title: R collapse and auto fill blanks in rows
body: <p>I have the following example dataset:</p>
  
  <pre><code>ID = c(123,123)
  NAmer = c("ABC","ABC")
  field1 =  c(1,NA)
  field2 =  c(NA,2)
  field3 = c(NA,NA)
  field4 = c(NA,NA)
  field5 = c(NA,NA)
  IHave &lt;- data.frame(ID,NAmer,field1,field2,field3,field4,field5)
  Iwant &lt;- c(123,"ABC",1,2,NA,NA,NA)
  </code></pre>
  
  <p>How do I go from <code>IHave</code> to <code>Iwant</code> using <code>data.table</code> or <code>tidyverse</code>?</p>
  
  <p>In practice I have some 000 rows.</p>