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.
all_wars_matrix), store the result as non_us_all.mean() on non_us_all to calculate the average non-US revenue for all movies. Simply print out the result.all_wars_matrix. Store the result as non_us_some.mean() again to print out the average of the values in non_us_some.
load(url("http://s3.amazonaws.com/assets.datacamp.com/course/intro_to_r/all_wars_matrix.RData"))
# all_wars_matrix is available in your workspace
all_wars_matrix
# Select the non-US revenue for all movies
non_us_all <-
# Average non-US revenue
# Select the non-US revenue for first two movies
non_us_some <-
# Average non-US revenue for first two movies
# all_wars_matrix is available in your workspace
all_wars_matrix
# Select the non-US revenue for all movies
non_us_all <- all_wars_matrix[,2]
# Average non-US revenue
mean(non_us_all)
# Select the non-US revenue for first two movies
non_us_some <- all_wars_matrix[1:2,2]
# Average non-US revenue for first two movies
mean(non_us_some)
msg = "Do not change the contents of `all_wars_matrix`; this matrix has already been created for you in the workspace."
test_object("all_wars_matrix", undefined_msg = msg, incorrect_msg = msg)
test_object("non_us_all",
incorrect_msg = "Did you assign to `non_us_all` the entire second column of `all_wars_matrix`? You can use `[, 2]` to do this!")
test_output_contains("mean(non_us_all)",
incorrect_msg = "Have you calculated the average of the values in `non_us_all` by calling `mean(non_us_all)`? Simply print out the result.")
test_object("non_us_some",
incorrect_msg = "Did you assign to `non_us_some` the non-US revenue for the first two movies? You can use `[1:2,2]` to do this!")
test_output_contains("mean(non_us_some)",
incorrect_msg = "Have you calculated the average of the values in `non_us_some` by calling `mean(non_us_some)`? Simply print out the result.")
success_msg("Nice one! Continue to the next exercise.")
You can select the entire second column of a matrix my_matrix with my_matrix[,2].