Exercise

# Selection of matrix elements

Similar to vectors, you can use the square brackets `[ ]`

to select one or multiple elements from a matrix. Whereas vectors have one dimension, matrices have two dimensions. You should therefore use a comma to separate the rows you want to select from the columns. For example:

`my_matrix[1,2]`

selects the element at the first row and second column.`my_matrix[1:3,2:4]`

results in a matrix with the data on the rows 1, 2, 3 and columns 2, 3, 4.

If you want to select all elements of a row or a column, no number is needed before or after the comma, respectively:

`my_matrix[,1]`

selects all elements of the first column.`my_matrix[1,]`

selects all elements of the first row.

Back to Star Wars with this newly acquired knowledge! As in the previous exercise, `all_wars_matrix`

is already available in your workspace.

Instructions

**100 XP**

- Select the non-US revenue for all movies (the entire second column of
`all_wars_matrix`

), store the result as`non_us_all`

. - Use
`mean()`

on`non_us_all`

to calculate the average non-US revenue for all movies. Simply print out the result. - This time, select the non-US revenue for the first two movies in
`all_wars_matrix`

. Store the result as`non_us_some`

. - Use
`mean()`

again to print out the average of the values in`non_us_some`

.