- Indexing involves picking out certain elements within a matrix. For a row or column matrix you simply have to specify the position in the unidirectional sequence. With larger matrices, there’s a need to specify the column and row of the element. Examples of these are seen below:

**CHECKPOINT**: *This is the “end” (No, not really)*

What would the following do to the matrices S and M above?

`P = M (2 : end, 2 : end)`

`J = S (4 : end-1)`

`T = S (end : -1 : 1)`

What is the key difference between matrices T and S?

- Summary on the use of the colon operator,
`:`

Format |
Purpose |

`A(:, j)` |
Outputs the jth column of A |

`A(i, : )` |
Outputs the ith row of A |

`A(:, : )` |
Outputs a two-dimensional array. If A were a matrix, output would be A |

`A(j : k)` |
Outputs jth to kth elements of A |

`A(:, j : k)` |
Outputs all the elements from the jth to kth columns of A |

`A(:)` |
Outputs all elements of A regarded as a single column |

- Empty square brackets
`[ ]`

are used to delete rows or columns in a matrix e.g.`A(:, j)`

deletes the jth column of matrix A.