title: How to convert week and year columns to a date column with lubridate in R
body: <p>I have been following along with Hadley Wickham's R for data science book. He has a lot of advice on using lubridate, but a lot of the functions assume that you have year, month, and day. How do you convert to date format when all you have is year and week using lubridate?</p>
  
  <pre><code>data.frame(
      year = c(2015, 2015, 2016, 2016, 2016, 2016, 2016),
      week = c(1, 20, 35, 49, 8, 4, 53)
    )
  
  #year   week
  #2015    1
  #2015    20
  #2016   35  
  #2016   49  
  #2016   8   
  #2016   4   
  #2016   53
  </code></pre>