title: expand date value within a group when NA
body: <p>The data below have two general groups that contain multiple observations, some of which are <code>NA</code> in the <code>DLA</code> field. The <code>DLA</code> date is the same for all records within a group. How can I expand the <code>DLA</code> value to 'fill in' the <code>NA</code> values with the respective date. I am working within <code>dplyr</code>, which I suspect has a solution that I have not been able to locate. These data are a small subset of a larger data set with ~5k rows and ~500 individuals. Many thanks. </p>
  
  <pre><code>dat &lt;- structure(list(GenIndID = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 
  1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("BHS_106", 
  "BHS_164"), class = "factor"), IndID = structure(c(1L, 1L, 1L, 
  1L, 2L, 2L, 2L, 2L, 3L, 4L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 6L, 7L, 
  8L), .Label = c("BHS_106_A", "BHS_106_B", "BHS_106_C", "BHS_106_D", 
  "BHS_164_A", "BHS_164_B", "BHS_164_C", "BHS_164_D"), class = "factor"), 
      DLA = structure(c(1507010400, 1507010400, 1507010400, 1507010400, 
      1507010400, 1507010400, 1507010400, 1507010400, NA, NA, 1499061600, 
      1499061600, 1499061600, 1499061600, 1499061600, 1499061600, 
      1499061600, NA, NA, NA), tzone = "", class = c("POSIXct", 
      "POSIXt"))), .Names = c("GenIndID", "IndID", "DLA"), row.names = c(411L, 
  412L, 413L, 414L, 415L, 416L, 417L, 418L, 419L, 420L, 442L, 443L, 
  444L, 445L, 446L, 447L, 448L, 449L, 450L, 451L), class = "data.frame")
  
  &gt; dat
      GenIndID     IndID        DLA
  411  BHS_106 BHS_106_A 2017-10-03
  412  BHS_106 BHS_106_A 2017-10-03
  413  BHS_106 BHS_106_A 2017-10-03
  414  BHS_106 BHS_106_A 2017-10-03
  415  BHS_106 BHS_106_B 2017-10-03
  416  BHS_106 BHS_106_B 2017-10-03
  417  BHS_106 BHS_106_B 2017-10-03
  418  BHS_106 BHS_106_B 2017-10-03
  419  BHS_106 BHS_106_C       &lt;NA&gt;
  420  BHS_106 BHS_106_D       &lt;NA&gt;
  442  BHS_164 BHS_164_A 2017-07-03
  443  BHS_164 BHS_164_A 2017-07-03
  444  BHS_164 BHS_164_A 2017-07-03
  445  BHS_164 BHS_164_A 2017-07-03
  446  BHS_164 BHS_164_A 2017-07-03
  447  BHS_164 BHS_164_A 2017-07-03
  448  BHS_164 BHS_164_A 2017-07-03
  449  BHS_164 BHS_164_B       &lt;NA&gt;
  450  BHS_164 BHS_164_C       &lt;NA&gt;
  451  BHS_164 BHS_164_D       &lt;NA&gt;
  </code></pre>