MatrixXd表示任意size的矩陣，元素類型爲double; VectorXd表示任意size的向量，元素類型爲double.html
//建立3*1的向量v，並賦值爲1,2,3 VectorXd v(3); v << 1, 2, 3;
使用固定尺寸的Matrix,Vector相比於可變尺寸的Matrix,Vector，例如Matrix3d m代替MatrixXd m(3,3)有如下優勢：ide
運行速度更快，編譯期間可實現更嚴格的錯誤檢查。函數
Eigen中，全部的矩陣，向量都是Matrix模板類的對象，向量只是矩陣的特例，行數或者列數爲1.性能
//便捷定義 typedef Matrix<float, 4, 4> Matrix4f; typedef Matrix<float, 3, 1> Vector3f; typedef Matrix<int, 1, 2> RowVector2i;
Eigen中經過（）獲取其元素，起始索引爲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.lua
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。spa
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. scala
matrix和array之間能夠相互進行轉換，經過調用matrix的.array()函數將matrix轉換爲array表達式；經過調用array的.matrix()函數將array轉換爲matrix表達式。3d
Eigen中禁止一個表達式中混合使用matrix和array,但容許賦值運算符這樣操做。指針
Eigen中爲matrix提供了cwiseProduct()函數以實現逐元素相乘。code
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。相比於使用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範數的平方，norm()計算2範數，lpNorm<p>()計算p範數，p設置爲Infinity可計算無窮範數。
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。