- In order to select randomly positioned elements (e.g. (2,3), (1,4) and (1,2)) within a matrix, you need to use the program’s linear indexing system. This numbers the elements as if they were placed in a
**line**. The numbering system starts from the top left moves downwards and continues at the top of the next column.

1 |
5 |
9 |
13 |

2 |
6 |
10 |
14 |

3 |
7 |
11 |
15 |

4 |
8 |
12 |
16 |

- For example, in matrix M (below), elements 1 to 4 are 16, 5, 9 and 4 and elements 5 to 8 would be 2, 11, 7 and 14 and so on. Thus, in order to select elements (2,3), (1,4) and (1,2):

**EXERCISE 3**

- From matrix J make a 4-by-4 matrix P that comprises the first four rows and last four columns of J
- From P make a 3-by-3 matrix K that comprises the first three rows and columns of P
- From K, make a 2-by-2 matrix C with the same elements it would have if it were declared by the code
`C=[21 ,27; 7, 26]`

. - By hand, write down the matrix, L made by the code:
`L=J([1, 7, 8; 22, 17 18; 9, 33, 4])`

- Using the attributes of J declare a scalar N, with a magnitude of 36.

**MISCELLANEOUS EXERCISE**

- Logical operators use a binary system (1 and 0) to act as some sort of
**filter**. Generally, a 1 signifies that the statement being analysed is true and a 0 implies it’s false. The 1 and 0 arrangement of a logical operator is usually used to harvest certain elements of a matrix. For example, a logical matrix B is shown below:

For a matrix A:

- What do you reckon would be the output of the code
`A(B)`

? Write down your guess by hand and run the code to see what it actually does. - What would be the output of the code
`C = isprime(A)`

? (Hint: it creates a logical matrix.) - Make a note of what the following do:
`A(~C)`

and`A(~C)=0`

. What is the difference between the two? If you had to characterise the action of the`~`

operator in words, what would you say is its function? (more on non-arithmetic operators such as this would be covered later)

(For more on Matrix indexing click on the link: Advanced Matrix indexing and Examples)