by default is a column vector, not a row vector.

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