  Course Content

# R Introduction: Part II

R Introduction: Part II

##   Matrix Indexation

Good. Now you know how to create a matrix in several ways. Let's work with already created objects. First, we need to learn how to access certain matrix elements.

Back to the Vectors section, you should remember that indexation in R starts with 1. Since the matrix is a two-dimensional structure, we need to refer to two positions: among rows and columns. Like in vectors, you need to use square brackets and put two integers (the first is the row number, the second is the column number) divided by a comma. For example, let's extract element `5` (row 2, column 2) and `3` (row 1, column 3) from the matrix below.  Also you can extract multiple rows and/or columns at once. Pass vector of indices instead of a single index. For example, we can extract the 2nd and 3rd columns for the 3rd row.  If you want to extract an entire row/column, do not set an index for the remaining part. For example, let's pull the first row and the third column.  Given matrix named `m`.
1. Extract the element `12`.
2. Extract the elements `4 6`. 