Selection of data frame elements

Similar to vectors and matrices, you select elements from a data frame with the help of square brackets [ ]. By using a comma, you can indicate what to select from the rows and the columns respectively. For example:

my_df[1,2] selects the value at the first row and select element in my_df.
my_df[1:3,2:4] selects rows 1, 2, 3 and columns 2, 3, 4 in my_df.

Sometimes you want to select all elements of a row or column. For example, my_df[1, ] selects all elements of the first row. Let us now apply this technique on planets_df!

Instructions

From planets_df, select the diameter of Mercury: this is the value at the first row and the third column. Simply print out the result.
From planets_df, select all data on Mars (the fourth row). Simply print out the result.

Take Hint (-30xp)