title: dplyr arrange() function sort by missing values
body: <p>I am attempting to work through Hadley Wickham's R for Data Science and have gotten tripped up on the following question: <em>"How could you use arrange() to sort all missing values to the start? (Hint: use is.na())"</em> I am using the <strong>flights</strong> dataset included in the <strong>nycflights13</strong> package. Given that arrange() sorts all unknown values to the bottom of the dataframe, I am not sure how one would do the opposite across the missing values of all variables. I realize that this question can be answered with base R code, but I am specifically interested in how this would be done using dplyr and a call to the arrange() and is.na() functions. Thanks.</p>