MatrixXd表示任意size的矩陣，元素類型為double; VectorXd表示任意size的向量，元素類型為double.

//創建3*1的向量v，并賦值為1,2,3 VectorXd v(3); v << 1, 2, 3;

使用固定尺寸的Matrix,Vector相比于可變尺寸的Matrix,Vector，例如Matrix3d?m代替MatrixXd?m(3,3)有以下優點：

運行速度更快，編譯期間可實現更嚴格的錯誤檢查。

Eigen中，所有的矩陣，向量都是Matrix模板類的對象，向量只是矩陣的特例，行數或者列數為1.

//便捷定義 typedef Matrix<float, 4, 4> Matrix4f; typedef Matrix<float, 3, 1> Vector3f; typedef Matrix<int, 1, 2> RowVector2i; ? typedef Matrix<double, Dynamic, Dynamic> MatrixXd;? //編譯期未知尺寸

?Eigen中通過（）獲取其元素，起始索引為0。The operator[] is also overloaded for index-based access in vectors, but keep in mind that C++ doesn't allow operator[] to take more than one argument.

?transpose()計算矩陣的轉置，?transpose()不支持就地轉置，使用transposeInPlace()來實現就地轉置。

?Array

We adopt the convention that typedefs of the form ArrayNt stand for 1-dimensional arrays, where N and t are the size and the scalar type.For 2-dimensional arrays, we use typedefs of the form ArrayNNt.?

matrix和array之間可以相互進行轉換，通過調用matrix的.array()函數將matrix轉換為array表達式；通過調用array的.matrix()函數將array轉換為matrix表達式。

Eigen中禁止一個表達式中混合使用matrix和array,但允許賦值運算符這樣操作。

Eigen中為matrix提供了cwiseProduct()函數以實現逐元素相乘。

?Block Operation

matrix.row(i) 獲取矩陣matrix的第i行，matrix.col(j)獲取矩陣matrix的第j列。相比于使用block()性能更好。

Eigen?also provides special methods for blocks that are flushed against one of the corners or sides of a matrix or array. For instance,?.topLeftCorner()?can be used to refer to a block in the top-left corner of a matrix.

?

squaredNorm() 計算2范數的平方，norm()計算2范數，lpNorm<p>()計算p范數，p設置為Infinity可計算無窮范數。

The following reductions operate on boolean values:

• all()?returns?true?if all of the coefficients in a given?Matrix?or?Array?evaluate to?true?.
• any()?returns?true?if at least one of the coefficients in a given?Matrix?or?Array?evaluates to?true?.
• count()?returns the number of coefficients in a given?Matrix?or?Array?that evaluate to?true.

?Visitors

Visitors are useful when one wants to obtain the location of a coefficient inside a?Matrix?or?Array. The simplest examples are?maxCoeff(&x,&y)?and?minCoeff(&x,&y), which can be used to find the location of the greatest or smallest coefficient in a?Matrix?or?Array.

The arguments passed to a visitor are pointers to the variables where the row and column position are to be stored. These variables should be of type?Index?

?matrix.data()函數返回一個指向矩陣內存地址的指針。Eigen中矩陣默認以column-major存儲元素值。

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轉載于:https://www.cnblogs.com/larry-xia/p/10666834.html

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