title: filling in columns with matching IDs from two dataframes in R
body: <p>I have two dataframes (df1, df2). I want to fill in the AGE and SEX values from df1 to df2 conditioned on having the same ID between the two. I tried several ways using for-loop and checking subject ID match between the two data frame but I failed. The result should be as in the df3. I have a huge dataset, so I want a piece of code in R that can do this easily. I would appreciate your assistance in this. Thank you.</p>
  
  <pre><code>df1:
  ID    AGE   SEX
  90901   39  0
  90902   28  0
  90903   40  1
  
  df2:
  ID     AGE  SEX  Conc
  90901   NA  NA    5
  90901   NA  NA    10
  90901   NA  NA    15
  90903   NA  NA    30
  90903   NA  NA    5
  90902   NA  NA    2.45
  90902   NA  NA    51
  90902   NA  NA    1
  70905   NA  NA    0.5
  
  result:
  df3:
  ID     AGE  SEX  Conc
  90901   39  0     5
  90901   39  0     10
  90901   39  0     15
  90903   40  1    30
  90903   40  1    5
  90902   28  1    2.45
  90902   28  0    51
  90902   28  0     1
  70905   NA  NA    0.5
  </code></pre>