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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.