title: How to fill NA with median?
body: <p>Example data:</p>
  
  <pre><code>set.seed(1)
  df &lt;- data.frame(years=sort(rep(2005:2010, 12)), 
                   months=1:12, 
                   value=c(rnorm(60),NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA,NA))
  
  head(df)
    years months      value
  1  2005      1 -0.6264538
  2  2005      2  0.1836433
  3  2005      3 -0.8356286
  4  2005      4  1.5952808
  5  2005      5  0.3295078
  6  2005      6 -0.8204684
  </code></pre>
  
  <p>Tell me please, how i can replace NA in df$value to median of others months? "value" must contain the median of value of all previous values for the same month. That is, if current month is May, "value" must contain the median value for all previous values of the month of May.</p>