title: Aggregate/Sum data set by week and by product in R
body: <p>I have a very large data set that I would like to aggregate both by week/month and by product(a few thousand). Is there a way to do so with a data set in the following format? </p>
  
  <pre><code>Date           product product2 product3
  03/03/2011       1        0        7
  04/08/2011       3        8        2
  03/05/2015       6        3       89
  03/01/2017       1        0        2
  03/03/2017       6        1        6
  </code></pre>
  
  <p>which would yield the following: </p>
  
  <pre><code>Date           product product2 product3
  wk1-032011       1        0        7
  wk2-042011       3        8        2
  wk1-032015       6        3       89
  wk1-032017       7        1        8
  
  
  df &lt;- structure(list(Date = c("03/03/2011", "04/04/2011", "03/05/2015", "03/01/2017", "03/03/2017"),
               product= c(1L, 3L, 6L, 1L, 6L),
               product2= c(0L, 8L, 3L, 0L, 1L), 
               product3= c(7L, 2L, 89L, 2L, 6L)), 
               .Names= c("Date", "product", "product2", "product3"), 
               class= "data.frame", row.names=c(NA, -5L))
  </code></pre>