# Let's reuse the data frame we created earlier (my_df)
# Extract the 1st row of the data frame
> my_df[1, ]

# Extract the 3rd columm
> my_df[, 3]

# Extract the 2nd element of the 4th column
> my_df[2, 4]

# Extract lines 2 to 4
> my_df[c(2,4), ]

# Extract the "site_id" column by referring directly to its name
# The dollar sign ($) allows such an operation!
> my_df$site_id

# Extract the "site_id" and "soil_pH" variables
> my_df[, c("site_id","soil_pH")]