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.
This is a part of the course
“Introduction to R”
Exercise instructions
- Select the non-US revenue for all movies (the entire second column of
all_wars_matrix
), store the result asnon_us_all
. - Use
mean()
onnon_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 asnon_us_some
. - Use
mean()
again to print out the average of the values innon_us_some
.
Hands-on interactive exercise
Have a go at this exercise by completing this sample code.
# 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