Another useful data structure is a matrix, which is a rectangular array or table of numeric values or missing values. You can use a matrix to store a single number, a column of numbers, (sometimes called a column vector,) a row of numbers, (or row vector,) or an entire table of numbers. A matrix can be empty -- that is, have no rows or columns. An empty matrix can be useful for collecting columns, rows, or other matrices of numbers together, and is also used to delete columns or rows from a matrix. And like with lists, you can store a matrix in a variable.
