--吴恩达 机器学习 笔记
Matrices are 2-dimensional arrays: [a b c d e f g h i j k l]
The above matrix has four rows and three columns, so it is a 4 x 3 matrix.
A vector is a matrix with one column and many rows: [w x y z]
So vectors are a subset of matrices. The above vector is a 4 x 1 matrix.
Notation and terms:
. Aij refers to the element in the ith row and jth column of matrix A. . A vector with 'n' rows is referred to as an 'n'-dimensional vector. . vi refers to the element in the ith row of the vector. . In general, all our vectors and matrices will be 1-indexed. Note that for some programming languages, the arrays are 0-indexed. . Matrices are usually denoted by uppercase names while vectors are lowercase. . "Scalar" means that an object is a single value, not a vector or matrix. . R refers to the set of scalar real numbers. . Rn refers to the set of n-dimensional vectors of real numbers.
