Moving the "Subsetting data" lesson to be before "Exploring dataframes"

[urmipoddar]

In lesson 5 ("Exploring dataframes"), learners are taught how to remove rows and columns using "dataframe[- row index, ]" or "dataframe[ ,- column index]". But the concept of indexing is not introduced until lesson 6 ("exploring dataframes"). So students may not actually understand how this method of removing rows and columns or what the word "index" means. Perhaps the lesson on subsetting data could come before the lesson on exploring dataframes?