title: Repeated measures in messy format, need help to tidy
body: <p>I have a very large data set containing weekly weights that have been coded with week of study and the weight at that visit. There are some missing visits and the data is not currently aligned.</p>
  
  <pre><code>df &lt;- data.frame(ID=1:3, Week_A=c(6,6,7), Weight_A=c(23,24,23), Week_B=c(7,7,8), 
                   Weight_B=c(25,26,27), Week_C=c(8,9,9), Weight_C=c(27,26,28)) 
  
  df
    ID Week_A Weight_A Week_B Weight_B Week_C Weight_C
  1  1      6       23      7       25      8       27
  2  2      6       24      7       26      9       26
  3  3      7       23      8       27      9       28
  </code></pre>
  
  <p>I would like to align the data by week number (ideal output below). </p>
  
  <pre><code>df_ideal &lt;- data.frame (ID=1:3, Week_6=c(23,24,NA), Week_7=c(25,26,23), 
                          Week_8=c(27,NA,27), Week_9=c(NA,26,28))
  
  df_ideal
    ID Week_6 Week_7 Week_8 Week_9
  1  1     23     25     27     NA
  2  2     24     26     NA     26
  3  3     NA     23     27     28
  </code></pre>
  
  <p>I would appreciate some help with this, even to find a starting point to manipulate this data to an easier to manage format.</p>