Only planets with rings

You will often want to select an entire column, namely one specific variable from a data frame. If you want to select all elements of the variable diameter, for example, both of these will do the trick:

planets_df[,3]
planets_df[,"diameter"]

However, there is a short-cut. If your columns have names, you can use the $ sign:

planets_df$diameter

Instructions

Use the $ sign to select the rings variable from planets_df. Store the vector that results as rings_vector.
Print out rings_vector to see if you got it right.

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